"""Generated regions inside a page body, found by delimiter rather than by prose.

`xref` owns the links block and `cite` owns the footnotes block. Both used to be
located by matching their **heading text** - `^## Beziehungen$` - which made a
translated heading a structural fact and put the KB language into the compiler.
It also made the region's *end* a guess: the footnotes block ran to the next
heading, and before that to the end of the file, which silently deleted whatever
sat after it on eight pages.

A marker pair answers both questions exactly:

    <!-- wikitool:links -->
    ## Beziehungen

    - **depends-on:** [[Hermes]]
    <!-- /wikitool:links -->

Everything between the markers is generated and is replaced wholesale on the
next write - heading included, which is why the heading text is a *rendering*
value from `kb/CONVENTIONS.md` rather than something the tool searches for. An
author never edits inside the markers; anything they put there is overwritten
without warning, exactly like `kb/index.md`.

The markers are HTML comments: invisible in every renderer this corpus is read
through, and the same convention `dist:strip-start`/`-end` already uses in
`AGENTS.md`. They cost a reader nothing and cost an LLM about twenty tokens a
page - the price of not having to guess where a generated region ends.
"""
from __future__ import annotations

import re
from typing import Optional

# The two regions the tool owns. `see-also` is deliberately absent: it was the
# reciprocal half of the old bidirectional `xref add`, and under authored
# directional edges it is a *label* (`see-also`) inside the links block, not a
# section of its own.
LINKS = "links"
FOOTNOTES = "footnotes"
BLOCKS = (LINKS, FOOTNOTES)

_NAME = r"[a-z][a-z0-9-]*"


def open_marker(name: str) -> str:
    return f"<!-- wikitool:{name} -->"


def close_marker(name: str) -> str:
    return f"<!-- /wikitool:{name} -->"


def _region_re(name: str) -> re.Pattern[str]:
    """The whole region including both markers and the blank line around it."""
    return re.compile(
        r"\n*"
        + re.escape(open_marker(name))
        + r".*?"
        + re.escape(close_marker(name))
        + r"\n*",
        re.DOTALL,
    )


_ANY_OPEN_RE = re.compile(rf"<!-- wikitool:({_NAME}) -->")
_ANY_CLOSE_RE = re.compile(rf"<!-- /wikitool:({_NAME}) -->")


def find(body: str, name: str) -> Optional[str]:
    """The generated content of `name`'s region, markers excluded, or None."""
    match = _region_re(name).search(body)
    if not match:
        return None
    text = match.group(0)
    start = text.index(open_marker(name)) + len(open_marker(name))
    end = text.index(close_marker(name))
    return text[start:end].strip("\n")


def render(name: str, heading: str, lines: list[str]) -> str:
    """A whole region, ready to place into a body. Empty `lines` renders "".

    An empty region is no region at all rather than a heading with nothing under
    it: a page that cites nothing should not carry an empty Footnotes section,
    and the same holds for a page with no declared edges.
    """
    if not lines:
        return ""
    parts = [open_marker(name), f"## {heading}", "", *lines, close_marker(name)]
    return "\n".join(parts)


def replace(body: str, name: str, region: str) -> str:
    """Put `region` where `name`'s region is, or append it if there is none.

    Appending at the end is right for both blocks: they are the page's trailing
    machine-owned material, and an author's prose never follows them. A region
    that is `""` removes what was there.
    """
    existing = _region_re(name).search(body)
    if existing:
        replacement = f"\n\n{region}\n" if region else "\n"
        return (body[: existing.start()] + replacement + body[existing.end():]).rstrip("\n") + "\n"
    if not region:
        return body
    return body.rstrip("\n") + "\n\n" + region + "\n"


def strip(body: str, name: str) -> str:
    """The body with `name`'s region removed entirely."""
    return replace(body, name, "")


def marker_pairs(body: str) -> dict[str, int]:
    """How many complete open/close pairs each region name has in `body`.

    The invariant `migrate verify` checks. An agent rewriting prose next to a
    boundary can drop or duplicate a marker, and the failure is otherwise silent:
    a lost opening marker turns a generated region into ordinary prose that the
    next write appends a second copy beside.
    """
    opens = [m.group(1) for m in _ANY_OPEN_RE.finditer(body)]
    closes = [m.group(1) for m in _ANY_CLOSE_RE.finditer(body)]
    names = set(opens) | set(closes)
    return {name: min(opens.count(name), closes.count(name)) for name in sorted(names)}


def unbalanced_markers(body: str) -> list[str]:
    """Region names whose open and close markers do not pair up."""
    opens = [m.group(1) for m in _ANY_OPEN_RE.finditer(body)]
    closes = [m.group(1) for m in _ANY_CLOSE_RE.finditer(body)]
    return sorted(
        name
        for name in set(opens) | set(closes)
        if opens.count(name) != closes.count(name)
    )