from calc.parser import Num, Unary, BinOp, Node


class EvalError(Exception):
    pass


def evaluate(node: Node):
    """Walk the AST and return an int or float.

    Result-type rule: if the result is mathematically whole-valued it is
    returned as int; non-whole results are returned as float.  This rule is
    applied after every operation so intermediate values are also normalised.
    """
    if isinstance(node, Num):
        return _normalise(node.value)
    if isinstance(node, Unary):
        return _normalise(-evaluate(node.operand))
    if isinstance(node, BinOp):
        left = evaluate(node.left)
        right = evaluate(node.right)
        if node.op == '+':
            return _normalise(left + right)
        if node.op == '-':
            return _normalise(left - right)
        if node.op == '*':
            return _normalise(left * right)
        if node.op == '/':
            if right == 0:
                raise EvalError("division by zero")
            return _normalise(left / right)
    raise EvalError(f"unknown node type: {type(node).__name__}")


def _normalise(value):
    """Return int when value is whole-valued, float otherwise."""
    if isinstance(value, float) and value == int(value):
        return int(value)
    return value