Add array.reverse(array) and strings.reverse(string) built-in fun…

…ctions. (open-policy-agent#4161)

The function `array.reverse` takes an array as an argument, and returns an array with a reversed order of elements.
The function `strings.reverse` takes a string as an argument, and returns a string with a reversed order of unicode code points.
WASM support is included for both built-ins.

Fixes open-policy-agent#3736

Signed-off-by: Kristian Svalland <kristian.svalland@gmail.com>

ast/builtins.go

@@ -80,6 +80,7 @@ var DefaultBuiltins = [...]*Builtin{
 	// Arrays
 	ArrayConcat,
 	ArraySlice,
+	ArrayReverse,
 
 	// Conversions
 	ToNumber,
@@ -127,6 +128,7 @@ var DefaultBuiltins = [...]*Builtin{
 	TrimSuffix,
 	TrimSpace,
 	Sprintf,
+	StringReverse,
 
 	// Numbers
 	NumbersRange,
@@ -717,6 +719,17 @@ var ArraySlice = &Builtin{
 	),
 }
 
+// ArrayReverse returns a given array, reversed
+var ArrayReverse = &Builtin{
+	Name: "array.reverse",
+	Decl: types.NewFunction(
+		types.Args(
+			types.NewArray(nil, types.A),
+		),
+		types.NewArray(nil, types.A),
+	),
+}
+
 /**
  * Conversions
  */
@@ -1080,6 +1093,17 @@ var Sprintf = &Builtin{
 	),
 }
 
+// StringReverse returns the given string, reversed.
+var StringReverse = &Builtin{
+	Name: "strings.reverse",
+	Decl: types.NewFunction(
+		types.Args(
+			types.S,
+		),
+		types.S,
+	),
+}
+
 /**
  * Numbers
  */

capabilities.json

@@ -123,6 +123,26 @@
         "type": "function"
       }
     },
+    {
+      "name": "array.reverse",
+      "decl": {
+        "args": [
+          {
+            "dynamic": {
+              "type": "any"
+            },
+            "type": "array"
+          }
+        ],
+        "result": {
+          "dynamic": {
+            "type": "any"
+          },
+          "type": "array"
+        },
+        "type": "function"
+      }
+    },
     {
       "name": "array.slice",
       "decl": {
@@ -2979,6 +2999,20 @@
         "type": "function"
       }
     },
+    {
+      "name": "strings.reverse",
+      "decl": {
+        "args": [
+          {
+            "type": "string"
+          }
+        ],
+        "result": {
+          "type": "string"
+        },
+        "type": "function"
+      }
+    },
     {
       "name": "substring",
       "decl": {

docs/content/policy-reference.md

@@ -324,6 +324,7 @@ complex types.
 | Built-in | Description | Wasm Support |
 | ------- |-------------|---------------|
 | <span class="opa-keep-it-together">``output := array.concat(array, array)``</span> | ``output`` is the result of concatenating the two input arrays together. | ✅ |
+| <span class="opa-keep-it-together">``output := array.reverse(array)``</span> | ``output`` is the result of reversing the order of the elements in ``array``. | ✅ |
   <span class="opa-keep-it-together">``output := array.slice(array, startIndex, stopIndex)``</span> | ``output`` is the part of the ``array`` from ``startIndex`` to ``stopIndex`` including the first but excluding the last. If `startIndex >= stopIndex` then `output == []`. If both `startIndex` and `stopIndex` are less than zero, `output == []`. Otherwise, `startIndex` and `stopIndex` are clamped to 0 and `count(array)` respectively. | ✅ |
 
 ### Sets
@@ -379,6 +380,7 @@ complex types.
 | <span class="opa-keep-it-together">``output := indexof(string, search)``</span> | ``output`` is the index inside ``string`` where ``search`` first occurs, or -1 if ``search`` does not exist | ✅ |
 | <span class="opa-keep-it-together">``output := lower(string)``</span> | ``output`` is ``string`` after converting to lower case | ✅ |
 | <span class="opa-keep-it-together">``output := replace(string, old, new)``</span> | ``output`` is a ``string`` representing ``string`` with all instances of ``old`` replaced by ``new`` | ✅ |
+| <span class="opa-keep-it-together">``output := strings.reverse(string)``</span> | ``output`` is ``string`` reversed | ✅ |
 | <span class="opa-keep-it-together">``output := strings.replace_n(patterns, string)``</span> | ``patterns`` is an object with old, new string key value pairs (e.g. ``{"old1": "new1", "old2": "new2", ...}``). ``output`` is a ``string`` with all old strings inside ``patterns`` replaced by the new strings | ✅ |
 | <span class="opa-keep-it-together">``output := split(string, delimiter)``</span> | ``output`` is ``array[string]`` representing elements of ``string`` separated by ``delimiter`` | ✅ |
 | <span class="opa-keep-it-together">``output := sprintf(string, values)``</span> | ``output`` is a ``string`` representing ``string`` formatted by the values in the ``array`` ``values``. | ``SDK-dependent`` |

