from calc.parser import Num, BinOp, Unary


class EvalError(Exception):
    pass


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

    Result type rule: returns int for whole-valued results (including whole-valued
    division), float for non-whole. Division by zero raises EvalError.
    """
    if isinstance(node, Num):
        return node.value
    if isinstance(node, Unary):
        v = evaluate(node.operand)
        if node.op == '-':
            return -v
        raise EvalError(f"unknown unary op {node.op!r}")
    if isinstance(node, BinOp):
        left = evaluate(node.left)
        right = evaluate(node.right)
        if node.op == '+':
            return left + right
        if node.op == '-':
            return left - right
        if node.op == '*':
            return left * right
        if node.op == '/':
            if right == 0:
                raise EvalError("division by zero")
            result = left / right
            if isinstance(result, float) and result.is_integer():
                return int(result)
            return result
        raise EvalError(f"unknown binary op {node.op!r}")
    raise EvalError(f"unknown node type {type(node).__name__!r}")