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

evaluate(node) -> int | float

Whole-valued results are returned as int ("4/2" -> 2).
Non-whole results are returned as float ("7/2" -> 3.5).
Division by zero raises EvalError.
"""
from __future__ import annotations

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


class EvalError(Exception):
    pass


def _normalize(val: int | float) -> int | float:
    """Return int if val is a whole-valued float, otherwise unchanged."""
    if isinstance(val, float) and val.is_integer():
        return int(val)
    return val


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