calc.swift

enum Token {
	case num(Int)
	case op(String)
}

func tokenize(_ s: String) -> [Token] {
	let scanner = Scanner(string: s)
	scanner.charactersToBeSkipped = CharacterSet.whitespaces
	var tokens = [Token]()
	while !scanner.isAtEnd {
		if let c = scanner.scanCharacters(from: CharacterSet(charactersIn: "+-*/")) {
			tokens.append(.op(String(c)))
		}
		else if let int = scanner.scanInt() {
			tokens.append(.num(int))
		}
	}
	return tokens
}

enum MyError : Error {
	case invalidToken
}

func calc(_ tokens: [Token]) throws -> Int {
	if tokens.count == 0 {
		return 0
	}
	guard case let .num(x) = tokens[0] else {
		throw MyError.invalidToken
	}
	var v = x
	var currentToken: Token = tokens[0]
	for token in tokens[1...] {
		switch token {
		case .num(let i):
			switch currentToken {
			case .num(_):
				throw MyError.invalidToken
			case .op(let c):
				switch c {
				case "+":
					v += i
				case "-":
					v -= i
				case "*":
					v *= i
				case "/":
					v /= i
				default:
					throw MyError.invalidToken
				}
				currentToken = token
			}
		case .op(_):
			switch currentToken {
			case .op(_):
				throw MyError.invalidToken
			case .num(_):
				currentToken = token
			}
			break
		}
	}
	return v
}

//let t = tokenize("1 / 1 * 2 + 100 - 80")
let t = tokenize("1 + 100 * 200")

print(t)
t
do {
	let result = try calc(t)
	result
} catch {
	error
}