train_single_lang.py

#!/usr/bin/env python3
"""

train_single_lang.py



Single-language fine-tuning of Nemotron-Speech-Streaming.

Adapted from train_multilingual_nemotron.py for per-language training.



This script is the entry point for every (language, hours, init) cell of

the main grid in the paper.  The init arm is selected by the combination

of --resume_from and --encoder_from / --reinit_encoder; the multilingual

tokenizer, decoder and joint always come from the multilingual base.



  ML init (paper's "ML" arm)

      --resume_from <multilingual_base.nemo>

      Encoder, decoder, joint and tokenizer all come from the multilingual

      base checkpoint.



  EN init (paper's "EN" arm)

      --resume_from <multilingual_base.nemo>

      --encoder_from nvidia/nemotron-speech-streaming-en-0.6b

      The decoder, joint and tokenizer are kept from the multilingual base

      (so the comparison isolates the encoder), and the encoder weights are

      overwritten layer-for-layer with the English-only Nemotron encoder.



  Random-encoder ablation (NOT the paper's EN arm)

      --resume_from <multilingual_base.nemo>

      --reinit_encoder

      Same as ML init except the encoder is freshly random-initialized.



  Direct from English checkpoint (legacy / not used in the main grid)

      --student nvidia/nemotron-speech-streaming-en-0.6b

      No --resume_from.  Everything (encoder, decoder, joint, tokenizer) is

      taken from the English checkpoint and only the prediction-network

      output layer is resized to the target tokenizer.



Example usage (paper ML init for German, 100 h):

  torchrun --nproc_per_node=1 train_single_lang.py \

    --lang de \

    --resume_from   <CKPT_DIR>/multilingual_base.nemo \

    --train_manifest <DATA_ROOT>/de/100h/train.jsonl \

    --val_manifest   <VAL_MANIFEST> \

    --output_dir ./out/de_100h_ml \

    --epochs 30 --batch_size 16 --grad_accum 3 --lr 1e-4 \

    --early_stop_patience 8 --decay_spec_augment --seed 42



Example usage (paper EN init for the same cell):

  torchrun --nproc_per_node=1 train_single_lang.py \

    --lang de \

    --resume_from   <CKPT_DIR>/multilingual_base.nemo \

    --encoder_from  nvidia/nemotron-speech-streaming-en-0.6b \

    --train_manifest <DATA_ROOT>/de/100h/train.jsonl \

    --val_manifest   <VAL_MANIFEST> \

    --output_dir ./out/de_100h_en \

    --epochs 30 --batch_size 16 --grad_accum 3 --lr 1e-4 \

    --early_stop_patience 8 --decay_spec_augment --seed 42



Manifest format (one JSON object per line):

  {"audio_filepath": "/abs/path/utt.wav", "duration": 8.4, "text": "reference transcript"}

  `duration` (seconds) is required: it drives min/max-duration filtering and

  the --max_train_hours subsampling.



Requirements:

  pip install nemo_toolkit[asr] soundfile jiwer tqdm

  Evaluation also requires Whisper's BasicMultilingualTextNormalizer from the

  Open ASR Leaderboard repo (clone https://github.com/huggingface/open_asr_leaderboard

  and add it to PYTHONPATH).

"""

import argparse
import json
import math
import os
import gc
import re
import sys
import unicodedata
from collections import defaultdict

import numpy as np
import torch
import torch.distributed as dist
import torch.nn as nn
import torch.nn.functional as F
from torch.nn.parallel import DistributedDataParallel as DDP
from torch.utils.data import DataLoader, DistributedSampler
from tqdm import tqdm

# Use Whisper's BasicMultilingualTextNormalizer for consistent eval
try:
    from normalizer import BasicMultilingualTextNormalizer
    _ml_normalizer = BasicMultilingualTextNormalizer()
    _has_real_normalizer = True
except ImportError:
    print(
        "ERROR: could not import BasicMultilingualTextNormalizer. "
        "This script requires Whisper's BasicMultilingualTextNormalizer, "
        "shipped in the Open ASR Leaderboard repo. "
        "Clone https://github.com/huggingface/open_asr_leaderboard and add it "
        "to PYTHONPATH (or set OPEN_ASR_LB_ROOT).",
        file=sys.stderr,
    )
    exit(1)
    _ml_normalizer = None
    _has_real_normalizer = False


# ═══════════════════════════════════════════════════════════
# DDP Utilities
# ═══════════════════════════════════════════════════════════

def setup_ddp():
    """Initialize distributed training. Returns (rank, world_size, local_rank, is_distributed)."""
    if "RANK" in os.environ:
        rank = int(os.environ["RANK"])
        world_size = int(os.environ["WORLD_SIZE"])
        local_rank = int(os.environ["LOCAL_RANK"])
        from datetime import timedelta
        dist.init_process_group("nccl", timeout=timedelta(minutes=60))
        torch.cuda.set_device(local_rank)
        return rank, world_size, local_rank, True
    else:
        return 0, 1, 0, False


def cleanup_ddp(is_distributed):
    if is_distributed:
        dist.destroy_process_group()


def is_main(rank):
    return rank == 0


def print_rank0(msg, rank=0):
    if is_main(rank):
        print(msg, flush=True)


# ═══════════════════════════════════════════════════════════
# Configuration
# ═══════════════════════════════════════════════════════════

def parse_args():
    p = argparse.ArgumentParser(
        description="Single-language Nemotron Streaming ASR Training",
        formatter_class=argparse.ArgumentDefaultsHelpFormatter,
    )
    # Language
    p.add_argument("--lang", type=str, required=True,
                   help="Language code (e.g., de, es, fr, it, nl, sv, pt, pl)")

    # Models
    p.add_argument("--teacher", default="nvidia/parakeet-tdt-0.6b-v3",
                   help="Teacher model for tokenizer extraction")
    p.add_argument("--student", default="nvidia/nemotron-speech-streaming-en-0.6b",
                   help="Student model (English, streaming)")

    # Data
    p.add_argument("--train_manifest", type=str, required=True,
                   help="Path to training manifest (JSONL)")
    p.add_argument("--val_manifest", type=str, required=True,
                   help="Path to validation manifest (JSONL)")

    # Training
    p.add_argument("--output_dir", default="./nemotron_lang")
    p.add_argument("--epochs", type=int, default=30)
    p.add_argument("--batch_size", type=int, default=16,
                   help="Per-GPU batch size")
    p.add_argument("--grad_accum", type=int, default=2,
                   help="Gradient accumulation steps")
    p.add_argument("--lr", type=float, default=1e-4)
    p.add_argument("--min_lr", type=float, default=1e-6)
    p.add_argument("--weight_decay", type=float, default=1e-3)
    p.add_argument("--warmup_epochs", type=int, default=1)
    p.add_argument("--max_duration", type=float, default=20.0)
    p.add_argument("--min_duration", type=float, default=0.3)

    # SpecAugment
    p.add_argument("--no_spec_augment", action="store_true", default=False)
    p.add_argument("--freq_masks", type=int, default=2)
    p.add_argument("--freq_width", type=int, default=27)
    p.add_argument("--time_masks", type=int, default=10)
    p.add_argument("--time_width", type=float, default=0.05)

    # Speed perturbation
    p.add_argument("--speed_perturb", action="store_true", default=True)
    p.add_argument("--speed_perturb_factors", type=float, nargs='+', default=[0.9, 1.0, 1.1])

