Merge pull request TheAlgorithms#5 from 5sw/shunting_yard

Implement shunting yard algorithm to evaluate simple math expressions

algorithms/parsing/shunting_yard/shunting_yard.swift

@@ -0,0 +1,97 @@
+import Foundation
+
+enum ShuntingYard {
+    enum Operator: String, CaseIterable {
+        case power = "^"
+        case plus = "+"
+        case minus = "-"
+        case times = "*"
+        case divide = "/"
+    }
+
+    static func evaluate(_ string: String) -> Double {
+        let scanner = Scanner(string: string)
+        var numberStack: [Double] = []
+        var operatorStack: [Operator] = []
+
+        func applyOperator(_ op: Operator) {
+            guard let a = numberStack.popLast(), let b = numberStack.popLast() else {
+                return
+            }
+
+            numberStack.append(op.apply(a, b))
+        }
+
+        while !scanner.isAtEnd {
+            if let op = scanner.scanOperator() {
+                while let last = operatorStack.last, last.precedence > op.precedence || (op.leftAssociative && last.precedence == op.precedence) {
+                    applyOperator(last)
+                    operatorStack.removeLast()
+                }
+                operatorStack.append(op)
+            } else if let number = scanner.scanDouble() {
+                numberStack.append(number)
+            } else {
+                break
+            }
+        }
+
+        while let op = operatorStack.popLast() {
+            applyOperator(op)
+        }
+
+        return numberStack.first ?? 0
+    }
+}
+
+extension ShuntingYard.Operator {
+    var precedence: Int {
+        switch self {
+        case .power: return 3
+        case .divide, .times: return 2
+        case .plus, .minus: return 1
+        }
+    }
+
+    var leftAssociative: Bool {
+        switch self {
+        case .power: return false
+        case .plus, .minus, .times, .divide: return true
+        }
+    }
+
+    func apply(_ a: Double, _ b: Double) -> Double {
+        switch self {
+        case .power: return pow(b, a)
+        case .divide: return b / a
+        case .times: return a * b
+        case .plus: return a + b
+        case .minus: return b - a
+        }
+    }
+}
+
+private extension Scanner {
+    func scanOperator() -> ShuntingYard.Operator? {
+        for op in ShuntingYard.Operator.allCases {
+            if scanString(op.rawValue) != nil {
+                return op
+            }
+        }
+        return nil
+    }
+}
+
+func testShuntingYard() {
+    func test(_ x: String) {
+        print(x,"=", ShuntingYard.evaluate(x))
+    }
+
+    test("3 + 4 * 5")
+    test("4 * 5 + 3")
+    test("2 ^ 3 ^ 4")
+    test("10.5 - 4 * 5")
+    test("2 + 3 ^ 4")
+    test("2 * 3 ^ 4")
+    test("3 ^ 4")
+}