[Expression]Add reverse prebuilt function

libraries/AdaptiveExpressions/BuiltinFunctions/Reverse.cs

@@ -0,0 +1,52 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+using System.Collections;
+using System.Linq;
+
+namespace AdaptiveExpressions.BuiltinFunctions
+{
+    /// <summary>
+    /// Reverses the order of the elements in a String or Array.
+    /// </summary>
+    internal class Reverse : ExpressionEvaluator
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Reverse"/> class.
+        /// </summary>
+        public Reverse()
+            : base(ExpressionType.Reverse, Evaluator(), ReturnType.String | ReturnType.Array, Validator)
+        {
+        }
+
+        private static EvaluateExpressionDelegate Evaluator()
+        {
+            return FunctionUtils.ApplyWithError(
+                        args =>
+                        {
+                            object result = null;
+                            string error = null;
+
+                            if (args[0] is string string0)
+                            {
+                                result = new string(string0.Reverse().ToArray());
+                            }
+                            else if (args[0] is IList list)
+                            {
+                                result = list.OfType<object>().Reverse().ToList();
+                            }
+                            else
+                            {
+                                error = $"{args[0]} is not a string or list.";
+                            }
+
+                            return (result, error);
+                        }, FunctionUtils.VerifyContainer);
+        }
+
+        private static void Validator(Expression expression)
+        {
+            FunctionUtils.ValidateOrder(expression, null, ReturnType.String | ReturnType.Array);
+        }
+    }
+}

libraries/AdaptiveExpressions/ExpressionFunctions.cs

@@ -137,6 +137,7 @@ private static IDictionary<string, ExpressionEvaluator> GetStandardFunctions()
                 new BuiltinFunctions.RemoveProperty(),
                 new BuiltinFunctions.Replace(),
                 new BuiltinFunctions.ReplaceIgnoreCase(),
+                new BuiltinFunctions.Reverse(),
                 new BuiltinFunctions.Round(),
                 new BuiltinFunctions.Select(),
                 new BuiltinFunctions.SentenceCase(),

libraries/AdaptiveExpressions/ExpressionType.cs

@@ -80,6 +80,7 @@ public static class ExpressionType
         public const string IndicesAndValues = "indicesAndValues";
         public const string Flatten = "flatten";
         public const string Unique = "unique";
+        public const string Reverse = "reverse";
 
         // DateTime
         public const string AddDays = "addDays";

tests/AdaptiveExpressions.Tests/BadExpressionTests.cs

@@ -356,6 +356,8 @@ public class BadExpressionTests
             Test("contains('hello world', 'hello', 'new')"), // should have 2 parameter
             Test("count(items, 1)"), // should have 1 parameter
             Test("count(1)"), // first param should be list or string
+            Test("reverse(items, 1)"), // should have 1 parameter
+            Test("reverse(1)"), // first param should be list or string
             Test("empty(1,2)"), // should have two params
             Test("first(items,2)"), // should have 1 param
             Test("last(items,2)"), // should have 1 param

tests/AdaptiveExpressions.Tests/ExpressionParserTests.cs

@@ -521,6 +521,10 @@ public class ExpressionParserTests
             Test("count('hello')", 5),
             Test("count(\"hello\")", 5),
             Test("count(concat(hello,world))", 10),
+            Test("reverse('hello')", "olleh"),
+            Test("reverse(reverse('hello'))", "hello"),
+            Test("reverse(\"hello\")", "olleh"),
+            Test("reverse(concat(hello,world))", "dlrowolleh"),
             Test("replace('hello', 'l', 'k')", "hekko"),
             Test("replace('hello', 'L', 'k')", "hello"),
             Test("replace(nullObj, 'L', 'k')", string.Empty),
@@ -934,6 +938,8 @@ public class ExpressionParserTests
             Test("concat(['a', 'b'], ['b', 'c'], ['c', 'd'])", new List<object> { "a", "b", "b", "c", "c", "d" }),
             Test("count(split(hello,'e'))", 2),
             Test("count(createArray('h', 'e', 'l', 'l', 'o'))", 5),
+            Test("reverse(split(hello,'e'))", new List<object> { "llo", "h" }),
+            Test("reverse(createArray('h', 'e', 'l', 'l', 'o'))", new List<object> { "o", "l", "l", "e", "h" }),
             Test("empty('')", true),
             Test("empty('a')", false),
             Test("empty(bag)", false),