    # Misc
    p.add_argument("--freeze_encoder_epochs", type=int, default=0,
                   help="Freeze encoder for first N epochs")
    p.add_argument("--reinit_encoder", action="store_true", default=False,
                   help="Randomly reinitialize all encoder weights (ablation). "
                        "NOT the paper's EN arm -- EN init uses --encoder_from "
                        "to copy the English encoder weights, not random init.")
    p.add_argument("--reinit_joint", action="store_true", default=False,
                   help="Randomly reinitialize the RNNT joint network weights (ablation study). "
                        "Useful after swapping encoders so the joint relearns the encoder->vocab mapping.")
    p.add_argument("--lr_decay_epochs", type=int, default=25,
                   help="Cosine decay reaches min_lr after N epochs (0=use total epochs)")
    p.add_argument("--constant_lr", action="store_true", default=False)
    p.add_argument("--log_every", type=int, default=50)
    p.add_argument("--eval_every_epoch", type=int, default=1)
    p.add_argument("--save_every_epoch", type=int, default=0,
                   help="Save training state every N epochs (0=disabled)")
    p.add_argument("--early_stop_patience", type=int, default=30,
                   help="Stop if WER doesn't improve for N evals (0=disabled)")
    p.add_argument("--grad_clip", type=float, default=1.0,
                   help="Gradient clipping max norm")
    p.add_argument("--rnnt_clamp", type=float, default=-1.0,
                   help="RNNT loss per-frame clamping value (-1=disabled, 1.0=recommended)")
    p.add_argument("--bf16", action="store_true", default=True)
    p.add_argument("--fp16", action="store_true", default=False)
    p.add_argument("--num_workers", type=int, default=4)
    p.add_argument("--max_train_hours", type=float, default=0,
                   help="Limit training data to N hours (0=use all data). Samples randomly.")
    p.add_argument("--data_seed", type=int, default=12345,
                   help="Seed for data subsampling (separate from training seed for reproducibility)")
    p.add_argument("--seed", type=int, default=42)
    p.add_argument("--resume_from", type=str, default=None,
                   help="Resume from .nemo checkpoint (the multilingual base for both "
                        "the ML and EN arms of the paper's main grid). Sets the "
                        "tokenizer, decoder and joint; the encoder is then either kept "
                        "(ML arm), overwritten via --encoder_from (EN arm), or "
                        "re-initialized via --reinit_encoder (ablation).")
    p.add_argument("--encoder_from", type=str, default=None,
                   help="Overwrite encoder weights with those of this checkpoint after "
                        "--resume_from has loaded the multilingual base. This is how "
                        "the paper's EN arm is built: multilingual tokenizer/decoder/joint "
                        "+ English encoder (nvidia/nemotron-speech-streaming-en-0.6b).")
    p.add_argument("--swap_joint_enc", action="store_true", default=False,
                   help="When using --encoder_from, also copy the joint network's encoder projection (enc linear + enc_hat) from the source model. Keeps encoder and joint.enc in sync.")
    p.add_argument("--encoder_from_layers", type=str, default="all",
                   help="Which Conformer layer indices to copy from --encoder_from. "
                        "Examples: 'all' (default, full encoder), 'none' (skip layers, only use preencode/postnorm flags), "
                        "'0:8' (Python slice, layers 0..7), '-8:' (last 8 layers), '0:8,16:24' (multiple ranges). "
                        "Negative indices count from the end. Layers outside the slice stay from --resume_from/--student.")
    p.add_argument("--encoder_from_preencode", type=str, default="off", choices=["auto", "on", "off"],
                   help="Copy the pre-encoder subsampling (conv frontend) and positional embeddings from --encoder_from. "
                        "Default 'off' keeps the destination model's preencode (ML baseline when --resume_from is set), "
                        "which is preferable for cross-lingual splices since the ML preencode has seen the target language. "
                        "'auto': on iff layer 0 is in --encoder_from_layers.")
    p.add_argument("--encoder_from_postnorm", type=str, default="off", choices=["auto", "on", "off"],
                   help="Copy the final encoder norm/output projection from --encoder_from. "
                        "Default 'off' keeps the destination model's postnorm (ML baseline when --resume_from is set), "
                        "which is the natural pairing when the top layers also come from the destination. "
                        "'auto': on iff the last layer is in --encoder_from_layers.")
    p.add_argument("--decay_spec_augment", action="store_true", default=False,
                   help="Linearly decay SpecAugment mask counts over training (time_masks: N->2, freq_masks: N->1)")
    p.add_argument("--resume_training", type=str, default=None,
                   help="Resume training from a training_state.pt checkpoint (saved in output_dir). Restores optimizer, scheduler, epoch, and all training state.")
    p.add_argument("--confidence_penalty", type=float, default=0.0,
                   help="Entropy regularization weight (0=off). Penalizes overconfident joint predictions. Try 0.1-0.3.")
    p.add_argument("--streaming_chunk_sec", type=float, default=0,
                   help="Enable chunk-aware streaming training. Chunk duration in seconds (e.g., 1.2). 0=full context training.")
    p.add_argument("--test_manifest", type=str, default=None,
                   help="Path to test manifest (JSONL) for final evaluation")
    p.add_argument("--decoder_hidden", type=int, default=0,
                   help="Override RNNT decoder (prediction network) LSTM hidden size. 0=keep original (640). E.g., 860, 1024.")
    p.add_argument("--decoder_layers", type=int, default=0,
                   help="Override RNNT decoder LSTM layer count. 0=keep original (2). E.g., 3, 4.")

    return p.parse_args()


# ═══════════════════════════════════════════════════════════
# Tokenizer Extraction (from teacher)
# ═══════════════════════════════════════════════════════════

def extract_tokenizer(model, tokenizer_dir):
    """Extract tokenizer .model file from a NeMo ASR model."""
    from pathlib import Path

    os.makedirs(tokenizer_dir, exist_ok=True)
    out_model = Path(tokenizer_dir) / "tokenizer.model"

    tok = getattr(model, "tokenizer", None)
    sp = getattr(tok, "tokenizer", None)

    if sp is not None and hasattr(sp, "serialized_model_proto"):
        blob = sp.serialized_model_proto()
        if blob:
            out_model.write_bytes(blob)
            _generate_vocab_txt(tokenizer_dir)
            vs = getattr(sp, "vocab_size", None)
            if callable(vs):
                vs = vs()
            return str(Path(tokenizer_dir)), int(vs) if vs else 0

    raise RuntimeError("Could not extract tokenizer from teacher model")


def _generate_vocab_txt(tokenizer_dir):
    import sentencepiece as spm_lib
    model_path = os.path.join(tokenizer_dir, "tokenizer.model")
    vocab_path = os.path.join(tokenizer_dir, "vocab.txt")
    if os.path.exists(vocab_path):
        return
    sp = spm_lib.SentencePieceProcessor()
    sp.load(model_path)
    with open(vocab_path, "w", encoding="utf-8") as f:
        for i in range(sp.get_piece_size()):
            f.write(sp.id_to_piece(i) + "\n")


# ═══════════════════════════════════════════════════════════
# Model Setup
# ═══════════════════════════════════════════════════════════

def setup_spec_augment(student, args):
    from nemo.collections.asr.modules.audio_preprocessing import SpectrogramAugmentation

    if args.no_spec_augment:
        student.spec_augmentation = None
        return

    spec_aug = SpectrogramAugmentation(
        freq_masks=args.freq_masks,
        time_masks=args.time_masks,
        freq_width=args.freq_width,
        time_width=args.time_width,
    )
    student.spec_augmentation = spec_aug.to(next(student.parameters()).device)


def update_spec_augment(student, args, epoch, total_epochs, rank):
    """Linearly decay SpecAugment mask counts over training."""
    if not args.decay_spec_augment or args.no_spec_augment:
        return
    from nemo.collections.asr.modules.audio_preprocessing import SpectrogramAugmentation

    progress = epoch / max(1, total_epochs - 1)
    new_time_masks = max(2, round(args.time_masks * (1 - progress)))
    new_freq_masks = max(1, round(args.freq_masks * (1 - 0.5 * progress)))

    model = student.module if isinstance(student, DDP) else student
    spec_aug = SpectrogramAugmentation(
        freq_masks=new_freq_masks,
        time_masks=new_time_masks,
        freq_width=args.freq_width,
        time_width=args.time_width,
    )
    model.spec_augmentation = spec_aug.to(next(model.parameters()).device)
    print_rank0(f"    SpecAug decay: freq={new_freq_masks}x{args.freq_width} time={new_time_masks}x{args.time_width}", rank)

    from omegaconf import open_dict
    with open_dict(model.cfg):
        model.cfg.spec_augment.freq_masks = new_freq_masks
        model.cfg.spec_augment.time_masks = new_time_masks
        model.cfg.spec_augment.freq_width = args.freq_width
        model.cfg.spec_augment.time_width = args.time_width


def _parse_layer_slice(spec, num_layers):
    """Parse a slice spec like 'all', 'none', '0:8', '-8:', '0:8,16:24' into a sorted set of indices.



    Supports Python-style slice ranges, comma-separated. Negative indices count from num_layers.

    Returns a set of ints in [0, num_layers).

    """
    if spec is None:
        return set()
    s = spec.strip().lower()
    if s in ("all", "*"):
        return set(range(num_layers))
    if s in ("none", ""):
        return set()
    out = set()
    for part in s.split(","):
        part = part.strip()
        if not part:
            continue
        if ":" in part:
            lo_s, hi_s = part.split(":", 1)
            lo = int(lo_s) if lo_s else 0
            hi = int(hi_s) if hi_s else num_layers
            if lo < 0:
                lo += num_layers
            if hi < 0:
                hi += num_layers
            lo = max(0, min(num_layers, lo))
            hi = max(0, min(num_layers, hi))
            out.update(range(lo, hi))
        else:
            idx = int(part)
            if idx < 0:
                idx += num_layers
            if 0 <= idx < num_layers:
                out.add(idx)
    return out


def _splice_encoder(dst_encoder, src_encoder, layer_indices, include_preencode, include_postnorm, rank):
    """Merge src_encoder weights into dst_encoder for the given layer indices, plus optional

    preencode (subsampling + positional encoding) and postnorm modules.



    Layer keys are expected to start with 'layers.<i>.'. Everything else is treated as

    'preencode-like' if its name matches pre_encode/pos_enc/embedding, or 'postnorm-like' otherwise.

    """
    src_sd = src_encoder.state_dict()
    dst_sd = dst_encoder.state_dict()

    PREENC_PREFIXES = ("pre_encode", "pos_enc", "pos_embedding", "pos_embed", "embedding")

    copied_layers = set()
    copied_pre = []
    copied_post = []
    skipped_shape = []
    skipped_missing = []

    for k, v in src_sd.items():
        if k.startswith("layers."):
            try:
                idx = int(k.split(".", 2)[1])
            except (ValueError, IndexError):
                continue
            if idx not in layer_indices:
                continue
            target_key = k
            bucket = ("layer", idx)
        elif any(k.startswith(p) for p in PREENC_PREFIXES):
            if not include_preencode:
                continue
            target_key = k
            bucket = ("pre", k)
        else:
            if not include_postnorm:
                continue
            target_key = k
            bucket = ("post", k)

        if target_key not in dst_sd:
            skipped_missing.append(target_key)
            continue
        if dst_sd[target_key].shape != v.shape:
            skipped_shape.append((target_key, tuple(v.shape), tuple(dst_sd[target_key].shape)))
            continue

        dst_sd[target_key] = v
        if bucket[0] == "layer":
            copied_layers.add(bucket[1])
        elif bucket[0] == "pre":
            copied_pre.append(target_key)
        else:
            copied_post.append(target_key)

    missing, unexpected = dst_encoder.load_state_dict(dst_sd, strict=False)
    print_rank0(
        f"  Encoder splice: copied layers {sorted(copied_layers)} "
        f"({len(copied_layers)} of {len(layer_indices)} requested)", rank
    )
    if copied_pre:
        print_rank0(f"  Encoder splice: copied {len(copied_pre)} preencode keys", rank)
    if copied_post:
        print_rank0(f"  Encoder splice: copied {len(copied_post)} postnorm/other keys", rank)
    if skipped_shape:
        print_rank0(f"  WARNING: skipped {len(skipped_shape)} keys due to shape mismatch (first: {skipped_shape[0]})", rank)
    if skipped_missing:
        print_rank0(f"  WARNING: skipped {len(skipped_missing)} keys missing in dst encoder (first: {skipped_missing[0]})", rank)


def load_student(args, device, rank):
    """Load student model, optionally swap tokenizer."""
    import nemo.collections.asr as nemo_asr

    if args.resume_from:
        print_rank0(f"  Resuming from: {args.resume_from}", rank)
        student = nemo_asr.models.ASRModel.restore_from(args.resume_from, map_location='cpu')
        print_rank0(f"  Vocab: {student.tokenizer.vocab_size} tokens", rank)
        args.freeze_encoder_epochs = 0

