"""Custom rule demonstrating pluggable extensibility (A1b)."""

import ast
from typing import Optional

from app.dataset.rules.base import Mutation, MutationRule


class UnusedVariableRule(MutationRule):
    """A custom rule: replace a variable read with an undefined name.

    This is intentionally simple and demonstrates that adding a new file under
    app/dataset/rules/<language>/ is enough to register a rule.
    """

    name = "unused_variable_demo"
    language = "python"
    defect_type = "custom_demo"

    def detect_and_mutate(self, source: str, filename: str = "") -> Optional[Mutation]:
        try:
            tree = ast.parse(source)
        except SyntaxError:
            return None

        for node in ast.walk(tree):
            if isinstance(node, ast.FunctionDef) and node.body:
                first = node.body[0]
                if isinstance(first, ast.Assign) and isinstance(first.targets[0], ast.Name):
                    var_name = first.targets[0].id
                    line_no = first.lineno
                    lines = source.splitlines(keepends=True)
                    line = lines[line_no - 1]
                    mutated_line = line.replace(var_name, "__undefined_" + var_name, 1)
                    if mutated_line == line:
                        continue
                    mutated = "".join(lines[:line_no - 1] + [mutated_line] + lines[line_no:])
                    return Mutation(
                        defect_type=self.defect_type,
                        language=self.language,
                        line_start=line_no,
                        line_end=line_no,
                        mutated_source=mutated,
                        reference_fix=f"Use the original variable name '{var_name}'.",
                        description=f"Custom rule: replaced '{var_name}' with an undefined placeholder.",
                    )
        return None