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

from __future__ import annotations

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


class EvalError(Exception):
    """Raised on a runtime evaluation error (e.g. division by zero)."""


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

    Result type rule: if the mathematical result is a whole number, return int;
    otherwise return float.  This guarantees '4/2' → 2 and '7/2' → 3.5.

    Raises EvalError on division by zero.
    """
    if isinstance(node, Num):
        return node.value

    if isinstance(node, Unary):
        v = evaluate(node.operand)
        return -v

    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}")

        if isinstance(result, float) and result == int(result):
            return int(result)
        return result

    raise EvalError(f"Unknown node type {type(node)!r}")