"""Layer 3: bounded text diff.

Runs on noise-stripped text (see noise_filters.py) so navigation/cookie/
timestamp churn doesn't register as a change. Bounded: only a capped number
of added/removed lines are kept, so a full page rewrite doesn't produce an
unbounded diff blob for storage or LLM consumption later.
"""

from __future__ import annotations

import difflib
from dataclasses import dataclass, field

from app.change_detection.noise_filters import strip_noise

_MAX_DIFF_LINES = 40


@dataclass(frozen=True)
class TextDiffResult:
    diff_ratio: float  # 0.0 = identical, 1.0 = completely different
    added_lines: list[str] = field(default_factory=list)
    removed_lines: list[str] = field(default_factory=list)

    @property
    def is_identical(self) -> bool:
        return self.diff_ratio == 0.0


def bounded_text_diff(previous_text: str, current_text: str) -> TextDiffResult:
    previous_clean = strip_noise(previous_text or "")
    current_clean = strip_noise(current_text or "")

    if previous_clean == current_clean:
        return TextDiffResult(diff_ratio=0.0)

    previous_lines = [line for line in previous_clean.splitlines() if line.strip()]
    current_lines = [line for line in current_clean.splitlines() if line.strip()]

    matcher = difflib.SequenceMatcher(a=previous_lines, b=current_lines, autojunk=False)
    similarity = matcher.ratio()
    diff_ratio = round(1.0 - similarity, 4)

    added: list[str] = []
    removed: list[str] = []
    for tag, i1, i2, j1, j2 in matcher.get_opcodes():
        if tag in ("replace", "delete"):
            removed.extend(previous_lines[i1:i2])
        if tag in ("replace", "insert"):
            added.extend(current_lines[j1:j2])

    return TextDiffResult(
        diff_ratio=diff_ratio,
        added_lines=added[:_MAX_DIFF_LINES],
        removed_lines=removed[:_MAX_DIFF_LINES],
    )