"""Split cleaned text into sentence-aware chunks for edge-tts.

~2600 chars per chunk keeps each synthesis call comfortably under the
service's limits while preserving sentence boundaries. Overlong sentences
are sub-split on commas. Also records each chunk's start char-offset (chunk_offsets.json)
so chapter positions can later be mapped onto audio time.
"""
import json
import os
import re


def chunk_text(text: str, outdir: str, chunk_size: int = 2600,
               offsets_path: str = None) -> int:
    text = re.sub(r"\s+", " ", text)
    parts = re.split(r"(?<=[.!?;:])\s+", text)

    chunks = []
    cur = ""
    for p in parts:
        if len(p) > chunk_size:
            if cur:
                chunks.append(cur.strip())
                cur = ""
            sub = re.split(r",\s+", p)
            s = ""
            for x in sub:
                if s and len(s) + len(x) + 1 > chunk_size:
                    chunks.append(s.strip())
                    s = x
                else:
                    s = (s + ", " + x) if s else x
            cur = s
            continue
        if cur and len(cur) + len(p) + 1 > chunk_size:
            chunks.append(cur.strip())
            cur = p
        else:
            cur = (cur + " " + p) if cur else p
    if cur.strip():
        chunks.append(cur.strip())

    # start char-offset of each chunk within the whitespace-normalized text
    offsets = []
    c = 0
    for ch in chunks:
        offsets.append(c)
        c += len(ch) + 1  # +1: chunks are joined with a single space

    os.makedirs(outdir, exist_ok=True)
    # wipe stale chunks from a previous run with a different chunk size
    for f in os.listdir(outdir):
        if f.endswith(".txt"):
            os.remove(os.path.join(outdir, f))

    for i, ch in enumerate(chunks):
        with open(os.path.join(outdir, f"{i:04d}.txt"), "w", encoding="utf-8") as f:
            f.write(ch)

    if offsets_path:
        with open(offsets_path, "w", encoding="utf-8") as f:
            json.dump(offsets, f)
    return len(chunks)