"""Evaluator for calc AST nodes.

evaluate(node) -> int | float

Result-type rule: whole-valued results are returned as int;
fractional results as float. So 4/2 -> 2, 7/2 -> 3.5.
"""

from calc.parser import Num, BinOp, Unary


class EvalError(Exception):
    pass


def _normalize(value):
    """Convert a whole-valued float to int; leave everything else alone."""
    if isinstance(value, float) and value.is_integer():
        return int(value)
    return value


def evaluate(node):
    """Walk an AST node and return its numeric value."""
    if isinstance(node, Num):
        return _normalize(node.value)
    if isinstance(node, Unary):
        return _normalize(-evaluate(node.operand))
    if isinstance(node, BinOp):
        left = evaluate(node.left)
        right = evaluate(node.right)
        if node.op == '+':
            return _normalize(left + right)
        if node.op == '-':
            return _normalize(left - right)
        if node.op == '*':
            return _normalize(left * right)
        if node.op == '/':
            if right == 0:
                raise EvalError('division by zero')
            return _normalize(left / right)
    raise EvalError(f'unknown node type: {type(node).__name__}')