"""Evaluator: walks an AST (from calc.parser) and returns int | float.

Result-type rule: if the result is whole-valued (no fractional part), return int;
otherwise return float. This means 4/2 → 2 (int) and 7/2 → 3.5 (float).
"""
from __future__ import annotations
from calc.parser import Num, BinOp, Unary, Node


class EvalError(Exception):
    pass


def _normalize(v: int | float) -> int | float:
    if isinstance(v, float) and v == int(v):
        return int(v)
    return v


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