"""
AST evaluator for the calc expression language.

evaluate(node) -> int | float

Result type rule:
  - Integer arithmetic returns int.
  - Division (/) always uses true division; if the result is whole-valued
    (e.g. 4/2 == 2.0) it is coerced to int, otherwise returned as float.
"""

from calc.parser import Num, BinOp, Unary


class EvalError(Exception):
    pass


def evaluate(node):
    """Walk an AST node and return an int or float result."""
    if isinstance(node, Num):
        return node.value

    if isinstance(node, Unary):
        val = evaluate(node.operand)
        if node.op == '-':
            return -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 binary operator: {node.op!r}")
        # Coerce whole-valued floats to int so "4/2" prints as "2" not "2.0"
        if isinstance(result, float) and result.is_integer():
            return int(result)
        return result

    raise EvalError(f"Unknown AST node type: {type(node).__name__!r}")