        # Optionally override encoder weights from a different model (e.g., English)
        if args.encoder_from:
            print_rank0(f"  Loading encoder from: {args.encoder_from}", rank)
            if args.encoder_from.endswith('.nemo'):
                encoder_model = nemo_asr.models.ASRModel.restore_from(args.encoder_from, map_location='cpu')
            else:
                encoder_model = nemo_asr.models.ASRModel.from_pretrained(args.encoder_from, map_location='cpu')

            # Determine number of Conformer layers from the source encoder
            src_layers_attr = getattr(encoder_model.encoder, "layers", None)
            num_layers = len(src_layers_attr) if src_layers_attr is not None else 0
            dst_layers_attr = getattr(student.encoder, "layers", None)
            dst_num_layers = len(dst_layers_attr) if dst_layers_attr is not None else 0
            if num_layers == 0 or dst_num_layers == 0:
                raise RuntimeError(
                    f"Could not locate '.encoder.layers' on src ({num_layers}) or dst ({dst_num_layers}); "
                    f"layer splicing requires a Conformer-style encoder with .layers nn.ModuleList."
                )
            if num_layers != dst_num_layers:
                print_rank0(
                    f"  WARNING: src encoder has {num_layers} layers but dst has {dst_num_layers}; "
                    f"only layers present in both will be copied.", rank
                )

            spec = args.encoder_from_layers or "all"
            effective_layers = min(num_layers, dst_num_layers)
            layer_indices = _parse_layer_slice(spec, effective_layers)

            # Auto-detect: preencode if EN provides layer 0; postnorm if EN provides last layer.
            auto_preencode = 0 in layer_indices
            auto_postnorm = (effective_layers - 1) in layer_indices

            def _resolve(flag, auto_value, name):
                if flag == "on":
                    return True
                if flag == "off":
                    return False
                return auto_value  # "auto"

            include_preencode = _resolve(args.encoder_from_preencode, auto_preencode, "preencode")
            include_postnorm = _resolve(args.encoder_from_postnorm, auto_postnorm, "postnorm")

            # Fast path: entire encoder copied (all layers + both boundaries) -> direct load_state_dict.
            full_copy = (
                len(layer_indices) == effective_layers
                and include_preencode
                and include_postnorm
                and num_layers == dst_num_layers
            )
            if full_copy:
                student.encoder.load_state_dict(encoder_model.encoder.state_dict())
                enc_params = sum(p.numel() for p in student.encoder.parameters()) / 1e6
                print_rank0(f"  Encoder fully swapped: {enc_params:.1f}M params from {args.encoder_from}", rank)
            else:
                print_rank0(
                    f"  Encoder splice spec='{spec}' ({len(layer_indices)}/{effective_layers} layers); "
                    f"preencode={include_preencode} ({args.encoder_from_preencode}"
                    f"{' -> auto=' + str(auto_preencode) if args.encoder_from_preencode == 'auto' else ''}), "
                    f"postnorm={include_postnorm} ({args.encoder_from_postnorm}"
                    f"{' -> auto=' + str(auto_postnorm) if args.encoder_from_postnorm == 'auto' else ''})", rank
                )
                _splice_encoder(
                    student.encoder,
                    encoder_model.encoder,
                    layer_indices,
                    include_preencode=include_preencode,
                    include_postnorm=include_postnorm,
                    rank=rank,
                )

            # Optionally also swap the joint network's encoder-side projection
            if args.swap_joint_enc:
                swapped_keys = []
                src_joint_sd = encoder_model.joint.state_dict()
                dst_joint_sd = student.joint.state_dict()
                for key in src_joint_sd:
                    # Match encoder-side projection layers (enc, enc_hat, etc.)
                    if 'enc' in key and key in dst_joint_sd and src_joint_sd[key].shape == dst_joint_sd[key].shape:
                        dst_joint_sd[key] = src_joint_sd[key]
                        swapped_keys.append(key)
                if swapped_keys:
                    student.joint.load_state_dict(dst_joint_sd)
                    print_rank0(f"  Joint encoder projection swapped: {swapped_keys}", rank)
                else:
                    print_rank0(f"  WARNING: --swap_joint_enc set but no matching joint.enc keys found", rank)

            del encoder_model
    else:
        # Extract teacher tokenizer (only rank 0 does this, then all read from disk)
        tokenizer_dir = os.path.join(args.output_dir, "teacher_tokenizer")
        if is_main(rank):
            print_rank0(f"  Loading teacher for tokenizer: {args.teacher}", rank)
            teacher = nemo_asr.models.ASRModel.from_pretrained(args.teacher)
            tokenizer_dir, teacher_vocab_size = extract_tokenizer(teacher, tokenizer_dir)
            print_rank0(f"  Teacher vocab: {teacher_vocab_size}", rank)
            del teacher
            torch.cuda.empty_cache()

        if dist.is_initialized():
            dist.barrier()

        # Load student
        print_rank0(f"  Loading student: {args.student}", rank)
        student = nemo_asr.models.ASRModel.from_pretrained(args.student)

        old_vocab = student.tokenizer.vocab_size
        student.change_vocabulary(new_tokenizer_dir=tokenizer_dir, new_tokenizer_type="bpe")
        new_vocab = student.tokenizer.vocab_size
        print_rank0(f"  Tokenizer swap: {old_vocab} → {new_vocab}", rank)

    # Optionally resize decoder (prediction network)
    if args.decoder_hidden > 0 or args.decoder_layers > 0:
        from omegaconf import open_dict, OmegaConf
        old_hidden = student.cfg.decoder.prednet.pred_hidden
        old_layers = student.cfg.decoder.prednet.pred_rnn_layers
        new_hidden = args.decoder_hidden if args.decoder_hidden > 0 else old_hidden
        new_layers = args.decoder_layers if args.decoder_layers > 0 else old_layers

        with open_dict(student.cfg):
            student.cfg.decoder.prednet.pred_hidden = new_hidden
            student.cfg.decoder.prednet.pred_rnn_layers = new_layers

        # Rebuild decoder + joint with new dimensions
        from nemo.collections.asr.modules import RNNTDecoder, RNNTJoint
        student.decoder = RNNTDecoder(
            prednet=OmegaConf.to_container(student.cfg.decoder.prednet, resolve=True),
            vocab_size=student.tokenizer.vocab_size,
            normalization_mode=student.cfg.decoder.get('normalization_mode', None),
            random_state_sampling=student.cfg.decoder.get('random_state_sampling', False),
            blank_as_pad=student.cfg.decoder.get('blank_as_pad', True),
        )

        # Rebuild joint network to match new decoder hidden
        with open_dict(student.cfg):
            student.cfg.joint.jointnet.pred_hidden = new_hidden
            student.cfg.joint.jointnet.encoder_hidden = student.cfg.encoder.d_model
            student.cfg.joint.num_classes = student.tokenizer.vocab_size
        joint_cfg = OmegaConf.to_container(student.cfg.joint, resolve=True)
        joint_cfg.pop('_target_', None)
        joint_cfg.pop('vocabulary', None)
        student.joint = RNNTJoint(**joint_cfg)

        dec_params = sum(p.numel() for p in student.decoder.parameters()) / 1e6
        joint_params = sum(p.numel() for p in student.joint.parameters()) / 1e6
        print_rank0(f"  Decoder resized: hidden {old_hidden}->{new_hidden}, layers {old_layers}->{new_layers}", rank)
        print_rank0(f"  New decoder params: {dec_params:.1f}M, joint params: {joint_params:.1f}M", rank)

    student = student.to(device)

    # Optionally reinitialize encoder weights (ablation study)
    if args.reinit_encoder:
        print_rank0("  REINITIALIZING ENCODER WEIGHTS (random init)", rank)
        for name, param in student.encoder.named_parameters():
            if param.dim() >= 2:
                torch.nn.init.xavier_uniform_(param)
            else:
                torch.nn.init.zeros_(param)
        # Also reinit batch norm running stats
        for module in student.encoder.modules():
            if isinstance(module, (torch.nn.BatchNorm1d, torch.nn.BatchNorm2d)):
                module.reset_running_stats()
        enc_params = sum(p.numel() for p in student.encoder.parameters())
        print_rank0(f"  Reinitialized {enc_params/1e6:.1f}M encoder parameters", rank)

    # Optionally reinitialize joint network weights (ablation study)
    if args.reinit_joint:
        print_rank0("  REINITIALIZING JOINT NETWORK WEIGHTS (random init)", rank)
        # Re-seed RNG with a fixed value so ML and EN arms get IDENTICAL joint init
        # (independent of prior RNG state consumed by different checkpoint loads).
        # Using args.seed (not args.seed+rank) ensures same init across DDP ranks too.
        _joint_seed = args.seed
        _gen_state_cpu = torch.random.get_rng_state()
        _gen_state_cuda = torch.cuda.get_rng_state_all() if torch.cuda.is_available() else None
        torch.manual_seed(_joint_seed)
        if torch.cuda.is_available():
            torch.cuda.manual_seed_all(_joint_seed)
        print_rank0(f"  Joint reinit RNG seeded with {_joint_seed} (identical across ML/EN arms)", rank)
        for name, param in student.joint.named_parameters():
            if param.dim() >= 2:
                torch.nn.init.xavier_uniform_(param)
            else:
                torch.nn.init.zeros_(param)
        for module in student.joint.modules():
            if isinstance(module, (torch.nn.BatchNorm1d, torch.nn.BatchNorm2d)):
                module.reset_running_stats()
        # Restore prior RNG state so training stochasticity is unaffected
        torch.random.set_rng_state(_gen_state_cpu)
        if _gen_state_cuda is not None:
            torch.cuda.set_rng_state_all(_gen_state_cuda)
        joint_params = sum(p.numel() for p in student.joint.parameters())
        print_rank0(f"  Reinitialized {joint_params/1e6:.1f}M joint parameters", rank)

