added Interpreter design pattern

Author: DmitryTsyvtsyn

src/main/kotlin/design_patterns/Interpreter.kt

@@ -0,0 +1,68 @@
+package design_patterns
+
+/**
+ *
+ * Interpreter is a behavioral design pattern that defines a simple language grammar for a problem domain,
+ *
+ * represents grammatical rules as language sentences and interprets them to solve commonly encountered problems
+ *
+ */
+
+// contains general information for the interpreter
+class InterpreterContext {
+    private val variables = mutableMapOf<String, Int>()
+
+    fun putVariable(key: String, value: Int) {
+        variables[key] = value
+    }
+
+    fun fetchVariable(key: String): Int {
+        return variables[key] ?: 0
+    }
+}
+
+// represents a specific interpreter grammar expression
+interface InterpreterExpression {
+    fun interpret(context: InterpreterContext)
+}
+
+class SetIntVariableExpression(
+    private val key: String,
+    private val intValue: Int
+) : InterpreterExpression {
+    override fun interpret(context: InterpreterContext) {
+        context.putVariable(key = key, value = intValue)
+    }
+}
+
+class PerformExpression(private vararg val expressions: InterpreterExpression) : InterpreterExpression {
+    override fun interpret(context: InterpreterContext) {
+        expressions.forEach { it.interpret(context) }
+    }
+}
+
+class AddVariablesExpression(
+    private val key0: String,
+    private val key1: String,
+    private val result: String
+) : InterpreterExpression {
+    override fun interpret(context: InterpreterContext) {
+        context.putVariable(
+            key = result,
+            value = context.fetchVariable(key0) + context.fetchVariable(key1)
+        )
+    }
+}
+
+class MultipleVariablesExpression(
+    private val key0: String,
+    private val key1: String,
+    private val result: String
+) : InterpreterExpression {
+    override fun interpret(context: InterpreterContext) {
+        context.putVariable(
+            key = result,
+            value = context.fetchVariable(key0) * context.fetchVariable(key1)
+        )
+    }
+}

src/test/kotlin/design_patterns/InterpreterTest.kt

@@ -0,0 +1,28 @@
+package design_patterns
+
+import org.junit.Test
+import org.junit.Assert.assertEquals
+
+class InterpreterTest {
+
+    @Test
+    fun test() {
+        // x = 10, y = 20, d = 5
+        // sum = x + y
+        // result = sum * d
+        val expression = PerformExpression(
+            SetIntVariableExpression("x", 10),
+            SetIntVariableExpression("y", 20),
+            AddVariablesExpression("x", "y", "sum"),
+            SetIntVariableExpression("d", 5),
+            MultipleVariablesExpression("sum", "d", "result")
+        )
+
+        val context = InterpreterContext()
+
+        expression.interpret(context)
+
+        assertEquals(150, context.fetchVariable("result"))
+    }
+
+}