from calc.parser import Num, BinOp, Unary


class EvalError(Exception):
    pass


def evaluate(node) -> "int | float":
    """Walk the AST and return a numeric result.

    Type rule: integer arithmetic stays int; division returns float, except when
    the quotient is whole-valued — then it is normalised to int so the CLI can
    print it without a trailing '.0'.
    """
    if isinstance(node, Num):
        return node.value
    if isinstance(node, Unary):
        return -evaluate(node.operand)
    if isinstance(node, BinOp):
        left = evaluate(node.left)
        right = evaluate(node.right)
        if node.op == 'PLUS':
            return left + right
        if node.op == 'MINUS':
            return left - right
        if node.op == 'STAR':
            return left * right
        if node.op == 'SLASH':
            if right == 0:
                raise EvalError("division by zero")
            result = left / right
            return int(result) if result == int(result) else result
    raise EvalError(f"unknown node: {type(node).__name__}")