    # SpecAugment
    setup_spec_augment(student, args)

    # Streaming chunk-aware training: restrict attention context
    if args.streaming_chunk_sec > 0:
        # Force single attention context mode [70, 13] instead of random multi-context
        # Default model has 4 modes: [70,13], [70,6], [70,1], [70,0] sampled randomly
        # This forces always using [70, 13] (largest context, lowest latency mode)
        student.encoder.att_context_size = [70, 13]
        student.encoder.att_context_size_all = [[70, 13]]
        student.encoder.att_context_probs = [1.0]
        print_rank0(f"  Streaming training: forced att_context=[70,13] only (no multi-context)", rank)

    # Disable CUDA graphs and typecheck
    from omegaconf import open_dict
    from nemo.core.classes.common import typecheck
    typecheck.set_typecheck_enabled(False)
    with open_dict(student.cfg):
        student.cfg.decoding.greedy.use_cuda_graph_decoder = False
    student.change_decoding_strategy(student.cfg.decoding)

    # Override RNNT loss clamping if requested
    if args.rnnt_clamp > 0:
        from nemo.collections.asr.losses.rnnt import RNNTLoss
        with open_dict(student.cfg):
            student.cfg.loss.warprnnt_numba_kwargs.clamp = args.rnnt_clamp
        student.loss = RNNTLoss(num_classes=student.decoder.vocab_size, loss_name='default',
                                loss_kwargs=dict(student.cfg.loss.warprnnt_numba_kwargs))
        print_rank0(f"  RNNT loss clamping: {args.rnnt_clamp}", rank)

    params = sum(p.numel() for p in student.parameters()) / 1e6
    print_rank0(f"  Student params: {params:.1f}M", rank)

    return student


# ═══════════════════════════════════════════════════════════
# Dataset & DataLoader
# ═══════════════════════════════════════════════════════════

class ASRManifestDataset(torch.utils.data.Dataset):
    def __init__(self, manifest_path, tokenizer, min_duration=0.3, max_duration=20.0,

                 speed_perturb=False, speed_perturb_factors=None,

                 max_train_hours=0, seed=42):
        self.tokenizer = tokenizer
        self.speed_perturb = speed_perturb
        self.speed_perturb_factors = speed_perturb_factors or [0.9, 1.0, 1.1]
        self.samples = []

        with open(manifest_path) as f:
            for line in f:
                item = json.loads(line)
                dur = item["duration"]
                if min_duration <= dur <= max_duration:
                    self.samples.append(item)

        # Subsample to max_train_hours if specified
        if max_train_hours > 0:
            target_seconds = max_train_hours * 3600
            rng = np.random.RandomState(seed)
            rng.shuffle(self.samples)
            selected = []
            total_dur = 0.0
            for s in self.samples:
                if total_dur >= target_seconds:
                    break
                selected.append(s)
                total_dur += s["duration"]
            self.samples = selected
            self.total_hours = total_dur / 3600

    def __len__(self):
        return len(self.samples)

    def __getitem__(self, idx):
        import soundfile as sf
        item = self.samples[idx]
        try:
            audio, sr = sf.read(item["audio_filepath"], dtype="float32")
        except Exception as e:
            print(f"    Corrupt audio: {item['audio_filepath']} ({e})", flush=True)
            return None

        if sr != 16000:
            ratio = 16000 / sr
            new_len = int(len(audio) * ratio)
            audio = np.interp(
                np.linspace(0, len(audio) - 1, new_len),
                np.arange(len(audio)), audio,
            ).astype(np.float32)

        # Speed perturbation
        if self.speed_perturb:
            import random
            speed = random.choice(self.speed_perturb_factors)
            if speed != 1.0:
                new_len = int(len(audio) / speed)
                audio = np.interp(
                    np.linspace(0, len(audio) - 1, new_len),
                    np.arange(len(audio)), audio,
                ).astype(np.float32)

        audio_tensor = torch.FloatTensor(audio)

        text = unicodedata.normalize("NFKC", item["text"])
        text = " ".join(text.split())
        tokens = torch.LongTensor(self.tokenizer.text_to_ids(text))

        return audio_tensor, tokens


def collate_asr(batch):
    batch = [b for b in batch if b is not None]
    if len(batch) == 0:
        return None
    audios = [b[0] for b in batch]
    tokens_list = [b[1] for b in batch]

    audio_lens = torch.LongTensor([len(a) for a in audios])
    token_lens = torch.LongTensor([len(t) for t in tokens_list])

    max_audio = audio_lens.max().item()
    max_tokens = token_lens.max().item()
    B = len(audios)

    padded_audio = torch.zeros(B, max_audio)
    padded_tokens = torch.zeros(B, max_tokens, dtype=torch.long)

    for i in range(B):
        padded_audio[i, :audio_lens[i]] = audios[i]
        padded_tokens[i, :token_lens[i]] = tokens_list[i]

    return padded_audio, audio_lens, padded_tokens, token_lens


# ═══════════════════════════════════════════════════════════
# Training Step
# ═══════════════════════════════════════════════════════════

def train_step(student, batch, device, confidence_penalty=0.0):
    """Single forward/backward step: RNNT loss + optional entropy regularization."""
    audio, audio_len, tokens, token_len = batch
    audio = audio.to(device)
    audio_len = audio_len.to(device)
    tokens = tokens.to(device)
    token_len = token_len.to(device)

    model = student.module if isinstance(student, DDP) else student

    # Mel spectrogram
    mel, mel_len = model.preprocessor(input_signal=audio, length=audio_len)

    # Spec augmentation
    if model.spec_augmentation is not None and model.training:
        mel = model.spec_augmentation(input_spec=mel, length=mel_len)

    # Encoder
    enc, enc_len = model.encoder(audio_signal=mel, length=mel_len)

    # Decoder
    dec_out = model.decoder(targets=tokens, target_length=token_len)
    if isinstance(dec_out, tuple):
        dec_out = dec_out[0]

    # Joint + RNNT loss
    if getattr(model.joint, 'fuse_loss_wer', False):
        result = model.joint(
            encoder_outputs=enc, decoder_outputs=dec_out,
            encoder_lengths=enc_len, transcripts=tokens,
            transcript_lengths=token_len, compute_wer=False,
        )
        loss = result[0]
    else:
        joint_out = model.joint(encoder_outputs=enc, decoder_outputs=dec_out)
        loss = model.loss(
            log_probs=joint_out, targets=tokens,
            input_lengths=enc_len, target_lengths=token_len,
        )

    if loss.dim() > 0:
        loss = loss.mean()

    # Confidence penalty: negative entropy regularization on joint output
    # Encourages less confident (more spread) predictions, acts like label smoothing
    if confidence_penalty > 0.0:
        # Extra forward pass through joint to get logits (works with fuse_loss_wer too)
        joint_logits = model.joint(encoder_outputs=enc, decoder_outputs=dec_out)
        # joint_logits: (B, T, U, V) log-probabilities
        probs = torch.exp(joint_logits)
        entropy = -(probs * joint_logits).sum(dim=-1)  # (B, T, U)
        # Maximize entropy = minimize negative entropy = subtract from loss
        loss = loss - confidence_penalty * entropy.mean()

    return loss


# ═══════════════════════════════════════════════════════════
# Evaluation
# ═══════════════════════════════════════════════════════════

def normalize_text(text):
    if _ml_normalizer is not None:
        return _ml_normalizer(text)
    text = unicodedata.normalize('NFKC', text)
    text = text.lower()
    text = re.sub(r'[^\w\s]', '', text)
    return ' '.join(text.split())


def normalize_text_fallback(text):
    """Always use fallback normalizer for consistency with older runs."""
    text = unicodedata.normalize('NFKC', text)
    text = text.lower()
    text = re.sub(r'[^\w\s]', '', text)
    return ' '.join(text.split())


def simple_wer(ref_words, hyp_words):
    n, m = len(ref_words), len(hyp_words)
    dp = [[0] * (m + 1) for _ in range(n + 1)]
    for i in range(n + 1): dp[i][0] = i
    for j in range(m + 1): dp[0][j] = j
    for i in range(1, n + 1):
        for j in range(1, m + 1):
            dp[i][j] = dp[i-1][j-1] if ref_words[i-1] == hyp_words[j-1] \
                else 1 + min(dp[i-1][j], dp[i][j-1], dp[i-1][j-1])
    return dp[n][m]


def compute_wer_ids(ref_words, hyp_words):
    """Edit distance with backtrace to get S/D/I counts."""
    n, m = len(ref_words), len(hyp_words)
    dp = [[0] * (m + 1) for _ in range(n + 1)]
    for i in range(n + 1): dp[i][0] = i
    for j in range(m + 1): dp[0][j] = j
    for i in range(1, n + 1):
        for j in range(1, m + 1):
            if ref_words[i - 1] == hyp_words[j - 1]:
                dp[i][j] = dp[i - 1][j - 1]
            else:
                dp[i][j] = 1 + min(dp[i - 1][j], dp[i][j - 1], dp[i - 1][j - 1])
    subs, dels, ins = 0, 0, 0
    i, j = n, m
    while i > 0 or j > 0:
        if i > 0 and j > 0 and ref_words[i - 1] == hyp_words[j - 1]:
            i -= 1; j -= 1
        elif i > 0 and j > 0 and dp[i][j] == dp[i - 1][j - 1] + 1:
            subs += 1; i -= 1; j -= 1
        elif i > 0 and dp[i][j] == dp[i - 1][j] + 1:
            dels += 1; i -= 1
        else:
            ins += 1; j -= 1
    return subs, dels, ins


