#!/usr/bin/env python3
"""
slice_n4_run.py
---------------
Slice the canonical RGC N4 logistic CSV into 253 roughly equal pieces
with small tail overlaps on every interior slice.

Design goals
------------
* Stream the source file — never load the full multi-GB CSV into RAM.
* Keep every slice self-contained (identical header row).
* Prefer cuts that fall on frame boundaries (multiples of 49 data rows)
  so a 49-core lattice snapshot is never split mid-frame.
* Add a configurable overlap (default 1.5 MB ≈ a few thousand frames)
  at the *start* of every slice except the first, and at the *end* of
  every slice except the last.  This gives contiguous analysis windows
  a small shared context for ACF / period-3 / order-parameter continuity.
* Produce deterministic, numbered output files:
      n4_slice_000.csv … n4_slice_252.csv

Usage
-----
  python slice_n4_run.py /path/to/logistic_20260606_145947.csv \\
                         [--outdir ./n4_slices] \\
                         [--n-slices 253] \\
                         [--overlap-mb 1.5] \\
                         [--cores 49]

The script only needs read access to the source and write access to
the output directory.  It prints a progress summary and a final
manifest (slice index, byte range, row estimate, file size).
"""

from __future__ import annotations

import argparse
import os
import sys
from pathlib import Path
from typing import List, Tuple


# ---------------------------------------------------------------------------
# Helpers
# ---------------------------------------------------------------------------

def human(n: float) -> str:
    """Pretty-print a byte count."""
    for unit in ("B", "KB", "MB", "GB", "TB"):
        if abs(n) < 1024.0:
            return f"{n:6.1f} {unit}"
        n /= 1024.0
    return f"{n:.1f} PB"


def find_frame_aligned_offset(
    fh,
    target_byte: int,
    cores: int,
    header_len: int,
    search_window: int = 512 * 1024,
) -> int:
    """
    Return a byte offset >= target_byte that sits on a clean frame boundary
    (i.e. after a complete set of `cores` data rows).

    Strategy: jump near the target, read a modest window, count newlines,
    and advance until (data_rows_seen % cores) == 0.
    """
    file_size = fh.seek(0, os.SEEK_END)
    if target_byte >= file_size:
        return file_size

    # Start a little before the target so we have context
    start = max(header_len, target_byte - search_window // 4)
    fh.seek(start)
    buf = fh.read(search_window)
    if not buf:
        return file_size

    # If we started mid-line, discard the partial first line
    if start > header_len and buf[0:1] != b"\n":
        nl = buf.find(b"\n")
        if nl < 0:
            return min(file_size, start + len(buf))
        buf = buf[nl + 1 :]
        start += nl + 1

    # Walk line-by-line until we are past target_byte *and* on a frame edge
    pos = start
    data_rows = 0
    for line in buf.splitlines(keepends=True):
        pos += len(line)
        data_rows += 1
        if pos >= target_byte and (data_rows % cores) == 0:
            return min(pos, file_size)

    # Fallback: just return the end of the window (still better than mid-line)
    return min(pos, file_size)


def compute_slice_plan(
    file_size: int,
    header_len: int,
    n_slices: int,
    overlap_bytes: int,
    cores: int,
    fh,
) -> List[Tuple[int, int, int, int]]:
    """
    Return a list of (slice_idx, start_byte, end_byte, pure_end_byte).

    start_byte  – inclusive, may include leading overlap
    end_byte    – exclusive, may include trailing overlap
    pure_end_byte – the nominal boundary *without* trailing overlap
                    (used so the next slice knows where its leading
                    overlap should begin)
    """
    body_size = file_size - header_len
    if body_size <= 0:
        raise ValueError("File contains only a header (or is empty).")

    nominal_chunk = body_size / n_slices
    plan = []

    # First pass: pure (non-overlapping) boundaries, frame-aligned
    pure_bounds = [header_len]
    for i in range(1, n_slices):
        target = header_len + int(i * nominal_chunk)
        aligned = find_frame_aligned_offset(fh, target, cores, header_len)
        pure_bounds.append(aligned)
    pure_bounds.append(file_size)

    # Second pass: expand each interior slice by overlap_bytes on both sides
    for i in range(n_slices):
        pure_start = pure_bounds[i]
        pure_end = pure_bounds[i + 1]

        if i == 0:
            start = pure_start          # no leading overlap on first slice
        else:
            # Walk backward from pure_start by ~overlap_bytes, stay frame-aligned
            target = max(header_len, pure_start - overlap_bytes)
            start = find_frame_aligned_offset(fh, target, cores, header_len)
            # Guarantee we do not go past the previous pure boundary in a
            # way that would create negative-length regions
            start = min(start, pure_start)

        if i == n_slices - 1:
            end = pure_end              # no trailing overlap on last slice
        else:
            target = min(file_size, pure_end + overlap_bytes)
            end = find_frame_aligned_offset(fh, target, cores, header_len)

