"""AST-level boundary error injection for Java using javalang."""

from typing import Optional

import javalang

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


class JavaBoundaryErrorRule(MutationRule):
    """Mutate array length boundary from `<` to `<=`.

    Uses `javalang` to locate a binary comparison against `.length` and flips
    the operator to introduce an off-by-one access.
    """

    name = "java_boundary_error"
    language = "java"
    defect_type = "boundary_condition_error"

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

        for path, node in tree:
            if not isinstance(node, javalang.tree.BinaryOperation):
                continue
            if node.operator != "<":
                continue
            right = node.operandr
            if isinstance(right, javalang.tree.MemberReference) and right.member == "length":
                # BinaryOperation itself may lack position; use enclosing statement
                if_statement = next((n for n in path if isinstance(n, javalang.tree.IfStatement)), None)
                pos = if_statement.position if if_statement else node.position
                if not pos:
                    continue
                lines = source.splitlines(keepends=True)
                line_no = pos.line
                line = lines[line_no - 1]
                mutated_line = line.replace("< ", "<= ", 1)
                if mutated_line == line:
                    mutated_line = line.replace("<", "<=", 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="Use strict `< length` to avoid ArrayIndexOutOfBoundsException.",
                    description="Changed array boundary check to off-by-one (<= length).",
                )
        return None