LibUnicode: Parse and generate case folding code point data

Case folding rules have a similar mapping style as special casing rules,
where one code point may map to zero or more case folding rules. These
will be used for case-insensitive string comparisons. To see how case
folding can differ from other casing rules, consider "ß" (U+00DF):

    >>> "ß".lower()
    'ß'

    >>> "ß".upper()
    'SS'

    >>> "ß".title()
    'Ss'

    >>> "ß".casefold()
    'ss'

Userland/Libraries/LibUnicode/CharacterTypes.cpp

@@ -61,6 +61,13 @@ ErrorOr<String> to_unicode_titlecase_full(StringView string, Optional<StringView
     return builder.to_string();
 }
 
+ErrorOr<String> to_unicode_casefold_full(StringView string)
+{
+    StringBuilder builder;
+    TRY(Detail::build_casefold_string(Utf8View { string }, builder));
+    return builder.to_string();
+}
+
 Optional<GeneralCategory> __attribute__((weak)) general_category_from_string(StringView) { return {}; }
 bool __attribute__((weak)) code_point_has_general_category(u32, GeneralCategory) { return {}; }
 Optional<Property> __attribute__((weak)) property_from_string(StringView) { return {}; }

Userland/Libraries/LibUnicode/CharacterTypes.h

@@ -44,6 +44,7 @@ u32 to_unicode_titlecase(u32 code_point);
 ErrorOr<DeprecatedString> to_unicode_lowercase_full(StringView, Optional<StringView> const& locale = {});
 ErrorOr<DeprecatedString> to_unicode_uppercase_full(StringView, Optional<StringView> const& locale = {});
 ErrorOr<String> to_unicode_titlecase_full(StringView, Optional<StringView> const& locale = {});
+ErrorOr<String> to_unicode_casefold_full(StringView);
 
 Optional<GeneralCategory> general_category_from_string(StringView);
 bool code_point_has_general_category(u32 code_point, GeneralCategory general_category);

Userland/Libraries/LibUnicode/UnicodeUtils.cpp

@@ -195,6 +195,19 @@ static SpecialCasing const* find_matching_special_case(u32 code_point, Utf8View
     return nullptr;
 }
 
+template<CaseFoldingStatus... StatusFilter>
+static CaseFolding const* find_matching_case_folding(u32 code_point)
+{
+    auto case_foldings = case_folding_mapping(code_point);
+
+    for (auto const* case_folding : case_foldings) {
+        if (((case_folding->status == StatusFilter) || ...))
+            return case_folding;
+    }
+
+    return nullptr;
+}
+
 #endif
 
 // https://www.unicode.org/versions/Unicode15.0.0/ch03.pdf#G34078
@@ -314,4 +327,32 @@ ErrorOr<void> build_titlecase_string([[maybe_unused]] Utf8View code_points, [[ma
 #endif
 }
 
+// https://www.unicode.org/versions/Unicode15.0.0/ch03.pdf#G53253
+ErrorOr<void> build_casefold_string([[maybe_unused]] Utf8View code_points, [[maybe_unused]] StringBuilder& builder)
+{
+#if ENABLE_UNICODE_DATA
+    // toCasefold(X): Map each character C in X to Case_Folding(C).
+    //
+    // Case_Folding(C) uses the mappings with the status field value “C” or “F” in the data file
+    // CaseFolding.txt in the Unicode Character Database.
+
+    using enum CaseFoldingStatus;
+
+    for (auto code_point : code_points) {
+        auto const* case_folding = find_matching_case_folding<Common, Full>(code_point);
+        if (!case_folding) {
+            TRY(builder.try_append_code_point(code_point));
+            continue;
+        }
+
+        for (size_t i = 0; i < case_folding->mapping_size; ++i)
+            TRY(builder.try_append_code_point(case_folding->mapping[i]));
+    }
+
+    return {};
+#else
+    return Error::from_string_literal("Unicode data has been disabled");
+#endif
+}
+
 }

Userland/Libraries/LibUnicode/UnicodeUtils.h

@@ -17,5 +17,6 @@ namespace Unicode::Detail {
 ErrorOr<void> build_lowercase_string(Utf8View code_points, StringBuilder& builder, Optional<StringView> const& locale);
 ErrorOr<void> build_uppercase_string(Utf8View code_points, StringBuilder& builder, Optional<StringView> const& locale);
 ErrorOr<void> build_titlecase_string(Utf8View code_points, StringBuilder& builder, Optional<StringView> const& locale);
+ErrorOr<void> build_casefold_string(Utf8View code_points, StringBuilder& builder);
 
 }