internal/compiler/wasm/wasm.go

@@ -91,6 +91,7 @@ var builtinsFunctions = map[string]string{
 	ast.Floor.Name:                      "opa_arith_floor",
 	ast.Rem.Name:                        "opa_arith_rem",
 	ast.ArrayConcat.Name:                "opa_array_concat",
+	ast.ArrayReverse.Name:               "opa_array_reverse",
 	ast.ArraySlice.Name:                 "opa_array_slice",
 	ast.SetDiff.Name:                    "opa_set_diff",
 	ast.And.Name:                        "opa_set_intersection",
@@ -149,6 +150,7 @@ var builtinsFunctions = map[string]string{
 	ast.Contains.Name:                   "opa_strings_contains",
 	ast.StartsWith.Name:                 "opa_strings_startswith",
 	ast.EndsWith.Name:                   "opa_strings_endswith",
+	ast.StringReverse.Name:              "opa_strings_reverse",
 	ast.Split.Name:                      "opa_strings_split",
 	ast.Replace.Name:                    "opa_strings_replace",
 	ast.ReplaceN.Name:                   "opa_strings_replace_n",

test/cases/testdata/array/test-array-0052.yaml

@@ -0,0 +1,43 @@
+cases:
+- note: 'array/reverse_123'
+  query: data.test.p = x
+  data:
+    foo:
+    - 1
+    - 2
+    - 3
+  modules:
+  - |
+    package test
+
+    p := array.reverse(data.foo)
+  want_result:
+  - x:
+    - 3
+    - 2
+    - 1
+- note: 'array/reverse_empty'
+  query: data.test.p = x
+  data:
+    foo: []
+  modules:
+  - |
+    package test
+
+    p := array.reverse(data.foo)
+  want_result:
+  - x: []
+- note: 'array/reverse_object_error'
+  query: data.test.p = x
+  data:
+    foo:
+      bar: baz
+      baz: bar
+  modules:
+  - |
+    package test
+
+    p := array.reverse(data.foo)
+  want_error: "array.reverse: operand 1 must be array but got object"
+  want_error_code: eval_type_error
+  strict_error: true

test/cases/testdata/strings/test-strings-0924.yaml

@@ -0,0 +1,73 @@
+cases:
+- note: 'strings/reverse_bar'
+  query: data.test.p = x
+  data:
+    foo: "bar"
+  modules:
+  - |
+    package test
+
+    p := strings.reverse(data.foo)
+  want_result:
+  - x: "rab"
+- note: 'strings/reverse_unicode_multi_char_emojii'
+  query: data.test.p = x
+  data:
+    # The "Keycap Digit Two" consists of three codepoints: [2 U+32, U+FE0F, U+20E3]
+    # Other examples of such emojies are smileys with defined skin-color
+    # In the current implementation, we reverse strings at the level of codepoints, not Glyphs
+    foo: "2️⃣"
+  modules:
+  - |
+    package test
+
+    p := strings.reverse(data.foo)
+  want_result:
+  - x: "⃣️2"
+- note: 'strings/reverse_unicode'
+  query: data.test.p = x
+  data:
+    foo: "1😀𝛾"
+  modules:
+  - |
+    package test
+
+    p := strings.reverse(data.foo)
+  want_result:
+  - x: "𝛾😀1"
+- note: 'strings/reverse_empty'
+  query: data.test.p = x
+  data:
+    foo: ""
+  modules:
+  - |
+    package test
+
+    p := strings.reverse(data.foo)
+  want_result:
+  - x: ""
+- note: 'strings/reverse_number_error'
+  query: data.test.p = x
+  data:
+    foo: 123
+  modules:
+  - |
+    package test
+
+    p := strings.reverse(data.foo)
+  want_error: "reverse: operand 1 must be string but got number"
+  want_error_code: eval_type_error
+  strict_error: true
+- note: 'strings/reverse_object_error'
+  query: data.test.p = x
+  data:
+    foo: 
+      bar: baz
+  modules:
+  - |
+    package test
+
+    p := strings.reverse(data.foo)
+  want_error: "reverse: operand 1 must be string but got object"
+  want_error_code: eval_type_error
+  strict_error: true