@torch.no_grad()
def evaluate_batch(student, manifest_path, device, max_samples=None, normalizer_fn=None, rank=0):
    """Evaluate WER using batch (non-streaming) inference."""
    import soundfile as sf_eval

    if normalizer_fn is None:
        normalizer_fn = normalize_text

    model = student.module if isinstance(student, DDP) else student
    model.eval()

    samples = []
    with open(manifest_path) as f:
        for line in f:
            samples.append(json.loads(line))
    if max_samples and len(samples) > max_samples:
        samples = samples[:max_samples]

    total_edits, total_words = 0, 0
    total_subs, total_dels, total_ins = 0, 0, 0
    errors = 0
    batch_size = 16
    examples = []

    total_batches = (len(samples) + batch_size - 1) // batch_size
    for start in range(0, len(samples), batch_size):
        batch_samples = samples[start:start + batch_size]
        batch_num = start // batch_size + 1
        if (batch_num % 10 == 0 or batch_num == 1) and rank == 0:
            print(f"    [eval batch {batch_num}/{total_batches}]", flush=True)
        try:
            audios = []
            for s in batch_samples:
                audio, sr = sf_eval.read(s["audio_filepath"], dtype="float32")
                if len(audio.shape) > 1:
                    audio = audio.mean(axis=1)
                audios.append(torch.FloatTensor(audio))

            audio_lens = torch.LongTensor([len(a) for a in audios])
            max_len = audio_lens.max().item()
            padded = torch.zeros(len(audios), max_len)
            for i, a in enumerate(audios):
                padded[i, :len(a)] = a

            padded = padded.to(device)
            audio_lens = audio_lens.to(device)

            mel, mel_len = model.preprocessor(input_signal=padded, length=audio_lens)
            enc, enc_len = model.encoder(audio_signal=mel, length=mel_len)

            best_hyps = model.decoding.rnnt_decoder_predictions_tensor(enc, enc_len)
            if isinstance(best_hyps, tuple):
                best_hyps = best_hyps[0]

            for s, hyp in zip(batch_samples, best_hyps):
                if hasattr(hyp, 'text') and hyp.text:
                    pred = hyp.text
                elif hasattr(hyp, 'y_sequence'):
                    tids = hyp.y_sequence.tolist() if torch.is_tensor(hyp.y_sequence) else list(hyp.y_sequence)
                    pred = model.tokenizer.ids_to_text(tids) if tids else ""
                else:
                    pred = str(hyp)

                ref_n = normalizer_fn(s["text"])
                pred_n = normalizer_fn(pred)
                ref_words = ref_n.split()
                pred_words = pred_n.split()
                if ref_words:
                    sub_c, del_c, ins_c = compute_wer_ids(ref_words, pred_words)
                    total_subs += sub_c
                    total_dels += del_c
                    total_ins += ins_c
                    total_edits += sub_c + del_c + ins_c
                    total_words += len(ref_words)

                if len(examples) < 5:
                    examples.append((s["text"][:55], pred[:55]))

        except Exception as e:
            errors += 1
            if errors <= 3 and rank == 0:
                print(f"    [batch eval error] {type(e).__name__}: {e}")

    wer_score = total_edits / max(total_words, 1) * 100

    if normalizer_fn is normalize_text and rank == 0:  # Only print examples for primary eval
        print(f"\n    {'Reference':<55} | {'Prediction':<55}")
        print(f"    {'-'*55} | {'-'*55}")
        for ref, pred in examples:
            print(f"    {ref:<55} | {pred:<55}")
        if errors:
            print(f"    ({errors} batch eval errors)")

    model.train()
    return wer_score, total_subs, total_dels, total_ins, total_words


@torch.no_grad()
def evaluate_streaming(student, manifest_path, device, max_samples=None, rank=0):
    """Evaluate WER using streaming inference."""
    import soundfile as sf_eval
    from nemo.collections.asr.parts.utils.streaming_utils import CacheAwareStreamingAudioBuffer

    model = student.module if isinstance(student, DDP) else student
    model.eval()

    right_context = 13
    chunk_frames = 1 + right_context
    model.encoder.setup_streaming_params(
        chunk_size=chunk_frames,
        shift_size=chunk_frames,
        left_chunks=70 // max(chunk_frames, 1),
    )

    samples = []
    with open(manifest_path) as f:
        for line in f:
            samples.append(json.loads(line))
    if max_samples and len(samples) > max_samples:
        samples = samples[:max_samples]

    total_edits, total_words = 0, 0
    total_subs, total_dels, total_ins = 0, 0, 0
    examples = []
    errors = 0

    for s in samples:
        try:
            audio, sr = sf_eval.read(s["audio_filepath"], dtype="float32")
            if len(audio.shape) > 1:
                audio = audio.mean(axis=1)

            buffer = CacheAwareStreamingAudioBuffer(model=model)
            buffer.append_audio(audio)

            cache_last_channel, cache_last_time, cache_last_channel_len = \
                model.encoder.get_initial_cache_state(batch_size=1, dtype=torch.float32, device=device)
            previous_hypotheses = None
            pred = ""

            for chunk_audio, chunk_len in buffer:
                if chunk_audio is None:
                    break
                result = model.conformer_stream_step(
                    processed_signal=chunk_audio,
                    processed_signal_length=chunk_len,
                    cache_last_channel=cache_last_channel,
                    cache_last_time=cache_last_time,
                    cache_last_channel_len=cache_last_channel_len,
                    previous_hypotheses=previous_hypotheses,
                    return_transcription=True,
                )
                if isinstance(result, tuple) and len(result) >= 6:
                    cache_last_channel = result[2]
                    cache_last_time = result[3]
                    cache_last_channel_len = result[4]
                    previous_hypotheses = result[5]
                    if result[5] and len(result[5]) > 0:
                        hyp = result[5][0]
                        new_text = ""
                        if hasattr(hyp, 'text') and hyp.text:
                            new_text = hyp.text
                        elif hasattr(hyp, 'y_sequence'):
                            tids = hyp.y_sequence.tolist() if torch.is_tensor(hyp.y_sequence) else list(hyp.y_sequence)
                            if tids:
                                new_text = model.tokenizer.ids_to_text(tids)
                        if new_text and len(new_text) > len(pred):
                            pred = new_text

            ref_n = normalize_text(s["text"])
            pred_n = normalize_text(pred)
            ref_words = ref_n.split()
            pred_words = pred_n.split()

            if ref_words:
                sub_c, del_c, ins_c = compute_wer_ids(ref_words, pred_words)
                total_subs += sub_c
                total_dels += del_c
                total_ins += ins_c
                total_edits += sub_c + del_c + ins_c
                total_words += len(ref_words)

            if len(examples) < 5:
                examples.append((s["text"][:55], pred[:55]))

        except Exception as e:
            errors += 1
            if errors <= 3 and rank == 0:
                print(f"    [streaming eval error] {type(e).__name__}: {e}")

    wer_score = total_edits / max(total_words, 1) * 100

    if rank == 0:
        print(f"\n    {'Reference':<55} | {'Prediction':<55}")
        print(f"    {'-'*55} | {'-'*55}")
        for ref, pred in examples:
            print(f"    {ref:<55} | {pred:<55}")
        if errors:
            print(f"    ({errors} samples failed)")

    model.train()
    return wer_score, total_subs, total_dels, total_ins, total_words


@torch.no_grad()
def compute_val_loss(student, manifest_path, device, max_samples=500):
    """Compute RNNT loss on validation set."""
    import soundfile as sf_val

    model = student.module if isinstance(student, DDP) else student
    model.eval()

    samples = []
    with open(manifest_path) as f:
        for line in f:
            samples.append(json.loads(line))
    if max_samples and len(samples) > max_samples:
        samples = samples[:max_samples]

    total_loss = 0.0
    count = 0

    for s in samples:
        try:
            audio, sr = sf_val.read(s["audio_filepath"], dtype="float32")
            if len(audio.shape) > 1:
                audio = audio.mean(axis=1)

            text = unicodedata.normalize("NFKC", s["text"])
            text = " ".join(text.split())
            tokens = model.tokenizer.text_to_ids(text)
            if not tokens:
                continue

            audio_tensor = torch.FloatTensor(audio).unsqueeze(0).to(device)
            audio_len = torch.LongTensor([len(audio)]).to(device)
            token_tensor = torch.LongTensor([tokens]).to(device)
            token_len = torch.LongTensor([len(tokens)]).to(device)

            mel, mel_len = model.preprocessor(input_signal=audio_tensor, length=audio_len)
            enc, enc_len = model.encoder(audio_signal=mel, length=mel_len)
            dec_out = model.decoder(targets=token_tensor, target_length=token_len)
            if isinstance(dec_out, tuple):
                dec_out = dec_out[0]

            if getattr(model.joint, 'fuse_loss_wer', False):
                result = model.joint(
                    encoder_outputs=enc, decoder_outputs=dec_out,
                    encoder_lengths=enc_len, transcripts=token_tensor,
                    transcript_lengths=token_len, compute_wer=False,
                )
                loss = result[0]
            else:
                joint_out = model.joint(encoder_outputs=enc, decoder_outputs=dec_out)
                loss = model.loss(log_probs=joint_out, targets=token_tensor,
                                  input_lengths=enc_len, target_lengths=token_len)

            if loss.dim() > 0:
                loss = loss.mean()
            total_loss += loss.item()
            count += 1
        except Exception:
            continue

    model.train()
    return total_loss / max(count, 1)


# ═══════════════════════════════════════════════════════════
# LR Schedule
# ═══════════════════════════════════════════════════════════

def get_cosine_schedule(optimizer, warmup_steps, total_steps, min_lr=1e-6):
    base_lr = optimizer.defaults["lr"]

    def lr_lambda(step):
        if step < warmup_steps:
            return max(1e-8 / base_lr, step / max(1, warmup_steps))
        progress = min(1.0, (step - warmup_steps) / max(1, total_steps - warmup_steps))
        return (min_lr + 0.5 * (base_lr - min_lr) * (1.0 + math.cos(math.pi * progress))) / base_lr

    return torch.optim.lr_scheduler.LambdaLR(optimizer, lr_lambda)


def get_constant_schedule(optimizer, warmup_steps):
    base_lr = optimizer.defaults["lr"]

    def lr_lambda(step):
        if step < warmup_steps:
            return max(1e-8 / base_lr, step / max(1, warmup_steps))
        return 1.0

    return torch.optim.lr_scheduler.LambdaLR(optimizer, lr_lambda)


