"""Evaluator for the arithmetic AST produced by calc.parser.

evaluate(node) -> int | float

Result-type rule: if the result is a whole number (no fractional part),
return an int; otherwise return a float.  This means 4/2 → 2 (int) and
7/2 → 3.5 (float).
"""

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


class EvalError(Exception):
    pass


def evaluate(node: Node) -> "int | float":
    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)
        op = node.op
        if op == '+':
            result = left + right
        elif op == '-':
            result = left - right
        elif op == '*':
            result = left * right
        elif op == '/':
            if right == 0:
                raise EvalError("division by zero")
            result = left / right
        else:
            raise EvalError(f"unknown operator: {op!r}")
        if isinstance(result, float) and result.is_integer():
            return int(result)
        return result
    raise EvalError(f"unknown node type: {type(node).__name__}")