"""Markdown chunking for embedding/index (phase 2).

Splits a note body into overlapping chunks along Markdown structure (headings →
paragraphs) so each embedded chunk is semantically coherent and bounded in
size. Frontmatter is assumed already stripped by the caller.
"""

from __future__ import annotations

import re

_HEADING_RE = re.compile(r"^#{1,6}\s+", re.MULTILINE)


def _split_paragraphs(text: str) -> list[str]:
    return [p.strip() for p in re.split(r"\n\s*\n", text) if p.strip()]


def chunk_markdown(content: str, *, max_chars: int = 800, overlap: int = 120) -> list[str]:
    """Return a list of text chunks for ``content``.

    Sections are cut at Markdown headings; long sections are further split into
    ~``max_chars`` windows with ``overlap`` characters of carry-over.
    """
    text = (content or "").strip()
    if not text:
        return []

    # Split into heading-delimited sections, keeping the heading with its body.
    sections: list[str] = []
    last = 0
    for m in _HEADING_RE.finditer(text):
        if m.start() > last:
            sections.append(text[last : m.start()].strip())
        last = m.start()
    sections.append(text[last:].strip())
    sections = [s for s in sections if s]

    chunks: list[str] = []
    for section in sections:
        if len(section) <= max_chars:
            chunks.append(section)
            continue
        # Pack paragraphs into windows, then hard-split anything still too long.
        buf = ""
        for para in _split_paragraphs(section):
            if len(buf) + len(para) + 2 <= max_chars:
                buf = f"{buf}\n\n{para}".strip()
            else:
                if buf:
                    chunks.append(buf)
                if len(para) <= max_chars:
                    buf = para
                else:
                    for i in range(0, len(para), max_chars - overlap):
                        chunks.append(para[i : i + max_chars])
                    buf = ""
        if buf:
            chunks.append(buf)
    return [c for c in chunks if c.strip()]