from __future__ import annotations

from difflib import unified_diff
from pathlib import Path

from engine.models import BaselineSnapshot, TaskSpec


class MutationValidationError(ValueError):
    pass


def _count_changed_lines(before: str, after: str, path: Path) -> int:
    diff = unified_diff(
        before.splitlines(keepends=True),
        after.splitlines(keepends=True),
        fromfile=f"{path.as_posix()} (before)",
        tofile=f"{path.as_posix()} (after)",
    )
    changed_lines = 0
    for line in diff:
        if line.startswith(("---", "+++", "@@")):
            continue
        if line.startswith(("+", "-")):
            changed_lines += 1
    return changed_lines


def validate_candidate_changes(task: TaskSpec, snapshot: BaselineSnapshot) -> None:
    changed_files = 0
    changed_lines = 0
    allowed_file_types = set(task.mutation.allowed_file_types)

    for path, baseline_text in snapshot.file_contents.items():
        current_text = path.read_text(encoding="utf-8") if path.exists() else ""
        if current_text == baseline_text:
            continue

        changed_files += 1
        if path.suffix not in allowed_file_types:
            raise MutationValidationError(f"disallowed file type: {path.suffix}")

        changed_lines += _count_changed_lines(baseline_text, current_text, path)

    if changed_files > task.artifacts.max_files_per_iteration:
        raise MutationValidationError(
            f"too many changed files: {changed_files} > {task.artifacts.max_files_per_iteration}"
        )

    if changed_lines > task.mutation.max_changed_lines:
        raise MutationValidationError(
            f"too many changed lines: {changed_lines} > {task.mutation.max_changed_lines}"
        )