# ═══════════════════════════════════════════════════════════
# Main Training Loop
# ═══════════════════════════════════════════════════════════

def train(student, train_loader, train_sampler, args, device, rank, is_distributed):
    os.makedirs(args.output_dir, exist_ok=True)

    lang = args.lang.upper()
    model = student.module if isinstance(student, DDP) else student

    trainable_params = [p for p in student.parameters() if p.requires_grad]
    optimizer = torch.optim.AdamW(
        trainable_params,
        lr=args.lr,
        weight_decay=args.weight_decay,
        betas=(0.9, 0.98),
        eps=1e-9,
    )

    steps_per_epoch = len(train_loader) // args.grad_accum
    total_steps = steps_per_epoch * args.epochs
    warmup_steps = steps_per_epoch * args.warmup_epochs

    if args.constant_lr:
        scheduler = get_constant_schedule(optimizer, warmup_steps)
    else:
        decay_epochs = args.lr_decay_epochs if args.lr_decay_epochs > 0 else args.epochs
        cosine_total_steps = steps_per_epoch * decay_epochs
        scheduler = get_cosine_schedule(optimizer, warmup_steps, cosine_total_steps, args.min_lr)

    use_amp = args.bf16 or args.fp16
    amp_dtype = torch.bfloat16 if args.bf16 else torch.float16
    scaler = torch.amp.GradScaler("cuda") if args.fp16 else None

    effective_batch = args.batch_size * args.grad_accum
    if is_distributed:
        world_size = dist.get_world_size()
        effective_batch *= world_size
    else:
        world_size = 1

    starting_from = args.resume_from if args.resume_from else args.student
    print_rank0(f"\n{'='*65}", rank)
    print_rank0(f"  {lang} Training Configuration", rank)
    print_rank0(f"{'='*65}", rank)
    print_rank0(f"  Language:           {lang}", rank)
    print_rank0(f"  Starting from:      {starting_from}", rank)
    print_rank0(f"  GPUs:               {world_size}", rank)
    print_rank0(f"  Train samples:      {len(train_loader.dataset)}", rank)
    print_rank0(f"  Per-GPU batch:      {args.batch_size}", rank)
    print_rank0(f"  Effective batch:    {args.batch_size} x {args.grad_accum} x {world_size} = {effective_batch}", rank)
    print_rank0(f"  Steps/epoch:        {steps_per_epoch}", rank)
    print_rank0(f"  Total steps:        {total_steps}", rank)
    print_rank0(f"  Warmup steps:       {warmup_steps}", rank)
    print_rank0(f"  Learning rate:      {args.lr} ({'constant' if args.constant_lr else 'cosine decay'})", rank)
    print_rank0(f"  Min LR:             {args.min_lr}", rank)
    print_rank0(f"  Freeze encoder:     first {args.freeze_encoder_epochs} epochs", rank)
    print_rank0(f"  Mixed precision:    {'bf16' if args.bf16 else 'fp16' if args.fp16 else 'off'}", rank)
    print_rank0(f"  Weight decay:       {args.weight_decay}", rank)
    print_rank0(f"  Grad clip norm:     {args.grad_clip}", rank)
    if args.no_spec_augment:
        print_rank0(f"  SpecAugment:        OFF", rank)
    else:
        print_rank0(f"  SpecAugment:        freq={args.freq_masks}x{args.freq_width} time={args.time_masks}x{args.time_width}", rank)
    print_rank0(f"  Speed perturb:      {args.speed_perturb_factors if args.speed_perturb else 'OFF'}", rank)
    print_rank0(f"  LR decay epochs:    {args.lr_decay_epochs}", rank)
    print_rank0(f"  Early stop:         {args.early_stop_patience} epochs", rank)
    if args.confidence_penalty > 0:
        print_rank0(f"  Confidence penalty: {args.confidence_penalty}", rank)
    if args.streaming_chunk_sec > 0:
        print_rank0(f"  Streaming train:    forced att_context=[70,13] only", rank)
    print_rank0(f"  Val manifest:       {args.val_manifest}", rank)
    print_rank0(f"{'='*65}\n", rank)

    global_step = 0
    best_wer = float("inf")
    best_val_loss = float("inf")
    patience_counter = 0
    start_epoch = 0

    # Track top-K best WER checkpoints
    top_k_wer = 3
    top_k_checkpoints = []  # list of (wer, epoch, path)

    # Track WER per epoch for convergence analysis
    wer_history = []

    import time as time_module

    # Resume from training checkpoint if specified
    if args.resume_training:
        ckpt_path = args.resume_training
        if os.path.isdir(ckpt_path):
            # Load latest model weights if available
            latest_model_path = os.path.join(ckpt_path, "latest_model.pt")
            if os.path.exists(latest_model_path):
                print_rank0(f"  Loading latest model weights from: {latest_model_path}", rank)
                sd = torch.load(latest_model_path, map_location=device, weights_only=False)
                model.load_state_dict(sd)
                del sd
                torch.cuda.empty_cache()
            ckpt_path = os.path.join(ckpt_path, "training_state.pt")
        if os.path.exists(ckpt_path):
            print_rank0(f"  Resuming training state from: {ckpt_path}", rank)
            ckpt = torch.load(ckpt_path, map_location=device, weights_only=False)
            optimizer.load_state_dict(ckpt["optimizer"])
            scheduler.load_state_dict(ckpt["scheduler"])
            start_epoch = ckpt["epoch"]
            global_step = ckpt["global_step"]
            best_wer = ckpt["best_wer"]
            best_val_loss = ckpt["best_val_loss"]
            patience_counter = ckpt["patience_counter"]
            wer_history = ckpt.get("wer_history", [])
            if scaler and "scaler" in ckpt:
                scaler.load_state_dict(ckpt["scaler"])
            print_rank0(f"  Resumed at epoch {start_epoch}, step {global_step}, best_wer={best_wer:.2f}%", rank)
            del ckpt
            torch.cuda.empty_cache()
        else:
            print_rank0(f"  WARNING: resume_training path not found: {ckpt_path}", rank)

    if is_distributed:
        dist.barrier()

    # ── Epoch 0: evaluate before any training ──
    if start_epoch == 0:
        print_rank0(f"\n  === Epoch 0 (pre-training baseline) ===", rank)
        import sys as _sys

        # Compute initial train loss + grad norm on first few batches (all ranks)
        student.train()
        e0_loss = 0.0
        e0_gnorm = 0.0
        e0_steps = 0
        e0_max_batches = 20
        for batch_idx, batch in enumerate(train_loader):
            if batch is None:
                continue
            if batch_idx >= e0_max_batches:
                break
            try:
                if use_amp:
                    with torch.amp.autocast("cuda", dtype=amp_dtype):
                        loss = train_step(student, batch, device, confidence_penalty=0.0)
                    loss.backward()
                else:
                    loss = train_step(student, batch, device, confidence_penalty=0.0)
                    loss.backward()
                trainable = [p for p in student.parameters() if p.requires_grad and p.grad is not None]
                gnorm = torch.nn.utils.clip_grad_norm_(trainable, 1e6).item()
                e0_loss += loss.item()
                if math.isfinite(gnorm):
                    e0_gnorm += gnorm
                e0_steps += 1
                optimizer.zero_grad()
            except Exception:
                optimizer.zero_grad()
                continue

        if e0_steps > 0:
            print_rank0(f"    Epoch 0 train_loss={e0_loss/e0_steps:.4f} gnorm={e0_gnorm/e0_steps:.3f} (avg over {e0_steps} batches)", rank)

        # Tear down DDP before eval (same as regular epoch eval)
        if is_distributed:
            optimizer.zero_grad(set_to_none=True)
            del student
            student = None
            torch.cuda.empty_cache()
            dist.barrier()

        # Free GPU memory before eval
        optimizer.zero_grad(set_to_none=True)
        opt_state_backup_e0 = {}
        for k, v in optimizer.state.items():
            opt_state_backup_e0[k] = {sk: sv.cpu() if torch.is_tensor(sv) else sv for sk, sv in v.items()}
        optimizer.state.clear()
        torch.cuda.empty_cache()
        gc.collect()

        # Evaluate WER + val_loss (all ranks run forward passes)
        print_rank0(f"\n  Evaluating {lang} (epoch 0)...", rank)
        _sys.stdout.flush()
        val_wer, _, _, _, _ = evaluate_batch(model, args.val_manifest, device, rank=rank)
        print_rank0(f"    [eval] WER done", rank); _sys.stdout.flush()
        val_loss = compute_val_loss(model, args.val_manifest, device)
        print_rank0(f"    [eval] val_loss done", rank); _sys.stdout.flush()

        if is_main(rank):
            print_rank0(f"    Epoch 0 WER: {val_wer:.2f}% | Val loss: {val_loss:.4f}", rank)
            wer_history.append({
                'epoch': 0,
                'wer': val_wer,
                'val_loss': val_loss,
                'lr': 0.0,
                'train_loss': e0_loss / max(1, e0_steps),
            })
        _sys.stdout.flush()

        # Restore optimizer states from CPU
        for k, v in opt_state_backup_e0.items():
            optimizer.state[k] = {sk: sv.to(device) if torch.is_tensor(sv) else sv for sk, sv in v.items()}
        del opt_state_backup_e0
        torch.cuda.empty_cache()

        # Rebuild DDP for training
        if is_distributed:
            student = DDP(model, device_ids=[int(os.environ.get("LOCAL_RANK", 0))], find_unused_parameters=True)
            optimizer.param_groups[0]["params"] = [p for p in student.parameters() if p.requires_grad]

        # Reset dataloader state
        if train_sampler is not None:
            train_sampler.set_epoch(0)

    for epoch in range(start_epoch, args.epochs):
        epoch_start = time_module.time()
        student.train()

        # Decay SpecAugment if enabled (all ranks need updated module)
        if args.decay_spec_augment:
            update_spec_augment(student, args, epoch, args.epochs, rank)

        if train_sampler is not None:
            train_sampler.set_epoch(epoch)

