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"""Generation-based val metrics after each Trainer evaluate() call."""

from __future__ import annotations

import json
import logging
import time
from pathlib import Path

import torch
from transformers import TrainerCallback

from physics_model.eval import generate_many, summarize
from physics_sft.metrics import flatten_eval_metrics, format_eval_table, score_prediction

logger = logging.getLogger(__name__)

_GLOO_GROUP = None


class PhysicsValMetricsCallback(TrainerCallback):
    """After loss eval, greedy-decode a mixed val subset and log answer metrics."""

    def __init__(
        self,
        processor,
        collator,
        eval_rows: list[dict],
        *,
        output_dir: Path,
        max_new_tokens: int = 512,
        max_samples: int | None = 32,
        gen_batch_size: int = 4,
    ) -> None:
        self.processor = processor
        self.collator = collator
        self.eval_rows = _take_mixed(list(eval_rows), max_samples)
        self.output_dir = Path(output_dir)
        self.max_new_tokens = int(max_new_tokens)
        self.gen_batch_size = max(1, int(gen_batch_size))
        self._trainer = None
        self._last_summary: dict = {}

    def set_trainer(self, trainer) -> None:
        self._trainer = trainer

    def on_evaluate(self, args, state, control, metrics=None, **kwargs):
        if self._trainer is None and hasattr(self, "trainer"):
            self._trainer = self.trainer
        trainer = self._trainer
        if trainer is None or not self.eval_rows:
            return

        extra: dict = {}
        try:
            logger.info(
                "step %s: starting generation val on %s rows (batch=%s, max_new_tokens=%s)",
                state.global_step,
                len(self.eval_rows),
                self.gen_batch_size,
                self.max_new_tokens,
            )
            t0 = time.time()
            extra = self._run(trainer, state.global_step)
            logger.info("step %s: generation val finished in %.1fs", state.global_step, time.time() - t0)
            if extra and trainer.accelerator.is_main_process:
                logger.info("\n%s", format_eval_table(self._last_summary, step=state.global_step))
        except Exception:
            logger.exception("generation val failed; continuing training")
            extra = {}
            _cpu_barrier()
        if metrics is not None and extra:
            metrics.update(extra)
        if extra and trainer.accelerator.is_main_process:
            trainer.log(extra)

    @torch.no_grad()
    def _run(self, trainer, step: int) -> dict:
        accelerator = trainer.accelerator
        rank = int(accelerator.process_index)
        world = int(accelerator.num_processes)
        model = accelerator.unwrap_model(trainer.model)
        restore = _enable_fast_generate(model)
        was_training = model.training
        model.eval()
        _gloo_group()
        local_error = None
        detailed_part: list[dict] = []
        my_rows = self.eval_rows[rank::world]
        try:
            preds = generate_many(
                model,
                self.processor,
                self.collator,
                my_rows,
                self.max_new_tokens,
                device=accelerator.device,
                batch_size=self.gen_batch_size,
                synced_gpus=False,
            )
            detailed_part = [score_prediction(row, pred) for row, pred in zip(my_rows, preds)]
            logger.info("rank %s val generate %s/%s", rank, len(my_rows), len(self.eval_rows))
        except Exception as exc:
            local_error = f"{type(exc).__name__}: {exc}"
            logger.exception("rank %s generation val failed", rank)
        finally:
            restore()
            if was_training:
                model.train()
                trainer.model.train()

        gathered = _all_gather_object({"rows": detailed_part, "error": local_error})
        if any(part.get("error") for part in gathered):
            logger.warning("skipping val metrics because at least one rank failed")
            return {}

        detailed = _merge_sharded_rows(self.eval_rows, gathered, world)
        summary = summarize(detailed)
        self._last_summary = summary
        flat = {k: (None if _is_nan(v) else v) for k, v in flatten_eval_metrics(summary, prefix="eval").items()}
        if accelerator.is_main_process:
            payload = {
                "step": step,
                "n": len(detailed),
                "summary": summary,
                "metrics": flat,
                "rows": detailed,
            }
            self.output_dir.mkdir(parents=True, exist_ok=True)
            path = self.output_dir / f"eval_step_{step}.json"
            path.write_text(json.dumps(payload, ensure_ascii=False, indent=2) + "\n", encoding="utf-8")
            logger.info("wrote val predictions %s", path)
        return flat


def _enable_fast_generate(model):
    """Turn on KV cache and turn off checkpointing for decode; restore afterwards."""
    modules = []
    seen = set()
    stack = [model]
    if hasattr(model, "get_base_model"):
        stack.append(model.get_base_model())
    if hasattr(model, "base_model"):
        stack.append(model.base_model)
    for module in stack:
        ident = id(module)
        if ident in seen:
            continue
        seen.add(ident)
        modules.append(module)

    snapshot = []
    for module in modules:
        cfg = getattr(module, "config", None)
        snapshot.append(
            (
                module,
                bool(getattr(cfg, "use_cache", False)) if cfg is not None else False,
                bool(getattr(module, "is_gradient_checkpointing", False)),
            )
        )
        if cfg is not None:
            cfg.use_cache = True
        if hasattr(module, "gradient_checkpointing_disable"):
            module.gradient_checkpointing_disable()

    def restore() -> None:
        for module, use_cache, had_gc in snapshot:
            cfg = getattr(module, "config", None)
            if cfg is not None:
                cfg.use_cache = use_cache
            if had_gc and hasattr(module, "gradient_checkpointing_enable"):
                module.gradient_checkpointing_enable()

    return restore


def _gloo_group():
    global _GLOO_GROUP
    import torch.distributed as dist

    if not dist.is_available() or not dist.is_initialized() or dist.get_world_size() <= 1:
        return None
    if _GLOO_GROUP is None:
        _GLOO_GROUP = dist.new_group(backend="gloo")
    return _GLOO_GROUP


def _all_gather_object(obj):
    import torch.distributed as dist

    if not dist.is_available() or not dist.is_initialized() or dist.get_world_size() <= 1:
        return [obj]
    group = _gloo_group()
    gathered = [None] * dist.get_world_size()
    dist.all_gather_object(gathered, obj, group=group)
    return gathered


def _cpu_barrier() -> None:
    import torch.distributed as dist

    if not dist.is_available() or not dist.is_initialized() or dist.get_world_size() <= 1:
        return
    dist.barrier(group=_gloo_group())


def _take_mixed(rows: list[dict], max_samples: int | None) -> list[dict]:
    if max_samples is None:
        return rows
    limit = max(0, int(max_samples))
    if limit == 0 or len(rows) <= limit:
        return rows[:limit]
    images = [r for r in rows if r.get("has_image")]
    texts = [r for r in rows if not r.get("has_image")]
    n_img = min(len(images), limit // 2)
    n_txt = min(len(texts), limit - n_img)
    picked = images[:n_img] + texts[:n_txt]
    if len(picked) < limit:
        leftover = [r for r in rows if r not in picked]
        picked.extend(leftover[: limit - len(picked)])
    return picked


def _merge_sharded_rows(eval_rows: list[dict], gathered: list[dict], world: int) -> list[dict]:
    merged: list[dict | None] = [None] * len(eval_rows)
    for rank, part in enumerate(gathered):
        rows = part.get("rows") or []
        idxs = range(rank, len(eval_rows), world)
        for idx, item in zip(idxs, rows):
            merged[idx] = item
    return [item for item in merged if item is not None]


def _is_nan(value) -> bool:
    try:
        return value != value
    except Exception:
        return False