"""AST-level null-pointer / None-reference injection for Python."""

import ast
from typing import Optional

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


class NoneReferenceRule(MutationRule):
    """Replace a checked variable access with an unchecked None dereference.

    This rule uses the standard library `ast` module to precisely locate a
    variable that is used after an `if x is not None:` guard, then removes the
    guard. The mutation position is derived from AST line numbers so it is
    exact and reproducible.
    """

    name = "none_reference"
    language = "python"
    defect_type = "null_pointer"

    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 not isinstance(node, ast.If):
                continue
            test = node.test
            # Match: if x is not None:
            if (
                isinstance(test, ast.Compare)
                and isinstance(test.left, ast.Name)
                and len(test.ops) == 1
                and isinstance(test.ops[0], ast.IsNot)
                and len(test.comparators) == 1
                and isinstance(test.comparators[0], ast.Constant)
                and test.comparators[0].value is None
            ):
                var_name = test.left.id
                lines = source.splitlines(keepends=True)
                start = node.lineno
                end = getattr(node, "end_lineno", node.lineno) or node.lineno
                body_lines = lines[start:end]
                dedented = []
                for line in body_lines[1:]:
                    if line.startswith("    "):
                        dedented.append(line[4:])
                    elif line.startswith("\t"):
                        dedented.append(line[1:])
                    else:
                        dedented.append(line)
                mutated = "".join(lines[: start - 1] + dedented + lines[end:])
                return Mutation(
                    defect_type=self.defect_type,
                    language=self.language,
                    line_start=start,
                    line_end=end,
                    mutated_source=mutated,
                    reference_fix=f"Add `if {var_name} is not None:` guard before use.",
                    description=f"Removed None-check guard for variable '{var_name}'.",
                )
        return None