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


class EvalError(Exception):
    pass


def evaluate(node: Node) -> int | float:
    """Walk an AST and return its numeric value.

    Uses true division (7/2 → 3.5). Division by zero raises EvalError.
    Integer operands with integer-only operations (+ - *) return int;
    any division always returns float. The CLI formats whole floats as int.
    """
    if isinstance(node, Num):
        return node.value
    if isinstance(node, Unary):
        if node.op == '-':
            return -evaluate(node.operand)
        raise EvalError(f"unknown unary operator: {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")
            return left / right
        raise EvalError(f"unknown binary operator: {node.op!r}")
    raise EvalError(f"unknown node type: {type(node)!r}")