        # Phase management
        if epoch < args.freeze_encoder_epochs:
            for p in model.encoder.parameters():
                p.requires_grad = False
            phase = f"Encoder frozen ({epoch+1}/{args.freeze_encoder_epochs})"
        else:
            for p in model.encoder.parameters():
                p.requires_grad = True
            phase = "Full training"

        optimizer.param_groups[0]["params"] = [
            p for p in student.parameters() if p.requires_grad
        ]

        epoch_loss = 0.0
        epoch_steps = 0
        epoch_grad_norm = 0.0
        grad_norm_steps = 0
        inf_grad_steps = 0

        pbar = tqdm(train_loader, desc=f"Epoch {epoch+1}/{args.epochs} [{lang}]",
                    leave=True, ncols=120, disable=not is_main(rank) or not sys.stderr.isatty())
        optimizer.zero_grad()

        max_batch_audio_sec = 0.0
        batch_lengths = []

        for batch_idx, batch in enumerate(pbar):
            if batch is None:
                continue

            # Track batch audio lengths
            audio, audio_len_t, _, _ = batch
            max_audio_samples = audio_len_t.max().item()
            total_audio_samples = audio_len_t.sum().item()
            max_sec = max_audio_samples / 16000
            total_sec = total_audio_samples / 16000
            batch_lengths.append(max_sec)
            if max_sec > max_batch_audio_sec:
                max_batch_audio_sec = max_sec

            try:
                if use_amp:
                    with torch.amp.autocast("cuda", dtype=amp_dtype):
                        loss = train_step(student, batch, device, confidence_penalty=args.confidence_penalty)
                    if scaler:
                        scaled_loss = loss / args.grad_accum
                        scaler.scale(scaled_loss).backward()
                    else:
                        (loss / args.grad_accum).backward()
                else:
                    loss = train_step(student, batch, device, confidence_penalty=args.confidence_penalty)
                    (loss / args.grad_accum).backward()

            except RuntimeError as e:
                if "out of memory" in str(e).lower():
                    torch.cuda.empty_cache()
                    print_rank0(f"\n    OOM at batch {batch_idx} (max_audio={max_sec:.1f}s, total={total_sec:.1f}s, batch_size={len(audio_len_t)}), skipping", rank)
                    optimizer.zero_grad()
                    continue
                raise

            epoch_loss += loss.item()
            epoch_steps += 1

            if (batch_idx + 1) % args.grad_accum == 0:
                if scaler:
                    scaler.unscale_(optimizer)
                trainable = [p for p in student.parameters() if p.requires_grad and p.grad is not None]
                grad_norm = torch.nn.utils.clip_grad_norm_(trainable, args.grad_clip).item()
                if scaler:
                    scaler.step(optimizer)
                    scaler.update()
                else:
                    optimizer.step()

                if math.isfinite(grad_norm):
                    epoch_grad_norm += grad_norm
                    grad_norm_steps += 1
                else:
                    inf_grad_steps += 1
                scheduler.step()
                optimizer.zero_grad()
                global_step += 1

            if epoch_steps % args.log_every == 0 and epoch_steps > 0 and is_main(rank):
                avg_loss = epoch_loss / epoch_steps
                avg_gnorm = epoch_grad_norm / max(1, grad_norm_steps)
                lr = optimizer.param_groups[0]["lr"]
                gnorm_str = f"{avg_gnorm:.2f}" if grad_norm_steps > 0 else "n/a"
                if inf_grad_steps > 0:
                    gnorm_str += f" ({inf_grad_steps} skipped)"
                pbar.set_postfix(loss=f"{avg_loss:.3f}", gnorm=gnorm_str,
                                 lr=f"{lr:.1e}", step=global_step)

        # End of epoch
        epoch_time = time_module.time() - epoch_start
        avg_loss = epoch_loss / max(1, epoch_steps)
        avg_gnorm = epoch_grad_norm / max(1, grad_norm_steps)
        gnorm_display = f"{avg_gnorm:.3f}" if grad_norm_steps > 0 else "n/a"
        if inf_grad_steps > 0:
            gnorm_display += f" ({inf_grad_steps} inf-skipped)"
        samples_per_sec = (epoch_steps * args.batch_size) / epoch_time
        gpu_mem = torch.cuda.max_memory_allocated(device) / 1e9 if torch.cuda.is_available() else 0

        # Batch length stats
        if batch_lengths:
            avg_max_sec = sum(batch_lengths) / len(batch_lengths)
            p95 = sorted(batch_lengths)[int(0.95 * len(batch_lengths))]
            print_rank0(f"    [batch stats] max_audio={max_batch_audio_sec:.1f}s avg_max={avg_max_sec:.1f}s p95={p95:.1f}s", rank)

        print_rank0(f"\n  Epoch {epoch+1} [{lang}] | {phase} | loss={avg_loss:.4f} "
                    f"gnorm={gnorm_display} lr={optimizer.param_groups[0]['lr']:.2e}"
                    f" | {epoch_time/60:.1f}min | {samples_per_sec:.0f} samples/s | GPU: {gpu_mem:.1f}GB", rank)

        # Tear down DDP before eval to remove forward hooks from model.
        # DDP with find_unused_parameters=True registers hooks on the underlying
        # model, so calling model.forward() during rank-0-only eval would trigger
        # DDP communication and deadlock the other ranks.
        if is_distributed:
            optimizer.zero_grad(set_to_none=True)
            del student
            student = None
            torch.cuda.empty_cache()
            dist.barrier()

        # Free GPU memory before eval: offload optimizer states to CPU
        optimizer.zero_grad(set_to_none=True)
        opt_state_backup = {}
        for k, v in optimizer.state.items():
            opt_state_backup[k] = {sk: sv.cpu() if torch.is_tensor(sv) else sv for sk, sv in v.items()}
        optimizer.state.clear()
        torch.cuda.empty_cache()
        gc.collect()

        mem_after = torch.cuda.memory_allocated(device) / 1e9
        print_rank0(f"    [pre-eval] GPU mem after offload: {mem_after:.1f}GB", rank)
        import sys as _sys; _sys.stdout.flush()

        # Evaluation — ALL ranks run forward passes (needed for SyncBatchNorm / distributed
        # layers inside NeMo encoder), but only rank 0 uses the results.
        if (epoch + 1) % args.eval_every_epoch == 0:
            try:
                print_rank0(f"\n  Evaluating {lang}...", rank)
                _sys.stdout.flush()

                val_wer, _, _, _, _ = evaluate_batch(model, args.val_manifest, device, rank=rank)
                print_rank0(f"    [eval] WER done", rank); _sys.stdout.flush()
                val_loss = compute_val_loss(model, args.val_manifest, device)
                print_rank0(f"    [eval] val_loss done", rank); _sys.stdout.flush()

                if is_main(rank):
                    print_rank0(f"    {lang} Batch WER: {val_wer:.2f}% | Val loss: {val_loss:.4f}", rank)

                    wer_history.append({
                        'epoch': epoch + 1,
                        'wer': val_wer,
                        'val_loss': val_loss,
                        'lr': optimizer.param_groups[0]["lr"],
                        'train_loss': avg_loss,
                    })

                    if val_wer < best_wer:
                        best_wer = val_wer
                        patience_counter = 0
                        save_path = os.path.join(args.output_dir, "best_model.nemo")
                        print_rank0(f"    [saving] best_model.nemo...", rank); _sys.stdout.flush()
                        model.save_to(save_path)
                        print_rank0(f"    New best WER! WER={best_wer:.2f}% -> {save_path}", rank)
                    else:
                        patience_counter += 1
                        print_rank0(f"    No WER improvement ({patience_counter}/{args.early_stop_patience})", rank)

                    # Save top-K best WER checkpoints for post-hoc evaluation
                    should_save_topk = len(top_k_checkpoints) < top_k_wer or val_wer < top_k_checkpoints[-1][0]
                    if should_save_topk:
                        topk_path = os.path.join(args.output_dir, f"best_model_wer_ep{epoch+1}.nemo")
                        print_rank0(f"    [saving] top-{top_k_wer} checkpoint (WER={val_wer:.2f}%)...", rank); _sys.stdout.flush()
                        model.save_to(topk_path)
                        top_k_checkpoints.append((val_wer, epoch + 1, topk_path))
                        top_k_checkpoints.sort(key=lambda x: x[0])  # sort by WER ascending
                        # Remove worst checkpoint if we exceed top_k_wer
                        while len(top_k_checkpoints) > top_k_wer:
                            _, _, old_path = top_k_checkpoints.pop()
                            if os.path.exists(old_path):
                                os.remove(old_path)
                                print_rank0(f"    [removed] {os.path.basename(old_path)}", rank)

                    if args.early_stop_patience > 0 and patience_counter >= args.early_stop_patience:
                        print_rank0(f"\n    Early stopping! No improvement for {args.early_stop_patience} epochs.", rank)
                        break

                    if val_loss < best_val_loss:
                        best_val_loss = val_loss
                        save_path = os.path.join(args.output_dir, "best_model_loss.nemo")
                        print_rank0(f"    [saving] best_model_loss.nemo...", rank); _sys.stdout.flush()
                        model.save_to(save_path)
                        print_rank0(f"    New best loss! loss={best_val_loss:.4f} -> {save_path}", rank)
            except Exception as e:
                print_rank0(f"    [eval error] {type(e).__name__}: {e} — skipping", rank)

        print_rank0(f"    [post-eval] reaching barrier...", rank); sys.stdout.flush()

        # Restore optimizer states from CPU
        for k, v in opt_state_backup.items():
            optimizer.state[k] = {sk: sv.to(device) if torch.is_tensor(sv) else sv for sk, sv in v.items()}
        del opt_state_backup
        torch.cuda.empty_cache()

        # Rebuild DDP for next training epoch (after eval is done)
        if is_distributed:
            student = DDP(model, device_ids=[int(os.environ.get("LOCAL_RANK", 0))], find_unused_parameters=True)
            optimizer.param_groups[0]["params"] = [p for p in student.parameters() if p.requires_grad]

