"""Field-level structured diff for CMOS 18 candidate outputs.

Approach: for each (field, value) in the canonical record, check whether the
candidate formatted string contains `str(value)` as a case-sensitive
substring. This is deliberately simpler than parsing the candidate — the
linter is responsible for structural checks (punctuation, ordering, italics),
so the diff only answers: "did the right facts survive the reformat?"

Frozen harness module. Edits here invalidate prior scores.
"""

from __future__ import annotations

from dataclasses import dataclass, field


@dataclass
class DiffResult:
    matched: list[str] = field(default_factory=list)
    missing: list[str] = field(default_factory=list)

    @property
    def total(self) -> int:
        return len(self.matched) + len(self.missing)

    @property
    def rate(self) -> float:
        if self.total == 0:
            return 1.0
        return len(self.matched) / self.total


def field_diff(candidate: str, canonical: dict) -> DiffResult:
    """Compare a formatted candidate string against a canonical field dict.

    Each canonical field counts as matched iff its stringified value appears
    as a case-sensitive substring of `candidate`.
    """
    result = DiffResult()
    for key, value in canonical.items():
        if str(value) in candidate:
            result.matched.append(key)
        else:
            result.missing.append(key)
    return result