AK: Add a somewhat naive implementation of String::reverse

This will reverse the String's code points (i.e. not just its bytes),
but is not aware of grapheme clusters.

AK/String.cpp

@@ -10,6 +10,7 @@
 #include <AK/String.h>
 #include <AK/StringBuilder.h>
 #include <AK/Utf8View.h>
+#include <AK/Vector.h>
 #include <stdlib.h>
 
 namespace AK {
@@ -319,6 +320,25 @@ ErrorOr<String> String::replace(StringView needle, StringView replacement, Repla
     return StringUtils::replace(*this, needle, replacement, replace_mode);
 }
 
+ErrorOr<String> String::reverse() const
+{
+    // FIXME: This handles multi-byte code points, but not e.g. grapheme clusters.
+    // FIXME: We could avoid allocating a temporary vector if Utf8View supports reverse iteration.
+    auto code_point_length = code_points().length();
+
+    Vector<u32> code_points;
+    TRY(code_points.try_ensure_capacity(code_point_length));
+
+    for (auto code_point : this->code_points())
+        code_points.unchecked_append(code_point);
+
+    auto builder = TRY(StringBuilder::create(code_point_length * sizeof(u32)));
+    while (!code_points.is_empty())
+        TRY(builder.try_append_code_point(code_points.take_last()));
+
+    return builder.to_string();
+}
+
 bool String::is_short_string() const
 {
     return has_short_string_bit(reinterpret_cast<uintptr_t>(m_data));

AK/String.h

@@ -69,6 +69,7 @@ public:
     [[nodiscard]] StringView bytes_as_string_view() const;
 
     ErrorOr<String> replace(StringView needle, StringView replacement, ReplaceMode replace_mode) const;
+    ErrorOr<String> reverse() const;
 
     [[nodiscard]] bool operator==(String const&) const;
     [[nodiscard]] bool operator!=(String const& other) const { return !(*this == other); }

Tests/AK/TestString.cpp

@@ -108,6 +108,23 @@ TEST_CASE(replace)
     }
 }
 
+TEST_CASE(reverse)
+{
+    auto test_reverse = [](auto test, auto expected) {
+        auto string = MUST(String::from_utf8(test));
+        auto result = MUST(string.reverse());
+
+        EXPECT_EQ(result, expected);
+    };
+
+    test_reverse(""sv, ""sv);
+    test_reverse("a"sv, "a"sv);
+    test_reverse("ab"sv, "ba"sv);
+    test_reverse("ab cd ef"sv, "fe dc ba"sv);
+    test_reverse("😀"sv, "😀"sv);
+    test_reverse("ab😀cd"sv, "dc😀ba"sv);
+}
+
 TEST_CASE(to_lowercase)
 {
     {