topdown/array.go

@@ -71,7 +71,24 @@ 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)
+
+	for index := 0; index < length; index++ {
+		reversedArr[index] = arr.Elem(length - index - 1)
+	}
+
+	return iter(ast.ArrayTerm(reversedArr...))
+}
+
 func init() {
 	RegisterFunctionalBuiltin2(ast.ArrayConcat.Name, builtinArrayConcat)
 	RegisterFunctionalBuiltin3(ast.ArraySlice.Name, builtinArraySlice)
+	RegisterBuiltinFunc(ast.ArrayReverse.Name, builtinArrayReverse)
 }

topdown/strings.go

@@ -412,6 +412,25 @@ 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)
+
+	for index, r := range sRunes {
+		reversedRunes[length-index-1] = r
+	}
+
+	reversedString := string(reversedRunes)
+
+	return iter(ast.StringTerm(reversedString))
+}
+
 func init() {
 	RegisterFunctionalBuiltin2(ast.FormatInt.Name, builtinFormatInt)
 	RegisterFunctionalBuiltin2(ast.Concat.Name, builtinConcat)
@@ -432,4 +451,5 @@ func init() {
 	RegisterFunctionalBuiltin2(ast.TrimSuffix.Name, builtinTrimSuffix)
 	RegisterFunctionalBuiltin1(ast.TrimSpace.Name, builtinTrimSpace)
 	RegisterFunctionalBuiltin2(ast.Sprintf.Name, builtinSprintf)
+	RegisterBuiltinFunc(ast.StringReverse.Name, builtinReverse)
 }

wasm/src/array.c

@@ -66,3 +66,24 @@ opa_value *opa_array_slice(opa_value *a, opa_value *i, opa_value *j)
 
     return &r->hdr;
 }
+
+OPA_BUILTIN
+opa_value *opa_array_reverse(opa_value *a)
+{
+    if (opa_value_type(a) != OPA_ARRAY)
+    {
+        return NULL;
+    }
+
+    opa_array_t *arr = opa_cast_array(a);
+
+    opa_array_t *reversed = opa_cast_array(opa_array_with_cap(arr->len));
+
+    int n = arr->len;
+
+    for (int i = 0; i < n; i++) {
+        opa_array_append(reversed, arr->elems[n - 1 - i].v);
+    }
+
+    return &reversed->hdr;
+}

wasm/src/array.h

@@ -3,5 +3,6 @@
 
 opa_value *opa_array_concat(opa_value *a, opa_value *b);
 opa_value *opa_array_slice(opa_value *a, opa_value *i, opa_value *j);
+opa_value *opa_array_reverse(opa_value *a);
 
 #endif

wasm/src/strings.c

@@ -351,6 +351,33 @@ opa_value *opa_strings_replace_n(opa_value *a, opa_value *b)
     return result;
 }
 
+OPA_BUILTIN
+opa_value *opa_strings_reverse(opa_value *a)
+{
+    if (opa_value_type(a) != OPA_STRING)
+    {
+        return NULL;
+    }
+
+    opa_string_t *s = opa_cast_string(a);
+
+    char *reversed = opa_malloc(s->len + 1);
+
+    for (int i = 0; i < s->len; )
+    {
+        int len = 0;
+        if (opa_unicode_decode_utf8(s->v, i, s->len, &len) == -1)
+        {
+            opa_abort("string: invalid unicode");
+        }
+        memcpy(&reversed[s->len - i - len], &s->v[i], len);
+        i += len;
+    }
+    reversed[s->len] = '\0';
+
+    return opa_string_allocated(reversed, s->len);
+}
+
 OPA_BUILTIN
 opa_value *opa_strings_split(opa_value *a, opa_value *b)
 {

wasm/src/strings.h

@@ -11,6 +11,7 @@ opa_value *opa_strings_indexof(opa_value *a, opa_value *b);
 opa_value *opa_strings_lower(opa_value *a);
 opa_value *opa_strings_replace(opa_value *a, opa_value *b, opa_value *c);
 opa_value *opa_strings_replace_n(opa_value *a, opa_value *b);
+opa_value *opa_strings_reverse(opa_value *a);
 opa_value *opa_strings_split(opa_value *a, opa_value *b);
 opa_value *opa_strings_startswith(opa_value *a, opa_value *b);
 opa_value *opa_strings_substring(opa_value *a, opa_value *b, opa_value *c);