topdown: Added a new array reverse function

ast: Updated the builtins list

partially fixes open-policy-agent#3736

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

ast/builtins.go

@@ -80,6 +80,7 @@ var DefaultBuiltins = [...]*Builtin{
 	// Arrays
 	ArrayConcat,
 	ArraySlice,
+	ArrayReverse,
 
 	// Conversions
 	ToNumber,
@@ -711,6 +712,16 @@ var ArraySlice = &Builtin{
 	),
 }
 
+var ArrayReverse = &Builtin{
+	Name: "array.reverse",
+	Decl: types.NewFunction(
+		types.Args(
+			types.NewArray(nil, types.A),
+		),
+		types.NewArray(nil, types.A),
+	),
+}
+
 /**
  * Conversions
  */

topdown/array.go

@@ -71,7 +71,27 @@ func builtinArraySlice(a, i, j ast.Value) (ast.Value, error) {
 	return arr.Slice(startIndex, stopIndex), nil
 }
 
+func builtinArrayReverse(bctx BuiltinContext, operands []*ast.Term, iter func(*ast.Term) error) error {
+	arr, err := builtins.ArrayOperand(operands[0].Value, 1)
+	if err != nil {
+		return err
+	}
+
+	length := arr.Len()
+	reversedArr := make([]*ast.Term, length)
+	index := 0
+
+	for index < length {
+		reversedArr[index] = arr.Elem(length - index - 1) //
+		index++
+	}
+
+	return iter(ast.ArrayTerm(reversedArr...))
+
+}
+
 func init() {
 	RegisterFunctionalBuiltin2(ast.ArrayConcat.Name, builtinArrayConcat)
 	RegisterFunctionalBuiltin3(ast.ArraySlice.Name, builtinArraySlice)
+	RegisterBuiltinFunc(ast.ArrayReverse.Name, builtinArrayReverse)
 }