LibUnicode: Implement grammar validators for Unicode TR-35

ECMA-402 requires validating user input against the EBNF grammar for
Unicode locales described in TR-35: https://www.unicode.org/reports/tr35

This commit adds validators for that grammar, as well as other helper to
e.g. canonicalize a locale string.

Userland/Libraries/LibUnicode/Locale.cpp

@@ -0,0 +1,152 @@
+/*
+ * Copyright (c) 2021, Tim Flynn <trflynn89@pm.me>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/AllOf.h>
+#include <AK/CharacterTypes.h>
+#include <AK/GenericLexer.h>
+#include <AK/QuickSort.h>
+#include <AK/StringBuilder.h>
+#include <LibUnicode/Locale.h>
+
+namespace Unicode {
+
+bool is_unicode_language_subtag(StringView subtag)
+{
+    // unicode_language_subtag = alpha{2,3} | alpha{5,8}
+    if ((subtag.length() < 2) || (subtag.length() == 4) || (subtag.length() > 8))
+        return false;
+    return all_of(subtag, is_ascii_alpha);
+}
+
+bool is_unicode_script_subtag(StringView subtag)
+{
+    // unicode_script_subtag = alpha{4}
+    if (subtag.length() != 4)
+        return false;
+    return all_of(subtag, is_ascii_alpha);
+}
+
+bool is_unicode_region_subtag(StringView subtag)
+{
+    // unicode_region_subtag = (alpha{2} | digit{3})
+    if (subtag.length() == 2)
+        return all_of(subtag, is_ascii_alpha);
+    if (subtag.length() == 3)
+        return all_of(subtag, is_ascii_digit);
+    return false;
+}
+
+bool is_unicode_variant_subtag(StringView subtag)
+{
+    // unicode_variant_subtag = (alphanum{5,8} | digit alphanum{3})
+    if ((subtag.length() >= 5) && (subtag.length() <= 8))
+        return all_of(subtag, is_ascii_alphanumeric);
+    if (subtag.length() == 4)
+        return is_ascii_digit(subtag[0]) && all_of(subtag.substring_view(1), is_ascii_alphanumeric);
+    return false;
+}
+
+Optional<LanguageID> parse_unicode_language_id(StringView language)
+{
+    // https://unicode.org/reports/tr35/#Unicode_language_identifier
+    //
+    // unicode_language_id = "root"
+    //     OR
+    // unicode_language_id = ((unicode_language_subtag (sep unicode_script_subtag)?) | unicode_script_subtag)
+    //                       (sep unicode_region_subtag)?
+    //                       (sep unicode_variant_subtag)*
+    LanguageID language_id {};
+
+    if (language == "root"sv) {
+        language_id.is_root = true;
+        return language_id;
+    }
+
+    auto segments = language.split_view_if(is_any_of("-_"sv), true); // keep_empty=true to ensure valid data follows a separator.
+    size_t index = 0;
+
+    if (segments.size() == index)
+        return {};
+
+    if (is_unicode_language_subtag(segments[index])) {
+        language_id.language = segments[index];
+        if (segments.size() == ++index)
+            return language_id;
+    }
+
+    if (is_unicode_script_subtag(segments[index])) {
+        language_id.script = segments[index];
+        if (segments.size() == ++index)
+            return language_id;
+    } else if (!language_id.language.has_value()) {
+        return {};
+    }
+
+    if (is_unicode_region_subtag(segments[index])) {
+        language_id.region = segments[index];
+        if (segments.size() == ++index)
+            return language_id;
+    }
+
+    while (index < segments.size()) {
+        if (!is_unicode_variant_subtag(segments[index]))
+            return {};
+        language_id.variants.append(segments[index++]);
+    }
+
+    return language_id;
+}
+
+Optional<LocaleID> parse_unicode_locale_id(StringView locale)
+{
+    LocaleID locale_id {};
+
+    // https://unicode.org/reports/tr35/#Unicode_locale_identifier
+    //
+    // unicode_locale_id = unicode_language_id
+    //                     extensions*
+    //                     pu_extensions?
+    auto language_id = parse_unicode_language_id(locale);
+    if (!language_id.has_value())
+        return {};
+
+    // FIXME: Handle extensions and pu_extensions.
+    return LocaleID { language_id.release_value() };
+}
+
+Optional<String> canonicalize_unicode_locale_id(LocaleID& locale_id)
+{
+    // https://unicode.org/reports/tr35/#Canonical_Unicode_Locale_Identifiers
+    StringBuilder builder;
+
+    if (!locale_id.language_id.language.has_value())
+        return {};
+
+    builder.append(locale_id.language_id.language->to_lowercase_string());
+
+    if (locale_id.language_id.script.has_value()) {
+        builder.append('-');
+        builder.append(locale_id.language_id.script->to_titlecase_string());
+    }
+
+    if (locale_id.language_id.region.has_value()) {
+        builder.append('-');
+        builder.append(locale_id.language_id.region->to_uppercase_string());
+    }
+
+    quick_sort(locale_id.language_id.variants);
+
+    for (auto const& variant : locale_id.language_id.variants) {
+        builder.append('-');
+        builder.append(variant.to_lowercase_string());
+    }
+
+    // FIXME: Handle extensions and pu_extensions.
+
+    return builder.build();
+}
+
+}