config: new "reverse" interpolation function

It reverses a list.

config/interpolate_funcs.go

@@ -120,6 +120,7 @@ func Funcs() map[string]ast.Function {
 		"pow":          interpolationFuncPow(),
 		"uuid":         interpolationFuncUUID(),
 		"replace":      interpolationFuncReplace(),
+		"reverse":      interpolationFuncReverse(),
 		"rsadecrypt":   interpolationFuncRsaDecrypt(),
 		"sha1":         interpolationFuncSha1(),
 		"sha256":       interpolationFuncSha256(),
@@ -955,6 +956,25 @@ func interpolationFuncReplace() ast.Function {
 	}
 }
 
+// interpolationFuncReverse implements the "reverse" function that does list reversal
+func interpolationFuncReverse() ast.Function {
+	return ast.Function{
+		ArgTypes:   []ast.Type{ast.TypeList},
+		ReturnType: ast.TypeList,
+		Variadic:   false,
+		Callback: func(args []interface{}) (interface{}, error) {
+			inputList := args[0].([]ast.Variable)
+
+			reversedList := make([]ast.Variable, len(inputList))
+			for idx := range inputList {
+				reversedList[len(inputList)-1-idx] = inputList[idx]
+			}
+
+			return reversedList, nil
+		},
+	}
+}
+
 func interpolationFuncLength() ast.Function {
 	return ast.Function{
 		ArgTypes:   []ast.Type{ast.TypeAny},

config/interpolate_funcs_test.go

@@ -1546,6 +1546,39 @@ func TestInterpolateFuncReplace(t *testing.T) {
 	})
 }
 
+func TestInterpolateFuncReverse(t *testing.T) {
+	testFunction(t, testFunctionConfig{
+		Vars: map[string]ast.Variable{
+			"var.inputlist": {
+				Type: ast.TypeList,
+				Value: []ast.Variable{
+					{Type: ast.TypeString, Value: "a"},
+					{Type: ast.TypeString, Value: "b"},
+					{Type: ast.TypeString, Value: "1"},
+					{Type: ast.TypeString, Value: "d"},
+				},
+			},
+			"var.emptylist": {
+				Type: ast.TypeList,
+				// Intentionally 0-lengthed list
+				Value: []ast.Variable{},
+			},
+		},
+		Cases: []testFunctionCase{
+			{
+				`${reverse(var.inputlist)}`,
+				[]interface{}{"d", "1", "b", "a"},
+				false,
+			},
+			{
+				`${reverse(var.emptylist)}`,
+				[]interface{}{},
+				false,
+			},
+		},
+	})
+}
+
 func TestInterpolateFuncLength(t *testing.T) {
 	testFunction(t, testFunctionConfig{
 		Cases: []testFunctionCase{