Add Interpreter pattern

Author: TheDmitry

source/behavioral/interpreter.swift

@@ -0,0 +1,90 @@
+/*:
+🎶 Interpreter
+--------------
+
+The interpreter pattern is used to evaluate sentences in a language.
+
+### Example
+*/
+
+protocol IntegerExp {
+    func evaluate(context: IntegerContext) -> Int
+    func replace(character: Character, integerExp: IntegerExp) -> IntegerExp
+    func copy() -> IntegerExp
+}
+
+class IntegerContext {
+    private var data: [Character:Int] = [:]
+    
+    func lookup(name: Character) -> Int {
+        return self.data[name]!
+    }
+    
+    func assign(integerVarExp: IntegerVarExp, value: Int) {
+        self.data[integerVarExp.name] = value
+    }
+}
+
+class IntegerVarExp: IntegerExp {
+    let name: Character
+    
+    init(name: Character) {
+        self.name = name
+    }
+    
+    func evaluate(context: IntegerContext) -> Int {
+        return context.lookup(self.name)
+    }
+    
+    func replace(name: Character, integerExp: IntegerExp) -> IntegerExp {
+        if name == self.name {
+            return integerExp.copy()
+        } else {
+            return IntegerVarExp(name: self.name)
+        }
+    }
+    
+    func copy() -> IntegerExp {
+        return IntegerVarExp(name: self.name)
+    }
+}
+
+class AddExp: IntegerExp {
+    private var operand1: IntegerExp
+    private var operand2: IntegerExp
+    
+    init(op1: IntegerExp, op2: IntegerExp) {
+        self.operand1 = op1
+        self.operand2 = op2
+    }
+    
+    func evaluate(context: IntegerContext) -> Int {
+        return self.operand1.evaluate(context) + self.operand2.evaluate(context)
+    }
+    
+    func replace(character: Character, integerExp: IntegerExp) -> IntegerExp {
+        return AddExp(op1: operand1.replace(character, integerExp: integerExp),
+            op2: operand2.replace(character, integerExp: integerExp))
+    }
+    
+    func copy() -> IntegerExp {
+        return AddExp(op1: self.operand1, op2: self.operand2)
+    }
+}
+/*:
+### Usage
+*/
+var expression: IntegerExp?
+var context = IntegerContext()
+
+var a = IntegerVarExp(name: "A")
+var b = IntegerVarExp(name: "B")
+var c = IntegerVarExp(name: "C")
+
+expression = AddExp(op1: a, op2: AddExp(op1: b, op2: c)) // a + (b + c)
+
+context.assign(a, value: 2)
+context.assign(b, value: 1)
+context.assign(c, value: 3)
+
+var result = expression?.evaluate(context)