AK: Add a couple of Utf16String factories

* Utf16String::from_utf8_with_replacement_character
* Utf16String::from_code_point

Author: trflynn89

AK/Utf16String.cpp

@@ -13,6 +13,28 @@ namespace AK {
 
 static_assert(sizeof(Detail::ShortString) == sizeof(Detail::Utf16StringData*));
 
+Utf16String Utf16String::from_utf8_with_replacement_character(StringView utf8_string, WithBOMHandling with_bom_handling)
+{
+    if (auto bytes = utf8_string.bytes(); with_bom_handling == WithBOMHandling::Yes && bytes.starts_with({ { 0xEF, 0xBB, 0xBF } }))
+        utf8_string = utf8_string.substring_view(3);
+
+    Utf8View utf8_view { utf8_string };
+
+    if (utf8_view.validate(AllowLonelySurrogates::No))
+        return Utf16String::from_utf8_without_validation(utf8_string);
+
+    StringBuilder builder(StringBuilder::Mode::UTF16);
+
+    for (auto code_point : utf8_view) {
+        if (is_unicode_surrogate(code_point))
+            builder.append_code_point(UnicodeUtils::REPLACEMENT_CODE_POINT);
+        else
+            builder.append_code_point(code_point);
+    }
+
+    return builder.to_utf16_string_without_validation();
+}
+
 Utf16String Utf16String::from_utf8_without_validation(StringView utf8_string)
 {
     if (utf8_string.length() <= Detail::MAX_SHORT_STRING_BYTE_COUNT && utf8_string.is_ascii()) {

AK/Utf16String.h

@@ -45,6 +45,12 @@ class [[nodiscard]] Utf16String : public Detail::Utf16StringBase {
         return from_utf8_without_validation(utf8_string);
     }
 
+    enum class WithBOMHandling {
+        No,
+        Yes,
+    };
+    static Utf16String from_utf8_with_replacement_character(StringView, WithBOMHandling = WithBOMHandling::Yes);
+
     ALWAYS_INLINE static ErrorOr<Utf16String> try_from_utf8(StringView utf8_string)
     {
         if (!Utf8View { utf8_string }.validate())
@@ -81,6 +87,18 @@ class [[nodiscard]] Utf16String : public Detail::Utf16StringBase {
     requires(IsOneOf<RemoveCVReference<T>, Utf16String, Utf16FlyString>)
     static Utf16String from_utf16_without_validation(T&&) = delete;
 
+    ALWAYS_INLINE static Utf16String from_code_point(u32 code_point)
+    {
+        Array<char16_t, 2> code_units;
+        size_t length_in_code_units = 0;
+
+        (void)UnicodeUtils::code_point_to_utf16(code_point, [&](auto code_unit) {
+            code_units[length_in_code_units++] = code_unit;
+        });
+
+        return from_utf16_without_validation({ code_units.data(), length_in_code_units });
+    }
+
     template<typename... Parameters>
     ALWAYS_INLINE static Utf16String formatted(CheckedFormatString<Parameters...>&& format, Parameters const&... parameters)
     {

Tests/AK/TestUtf16String.cpp

@@ -96,6 +96,27 @@ TEST_CASE(from_utf8)
     }
 }
 
+TEST_CASE(from_utf8_with_replacement_character)
+{
+    auto string1 = Utf16String::from_utf8_with_replacement_character("long string \xf4\x8f\xbf\xc0"sv, Utf16String::WithBOMHandling::No); // U+110000
+    EXPECT_EQ(string1, u"long string \ufffd\ufffd\ufffd\ufffd"sv);
+
+    auto string3 = Utf16String::from_utf8_with_replacement_character("A valid string!"sv, Utf16String::WithBOMHandling::No);
+    EXPECT_EQ(string3, "A valid string!"sv);
+
+    auto string4 = Utf16String::from_utf8_with_replacement_character(""sv, Utf16String::WithBOMHandling::No);
+    EXPECT_EQ(string4, ""sv);
+
+    auto string5 = Utf16String::from_utf8_with_replacement_character("\xEF\xBB\xBFWHF!"sv, Utf16String::WithBOMHandling::Yes);
+    EXPECT_EQ(string5, "WHF!"sv);
+
+    auto string6 = Utf16String::from_utf8_with_replacement_character("\xEF\xBB\xBFWHF!"sv, Utf16String::WithBOMHandling::No);
+    EXPECT_EQ(string6, u"\ufeffWHF!"sv);
+
+    auto string7 = Utf16String::from_utf8_with_replacement_character("\xED\xA0\x80WHF!"sv); // U+D800
+    EXPECT_EQ(string7, u"\ufffdWHF!"sv);
+}
+
 TEST_CASE(from_utf16)
 {
     {
@@ -235,6 +256,32 @@ TEST_CASE(from_utf32)
     }
 }
 
+TEST_CASE(from_code_point)
+{
+    u32 code_point = 0;
+
+    for (; code_point < AK::UnicodeUtils::FIRST_SUPPLEMENTARY_PLANE_CODE_POINT; ++code_point) {
+        auto string = Utf16String::from_code_point(code_point);
+        EXPECT_EQ(string.length_in_code_units(), 1uz);
+        EXPECT_EQ(string.length_in_code_points(), 1uz);
+        EXPECT_EQ(string.code_point_at(0), code_point);
+        EXPECT_EQ(string.code_unit_at(0), code_point);
+    }
+
+    for (; code_point < AK::UnicodeUtils::FIRST_SUPPLEMENTARY_PLANE_CODE_POINT + 10'000; ++code_point) {
+        auto string = Utf16String::from_code_point(code_point);
+        EXPECT_EQ(string.length_in_code_units(), 2uz);
+        EXPECT_EQ(string.length_in_code_points(), 1uz);
+        EXPECT_EQ(string.code_point_at(0), code_point);
+
+        size_t i = 0;
+        (void)AK::UnicodeUtils::code_point_to_utf16(code_point, [&](auto code_unit) {
+            EXPECT_EQ(string.code_unit_at(i++), code_unit);
+        });
+        EXPECT_EQ(i, 2uz);
+    }
+}
+
 TEST_CASE(formatted)
 {
     {