Completed task 6 and closes #6

Calculator/Calculator/CalculatorBrain.swift

@@ -11,28 +11,29 @@ import Foundation
 struct CalculatorBrain {
 
     private var accumulator: Double?
+    private var accumulatorString: String? // todo tuple
 
     private enum Operation {
         case constant(Double)
-        case unaryOperation((Double) -> Double)
-        case binaryOperation((Double,Double) -> Double)
+        case unaryOperation((Double) -> Double, (String) -> String)
+        case binaryOperation((Double,Double) -> Double, (String,String) -> String)
         case equals
     }
 
     private var operations: Dictionary<String,Operation> = [
         "π" : Operation.constant(Double.pi),
         "e" : Operation.constant(M_E),
-        "√" : Operation.unaryOperation(sqrt),
-        "cos" : Operation.unaryOperation(cos),
-        "sin" : Operation.unaryOperation(sin),
-        "tan" : Operation.unaryOperation(tan),
-        "x²" : Operation.unaryOperation({ $0 * $0 }),
-        "x⁻¹" : Operation.unaryOperation({ 1 / $0 }),
-        "±" : Operation.unaryOperation({ -$0 }),
-        "×" : Operation.binaryOperation({ $0 * $1 }),
-        "÷" : Operation.binaryOperation({ $0 / $1 }),
-        "+" : Operation.binaryOperation({ $0 + $1 }),
-        "−" : Operation.binaryOperation({ $0 - $1 }),
+        "√" : Operation.unaryOperation(sqrt, {"√(\($0))"}),
+        "cos" : Operation.unaryOperation(cos, {"cos(\($0))"}),
+        "sin" : Operation.unaryOperation(sin, {"sin(\($0))"}),
+        "tan" : Operation.unaryOperation(tan, {"tan(\($0))"}),
+        "x²" : Operation.unaryOperation({ $0 * $0 }, {"\($0)²"}),
+        "x⁻¹" : Operation.unaryOperation({ 1 / $0 }, {"\($0)⁻¹"}),
+        "±" : Operation.unaryOperation({ -$0 }, {"-\($0)"}),
+        "×" : Operation.binaryOperation({ $0 * $1 }, {"\($0) × \($1)"}),
+        "÷" : Operation.binaryOperation({ $0 / $1 }, {"\($0) ÷ \($1)"}),
+        "+" : Operation.binaryOperation({ $0 + $1 }, {"\($0) + \($1)"}),
+        "−" : Operation.binaryOperation({ $0 - $1 }, {"\($0) − \($1)"}),
         "=" : Operation.equals
     ]
 
@@ -41,15 +42,18 @@ struct CalculatorBrain {
             switch operation {
             case .constant(let value):
                 accumulator = value
-            case .unaryOperation(let function):
+                accumulatorString = symbol
+            case .unaryOperation(let function, let descriptionFunction):
                 if accumulator != nil {
                     accumulator = function(accumulator!)
+                    accumulatorString = descriptionFunction(accumulatorString!)
                 }
-            case .binaryOperation(let function):
+            case .binaryOperation(let function, let descriptionFunction):
                 performPendingBinaryOperation()
                 if accumulator != nil {
-                    pendingBinaryOperation = PendingBinaryOperation(function: function, firstOperand: accumulator!)
+                    pendingBinaryOperation = PendingBinaryOperation(function: function, firstOperand: accumulator!, descriptionFunction: descriptionFunction, descriptionOperand: accumulatorString!)
                     accumulator = nil
+                    accumulatorString = nil
                 }
                 break
             case .equals:
@@ -61,6 +65,7 @@ struct CalculatorBrain {
     private mutating func performPendingBinaryOperation() {
         if pendingBinaryOperation != nil && accumulator != nil {
             accumulator = pendingBinaryOperation!.perform(with: accumulator!)
+            accumulatorString = pendingBinaryOperation!.buildDescription(with: accumulatorString!)
             pendingBinaryOperation = nil
         }
     }
@@ -70,14 +75,22 @@ struct CalculatorBrain {
     private struct PendingBinaryOperation {
         let function: (Double,Double) -> Double
         let firstOperand: Double
+
+        let descriptionFunction: (String, String) -> String
+        let descriptionOperand: String
 
         func perform(with secondOperand: Double) -> Double {
             return function(firstOperand, secondOperand)
         }
+
+        func buildDescription(with secondOperand: String) -> String {
+            return descriptionFunction(descriptionOperand, secondOperand)
+        }
     }
 
     mutating func setOperand(_ operand: Double) {
         accumulator = operand
+        accumulatorString = String(format: "%g", operand)
     }
 
     var result: Double? {
@@ -92,5 +105,13 @@ struct CalculatorBrain {
         }
     }
 
-    var description: String = "Not implemented"
+    var description: String? {
+        get {
+            if pendingBinaryOperation != nil {
+                return pendingBinaryOperation!.descriptionFunction(pendingBinaryOperation!.descriptionOperand, accumulatorString ?? "")
+            } else {
+                return accumulatorString
+            }
+        }
+    }
 }