"""Patch rendering — an overlay as a reviewable diff (FederationRequirements ADR-05).

A pure function over (base body, overlay body): the auditable change proposal that an overlay
becomes before it is applied. Markdown/native text in this slice (lossless); a lossy
native-syntax-with-fidelity-report variant is later (TSD §A.6).
"""

from __future__ import annotations

import difflib
from dataclasses import dataclass

from shard_wiki.coordination.overlay import Overlay
from shard_wiki.model import Identity

__all__ = ["Patch", "render_patch"]


@dataclass(frozen=True, slots=True)
class Patch:
    target: Identity
    diff: str

    @property
    def is_empty(self) -> bool:
        return self.diff == ""


def render_patch(overlay: Overlay, base_body: str) -> Patch:
    """Render ``base_body`` → ``overlay.body`` as a unified diff against the overlay target."""
    label = str(overlay.target)
    lines = difflib.unified_diff(
        base_body.splitlines(keepends=True),
        overlay.body.splitlines(keepends=True),
        fromfile=f"a/{label}",
        tofile=f"b/{label}",
    )
    return Patch(target=overlay.target, diff="".join(lines))