        plan.append((i, start, end, pure_end))

    return plan


# ---------------------------------------------------------------------------
# Main slicing routine
# ---------------------------------------------------------------------------

def slice_file(
    src: Path,
    outdir: Path,
    n_slices: int = 253,
    overlap_mb: float = 1.5,
    cores: int = 49,
    bufsize: int = 8 * 1024 * 1024,
) -> None:
    if not src.is_file():
        sys.exit(f"ERROR: source file not found: {src}")

    outdir.mkdir(parents=True, exist_ok=True)
    overlap_bytes = int(overlap_mb * 1024 * 1024)
    file_size = src.stat().st_size

    print(f"Source      : {src}")
    print(f"Size        : {human(file_size)}")
    print(f"Slices      : {n_slices}")
    print(f"Overlap     : {overlap_mb} MB  ({human(overlap_bytes)})")
    print(f"Cores/frame : {cores}")
    print(f"Output dir  : {outdir}")
    print()

    with open(src, "rb") as fh:
        # Read header (first line)
        header = fh.readline()
        if not header:
            sys.exit("ERROR: empty file")
        header_len = len(header)
        print(f"Header ({header_len} bytes): {header.decode('utf-8', errors='replace').rstrip()}")
        print()

        plan = compute_slice_plan(
            file_size, header_len, n_slices, overlap_bytes, cores, fh
        )

        manifest = []
        for idx, start, end, pure_end in plan:
            out_path = outdir / f"n4_slice_{idx:03d}.csv"
            length = end - start
            # Leading / trailing overlap sizes for the log
            lead = max(0, pure_end - start) if idx > 0 else 0
            # Actually lead = pure_start - start; we approximate via plan
            pure_start = plan[idx][1] if idx == 0 else plan[idx - 1][3]
            # Recompute cleanly
            pure_start_i = header_len if idx == 0 else plan[idx - 1][3]
            lead_ov = max(0, pure_start_i - start) if idx > 0 else 0
            trail_ov = max(0, end - pure_end) if idx < n_slices - 1 else 0

            with open(out_path, "wb") as out:
                out.write(header)          # every slice is self-describing
                fh.seek(start)
                remaining = length
                while remaining > 0:
                    chunk = fh.read(min(bufsize, remaining))
                    if not chunk:
                        break
                    out.write(chunk)
                    remaining -= len(chunk)

            final_size = out_path.stat().st_size
            manifest.append(
                {
                    "idx": idx,
                    "file": out_path.name,
                    "start": start,
                    "end": end,
                    "bytes": final_size,
                    "lead_ov": lead_ov,
                    "trail_ov": trail_ov,
                }
            )
            print(
                f"  [{idx:03d}] {out_path.name}  "
                f"{human(final_size)}  "
                f"bytes {start:,}–{end:,}  "
                f"lead_ov={human(lead_ov)}  trail_ov={human(trail_ov)}"
            )

    # Write a small manifest CSV for later bookkeeping
    man_path = outdir / "slice_manifest.csv"
    with open(man_path, "w", encoding="utf-8") as m:
        m.write("slice_idx,filename,start_byte,end_byte,file_bytes,lead_overlap_bytes,trail_overlap_bytes\n")
        for row in manifest:
            m.write(
                f"{row['idx']},{row['file']},{row['start']},{row['end']},"
                f"{row['bytes']},{row['lead_ov']},{row['trail_ov']}\n"
            )
    print()
    print(f"Manifest written to {man_path}")
    print(f"Done — {n_slices} slices.")


# ---------------------------------------------------------------------------
# CLI
# ---------------------------------------------------------------------------

def main() -> None:
    p = argparse.ArgumentParser(
        description="Slice canonical RGC N4 logistic CSV into overlapping pieces."
    )
    p.add_argument(
        "source",
        type=Path,
        help="Path to the full logistic_*.csv (canonical N4 run)",
    )
    p.add_argument(
        "--outdir",
        type=Path,
        default=Path("./n4_slices"),
        help="Directory for the slice files (default: ./n4_slices)",
    )
    p.add_argument(
        "--n-slices",
        type=int,
        default=253,
        help="Number of slices (default: 253)",
    )
    p.add_argument(
        "--overlap-mb",
        type=float,
        default=1.5,
        help="Overlap size in MiB at each interior boundary (default: 1.5)",
    )
    p.add_argument(
        "--cores",
        type=int,
        default=49,
        help="Cores per lattice frame (default: 49)",
    )
    args = p.parse_args()

    if args.n_slices < 1:
        sys.exit("--n-slices must be >= 1")
    if args.overlap_mb < 0:
        sys.exit("--overlap-mb must be >= 0")

    slice_file(
        src=args.source,
        outdir=args.outdir,
        n_slices=args.n_slices,
        overlap_mb=args.overlap_mb,
        cores=args.cores,
    )


if __name__ == "__main__":
    main()