Creating a Function that Reverses a String (#796)

Creating a reverse function for CEL that takes a string and returns a
new string whose characters are in the reverse order of the original.

ext/README.md

@@ -414,3 +414,17 @@ Examples:
 
      'TacoCat'.upperAscii()      // returns 'TACOCAT'
      'TacoCÆt Xii'.upperAscii()  // returns 'TACOCÆT XII'
+
+### Reverse
+
+Returns a new string whose characters are the same as the target string, only formatted in
+reverse order.
+This function relies on converting strings to rune arrays in order to reverse.
+It can be located in Version 3 of strings.
+
+    <string>.reverse() -> <string>
+
+Examples:
+
+    'gums'.reverse() // returns 'smug'
+    'John Smith'.reverse() // returns 'htimS nhoJ'

ext/strings.go

@@ -267,6 +267,19 @@ const (
 //
 //	'TacoCat'.upperAscii()      // returns 'TACOCAT'
 //	'TacoCÆt Xii'.upperAscii()  // returns 'TACOCÆT XII'
+//
+// # Reverse
+//
+// Returns a new string whose characters are the same as the target string, only formatted in
+// reverse order.
+// This function relies on converting strings to rune arrays in order to reverse
+//
+//	<string>.reverse() -> <string>
+//
+// Examples:
+//
+//	'gums'.reverse() // returns 'smug'
+//	'John Smith'.reverse() // returns 'htimS nhoJ'
 func Strings(options ...StringsOption) cel.EnvOption {
 	s := &stringLib{
 		version:        math.MaxUint32,
@@ -500,6 +513,16 @@ func (lib *stringLib) CompileOptions() []cel.EnvOption {
 					}))),
 		)
 	}
+	if lib.version >= 3 {
+		opts = append( opts,
+			cel.Function("reverse",
+				cel.MemberOverload("reverse", []*cel.Type{cel.StringType}, cel.StringType,
+					cel.UnaryBinding(func(str ref.Val) ref.Val {
+						s := str.(types.String)
+						return stringOrError(reverse(string(s)))
+					}))),
+		)
+	}
 	if lib.validateFormat {
 		opts = append(opts, cel.ASTValidators(stringFormatValidator{}))
 	}
@@ -653,6 +676,14 @@ func upperASCII(str string) (string, error) {
 	return string(runes), nil
 }
 
+func reverse(str string) (string, error) {
+	chars := []rune(str)
+	for i, j := 0, len(chars)-1; i < j; i, j = i+1, j-1 {
+		chars[i], chars[j] = chars[j], chars[i]
+	}
+	return string(chars), nil
+}
+
 func joinSeparator(strs []string, separator string) (string, error) {
 	return strings.Join(strs, separator), nil
 }

ext/strings_test.go

@@ -105,6 +105,13 @@ var stringTests = []struct {
 	// Upper ASCII tests.
 	{expr: `'tacoCat'.upperAscii() == 'TACOCAT'`},
 	{expr: `'tacoCαt'.upperAscii() == 'TACOCαT'`},
+	// Reverse tests.
+	{expr: `'gums'.reverse() == 'smug'`},
+	{expr: `'palindromes'.reverse() == 'semordnilap'`},
+	{expr: `'John Smith'.reverse() == 'htimS nhoJ'`},
+	{expr: `'u180etext'.reverse() == 'txete081u'`},
+	{expr: `'2600+U'.reverse() == 'U+0062'`},
+	{expr: `'\u180e\u200b\u200c\u200d\u2060\ufeff'.reverse() == '\ufeff\u2060\u200d\u200c\u200b\u180e'`},
 	// Join tests.
 	{expr: `['x', 'y'].join() == 'xy'`},
 	{expr: `['x', 'y'].join('-') == 'x-y'`},