        # Save full training state for resumability
        if is_main(rank) and args.save_every_epoch > 0 and (epoch + 1) % args.save_every_epoch == 0:
            # Save latest model weights as raw state dict (fast, avoids NeMo save_to overhead)
            latest_path = os.path.join(args.output_dir, "latest_model.pt")
            torch.save(model.state_dict(), latest_path + ".tmp")
            os.replace(latest_path + ".tmp", latest_path)

            state = {
                "epoch": epoch + 1,
                "global_step": global_step,
                "optimizer": optimizer.state_dict(),
                "scheduler": scheduler.state_dict(),
                "best_wer": best_wer,
                "best_val_loss": best_val_loss,
                "patience_counter": patience_counter,
                "wer_history": wer_history,
            }
            if scaler:
                state["scaler"] = scaler.state_dict()
            state_path = os.path.join(args.output_dir, "training_state.pt")
            torch.save(state, state_path + ".tmp")
            os.replace(state_path + ".tmp", state_path)

        # Clear GPU memory fragmentation from eval/save before next training epoch
        torch.cuda.empty_cache()

        if is_distributed:
            dist.barrier()

    # Save WER history for convergence analysis
    if is_main(rank):
        history_path = os.path.join(args.output_dir, "wer_history.json")
        with open(history_path, "w") as f:
            json.dump(wer_history, f, indent=2)
        print_rank0(f"\n  WER history saved to {history_path}", rank)

        # Final save
        save_path = os.path.join(args.output_dir, "final_model.nemo")
        model.save_to(save_path)
        print_rank0(f"  Final model -> {save_path}", rank)
        print_rank0(f"  Best {lang} WER: {best_wer:.2f}%", rank)

    return student


# ═══════════════════════════════════════════════════════════
# Entry Point
# ═══════════════════════════════════════════════════════════

def main():
    args = parse_args()

    rank, world_size, local_rank, is_distributed = setup_ddp()
    device = torch.device(f"cuda:{local_rank}")

    torch.manual_seed(args.seed + rank)
    np.random.seed(args.seed + rank)
    torch.cuda.manual_seed_all(args.seed + rank)
    import random
    random.seed(args.seed + rank)

    os.makedirs(args.output_dir, exist_ok=True)

    lang = args.lang.upper()
    starting_from = "multilingual base" if args.resume_from else "English checkpoint"

    print_rank0(f"\n{'='*65}", rank)
    print_rank0(f"  Nemotron Streaming ASR — {lang} Training", rank)
    print_rank0(f"{'='*65}", rank)
    print_rank0(f"  Language:  {lang}", rank)
    print_rank0(f"  Path:      {starting_from}", rank)
    print_rank0(f"  GPUs:      {world_size}", rank)
    if args.resume_from:
        print_rank0(f"  Base:      {args.resume_from}", rank)
    else:
        print_rank0(f"  Student:   {args.student}", rank)
    print_rank0(f"  Train:     {args.train_manifest}", rank)
    print_rank0(f"  Val:       {args.val_manifest}", rank)
    print_rank0(f"{'='*65}", rank)

    # Load model
    print_rank0(f"\n[1/3] Loading model...", rank)
    student = load_student(args, device, rank)

    # Wrap in DDP
    if is_distributed:
        student = DDP(student, device_ids=[local_rank], find_unused_parameters=True)
        print_rank0(f"  Wrapped in DDP (find_unused_parameters=True)", rank)

    # Create dataloader
    print_rank0(f"\n[2/3] Creating data loaders...", rank)
    model_for_tok = student.module if isinstance(student, DDP) else student
    train_dataset = ASRManifestDataset(
        args.train_manifest,
        model_for_tok.tokenizer,
        min_duration=args.min_duration,
        max_duration=args.max_duration,
        speed_perturb=args.speed_perturb,
        speed_perturb_factors=args.speed_perturb_factors,
        max_train_hours=args.max_train_hours,
        seed=args.data_seed,
    )
    print_rank0(f"  Train dataset: {len(train_dataset)} samples", rank)
    if args.max_train_hours > 0:
        print_rank0(f"  Subsampled to {train_dataset.total_hours:.1f}h (requested {args.max_train_hours}h)", rank)

    # Offset by -42 so --seed=42 (default) keeps PyTorch's default sampler seed=0
    # for backward compat with prior runs. --seed=43 -> sampler seed=1, etc.
    train_sampler = DistributedSampler(train_dataset, shuffle=True, seed=args.seed - 42) if is_distributed else None
    train_loader = DataLoader(
        train_dataset,
        batch_size=args.batch_size,
        shuffle=(train_sampler is None),
        sampler=train_sampler,
        num_workers=args.num_workers,
        collate_fn=collate_asr,
        pin_memory=True,
        drop_last=True,
    )

    # Train
    print_rank0(f"\n[3/3] Starting {lang} training...", rank)
    student = train(
        student, train_loader, train_sampler,
        args, device, rank, is_distributed,
    )

    # Final eval on best model — all ranks run eval (NeMo model has distributed internals),
    # only rank 0 prints/uses results
    import nemo.collections.asr as nemo_asr

    best_path = os.path.join(args.output_dir, "best_model.nemo")
    if os.path.exists(best_path):
        print_rank0(f"\n  Loading best model from {best_path}...", rank)
        best_model = nemo_asr.models.ASRModel.restore_from(best_path, map_location=device)
        best_model = best_model.to(device)
        best_model.eval()
    else:
        print_rank0(f"\n  Best model not found, using final model.", rank)
        best_model = model

    print_rank0(f"\n{'='*65}", rank)
    print_rank0(f"  Final Evaluation — {lang} (best checkpoint)", rank)
    print_rank0(f"{'='*65}", rank)

    batch_wer, b_s, b_d, b_i, b_w = evaluate_batch(best_model, args.val_manifest, device, rank=rank)
    if is_main(rank):
        print_rank0(f"  {lang} Val Batch WER:     {batch_wer:.2f}%  (S={b_s/max(b_w,1)*100:.2f}% D={b_d/max(b_w,1)*100:.2f}% I={b_i/max(b_w,1)*100:.2f}%)", rank)
        print_rank0(f"    Counts: subs={b_s} dels={b_d} ins={b_i} / {b_w} ref words", rank)

    print_rank0(f"\n  Running streaming eval...", rank)
    stream_wer, s_s, s_d, s_i, s_w = evaluate_streaming(best_model, args.val_manifest, device, rank=rank)
    if is_main(rank):
        print_rank0(f"  {lang} Val Streaming WER:  {stream_wer:.2f}%  (S={s_s/max(s_w,1)*100:.2f}% D={s_d/max(s_w,1)*100:.2f}% I={s_i/max(s_w,1)*100:.2f}%)", rank)
        print_rank0(f"    Counts: subs={s_s} dels={s_d} ins={s_i} / {s_w} ref words", rank)

    if args.test_manifest:
        print_rank0(f"\n{'='*65}", rank)
        print_rank0(f"  Test Evaluation — {lang} (best checkpoint)", rank)
        print_rank0(f"{'='*65}", rank)

        test_batch_wer, tb_s, tb_d, tb_i, tb_w = evaluate_batch(best_model, args.test_manifest, device, rank=rank)
        if is_main(rank):
            print_rank0(f"  {lang} Test Batch WER:     {test_batch_wer:.2f}%  (S={tb_s/max(tb_w,1)*100:.2f}% D={tb_d/max(tb_w,1)*100:.2f}% I={tb_i/max(tb_w,1)*100:.2f}%)", rank)
            print_rank0(f"    Counts: subs={tb_s} dels={tb_d} ins={tb_i} / {tb_w} ref words", rank)

        print_rank0(f"\n  Running streaming test eval...", rank)
        test_stream_wer, ts_s, ts_d, ts_i, ts_w = evaluate_streaming(best_model, args.test_manifest, device, rank=rank)
        if is_main(rank):
            print_rank0(f"  {lang} Test Streaming WER:  {test_stream_wer:.2f}%  (S={ts_s/max(ts_w,1)*100:.2f}% D={ts_d/max(ts_w,1)*100:.2f}% I={ts_i/max(ts_w,1)*100:.2f}%)", rank)
            print_rank0(f"    Counts: subs={ts_s} dels={ts_d} ins={ts_i} / {ts_w} ref words", rank)

    del best_model
    torch.cuda.empty_cache()

    # Evaluate best-by-loss model if it exists
    best_loss_path = os.path.join(args.output_dir, "best_model_loss.nemo")
    if os.path.exists(best_loss_path):
        print_rank0(f"\n{'='*65}", rank)
        print_rank0(f"  Final Evaluation — {lang} (best loss checkpoint)", rank)
        print_rank0(f"{'='*65}", rank)

        print_rank0(f"  Loading best-by-loss model from {best_loss_path}...", rank)
        best_loss_model = nemo_asr.models.ASRModel.restore_from(best_loss_path, map_location=device)
        best_loss_model = best_loss_model.to(device)
        best_loss_model.eval()

        bl_wer, bl_s, bl_d, bl_i, bl_w = evaluate_batch(best_loss_model, args.val_manifest, device, rank=rank)
        if is_main(rank):
            print_rank0(f"  {lang} Val Batch WER (loss-best):     {bl_wer:.2f}%  (S={bl_s/max(bl_w,1)*100:.2f}% D={bl_d/max(bl_w,1)*100:.2f}% I={bl_i/max(bl_w,1)*100:.2f}%)", rank)

        if args.test_manifest:
            tbl_wer, tbl_s, tbl_d, tbl_i, tbl_w = evaluate_batch(best_loss_model, args.test_manifest, device, rank=rank)
            if is_main(rank):
                print_rank0(f"  {lang} Test Batch WER (loss-best):    {tbl_wer:.2f}%  (S={tbl_s/max(tbl_w,1)*100:.2f}% D={tbl_d/max(tbl_w,1)*100:.2f}% I={tbl_i/max(tbl_w,1)*100:.2f}%)", rank)

        del best_loss_model
        torch.cuda.empty_cache()

    # All ranks must wait for rank 0's final eval before destroying process group
    if is_distributed:
        dist.barrier()

    cleanup_ddp(is_distributed)


if __name__ == "__main__":
    main()