topdown: Added a new string reverse function

ast: Updated the builtins list

partially fixes open-policy-agent#3736

Signed-off-by: Olamide Omolola <omololaolamidex@gmail.com>

ast/builtins.go

@@ -128,6 +128,7 @@ var DefaultBuiltins = [...]*Builtin{
 	TrimSuffix,
 	TrimSpace,
 	Sprintf,
+	Reverse,
 
 	// Numbers
 	NumbersRange,
@@ -1085,6 +1086,17 @@ var Sprintf = &Builtin{
 	),
 }
 
+// Reverse returns the given string, reversed.
+var Reverse = &Builtin{
+	Name: "reverse",
+	Decl: types.NewFunction(
+		types.Args(
+			types.S,
+		),
+		types.S,
+	),
+}
+
 /**
  * Numbers
  */

topdown/strings.go

@@ -412,6 +412,29 @@ func builtinSprintf(a, b ast.Value) (ast.Value, error) {
 	return ast.String(fmt.Sprintf(string(s), args...)), nil
 }
 
+func builtinReverse(bctx BuiltinContext, operands []*ast.Term, iter func(*ast.Term) error) error {
+	s, err := builtins.StringOperand(operands[0].Value, 1)
+	if err != nil {
+		return err
+	}
+
+	sRunes := []rune(string(s))
+	length := len(sRunes)
+	reversedRunes := make([]rune, length)
+	index := 0
+
+	for index < length {
+		reversedRunes[index] = sRunes[length-index-1]
+		index++
+	}
+
+	var reversedString ast.String
+	reversedString = ast.String(sRunes)
+
+	return iter(ast.StringTerm(reversedString.String()))
+
+}
+
 func init() {
 	RegisterFunctionalBuiltin2(ast.FormatInt.Name, builtinFormatInt)
 	RegisterFunctionalBuiltin2(ast.Concat.Name, builtinConcat)
@@ -432,4 +455,5 @@ func init() {
 	RegisterFunctionalBuiltin2(ast.TrimSuffix.Name, builtinTrimSuffix)
 	RegisterFunctionalBuiltin1(ast.TrimSpace.Name, builtinTrimSpace)
 	RegisterFunctionalBuiltin2(ast.Sprintf.Name, builtinSprintf)
+	RegisterBuiltinFunc(ast.Reverse.Name, builtinReverse)
 }