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

evaluate(node) -> int | float

Result-type rule: if the computed value is a whole number (no fractional
part), it is returned as int; otherwise as float.  This ensures "4/2" -> 2
and "7/2" -> 3.5 without callers having to special-case anything.

Division-by-zero raises EvalError, not a bare ZeroDivisionError.
"""
from __future__ import annotations
from calc.parser import Num, BinOp, Unary, Node


class EvalError(Exception):
    pass


def evaluate(node: Node) -> int | float:
    result = _eval(node)
    if isinstance(result, float) and result == int(result):
        return int(result)
    return result


def _eval(node: Node) -> int | float:
    if isinstance(node, Num):
        return node.value
    if isinstance(node, Unary):
        return -_eval(node.operand)
    if isinstance(node, BinOp):
        left = _eval(node.left)
        right = _eval(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")
            return left / right
    raise EvalError(f"unknown node type: {type(node).__name__}")