CS193P Professor Paul Hegarty iTuneU course

CalculatorBrain-Enum-Example

//: Playground - noun: a place where people can play

import UIKit
import Foundation

class CalculatorBrain {
    
    enum Op: CustomStringConvertible {
        case Operand(Double)
        case BinaryOperation(String, (Double, Double) -> Double)
        
        var description: String {
            get {
                switch self {
                case .Operand(let operand):
                    return "\(operand)"
                case .BinaryOperation(let symbol, _):
                    return symbol
                }
            }
        }
    }
    
    var opStack = [Op]()
    var knownOps = [String:Op]()
    
    init() {
        func learnOp (op: Op) {
            knownOps[op.description] = op
        }
        
        learnOp(Op.BinaryOperation("*", *))
        learnOp(Op.BinaryOperation("+", +))
        learnOp(Op.BinaryOperation("-", {$1 - $0}))
        learnOp(Op.BinaryOperation("/", {$1 / $0}))
    }
    
    func evaluate() -> Double? {
        let (result, _) = evaluate(opStack)
        return result
    }
    
    func evaluate(ops: [Op]) -> (result: Double?, remainingOps: [Op]) {
        if !ops.isEmpty {
            var remainingOps = ops
            let op = remainingOps.removeLast()
            
            switch op {
            case .Operand(let operand):
                return (operand, remainingOps)
            case .BinaryOperation(_, let operation):
                let op1Evaluation = evaluate(remainingOps)
                if let operand1 = op1Evaluation.result {
                    let op2Evaluation = evaluate(op1Evaluation.remainingOps)
                    if let operand2 = op2Evaluation.result {
                        return (operation(operand1, operand2), op2Evaluation.remainingOps)
                    }
                }
            }
        }
        return (nil, ops)
    }
    
    func pushOperand(operand: Double) -> Double? {
        opStack.append(Op.Operand(operand))
        return evaluate()
    }
    
    func popOperand() -> Double? {
        if !opStack.isEmpty {
            opStack.removeLast()
        }
        return evaluate()
    }
    
    func performOperation(symbol: String) -> Double? {
        if let operation = knownOps[symbol] {
            opStack.append(operation);
        }
        return evaluate()
    }
    
    func showStack() -> String? {
        return opStack.map{ "\($0)" }.joinWithSeparator(" ")
    }
}

var cb = CalculatorBrain()

print(cb.knownOps)

cb.showStack()
cb.pushOperand(1)
cb.pushOperand(2)
cb.performOperation("+")
cb.pushOperand(3)
cb.pushOperand(4)
cb.performOperation("/")
cb.pushOperand(5)
cb.pushOperand(6)
cb.performOperation("*")
cb.pushOperand(7)
cb.pushOperand(8)
cb.performOperation("-")
cb.showStack()