LibUnicode: Parse and generate relative-time format patterns

Relative-time format patterns are of one of two forms:

    * Tensed - refer to the past or the future, e.g. "N years ago" or
      "in N years".
    * Numbered - refer to a specific numeric value, e.g. "in 1 year"
      becomes "next year" and "in 0 years" becomes "this year".

In ECMA-402, tensed and numbered refer to the numeric formatting options
of "always" and "auto", respectively.

Userland/Libraries/LibUnicode/CMakeLists.txt

@@ -13,6 +13,7 @@ set(SOURCES
     DateTimeFormat.cpp
     Locale.cpp
     NumberFormat.cpp
+    RelativeTimeFormat.cpp
 )
 
 serenity_lib(LibUnicode unicode)

Userland/Libraries/LibUnicode/RelativeTimeFormat.cpp

@@ -0,0 +1,58 @@
+/*
+ * Copyright (c) 2022, Tim Flynn <trflynn89@pm.me>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibUnicode/RelativeTimeFormat.h>
+
+namespace Unicode {
+
+Optional<TimeUnit> time_unit_from_string(StringView time_unit)
+{
+    if (time_unit == "second"sv)
+        return TimeUnit::Second;
+    if (time_unit == "minute"sv)
+        return TimeUnit::Minute;
+    if (time_unit == "hour"sv)
+        return TimeUnit::Hour;
+    if (time_unit == "day"sv)
+        return TimeUnit::Day;
+    if (time_unit == "week"sv)
+        return TimeUnit::Week;
+    if (time_unit == "month"sv)
+        return TimeUnit::Month;
+    if (time_unit == "quarter"sv)
+        return TimeUnit::Quarter;
+    if (time_unit == "year"sv)
+        return TimeUnit::Year;
+    return {};
+}
+
+StringView time_unit_to_string(TimeUnit time_unit)
+{
+    switch (time_unit) {
+    case TimeUnit::Second:
+        return "second"sv;
+    case TimeUnit::Minute:
+        return "minute"sv;
+    case TimeUnit::Hour:
+        return "hour"sv;
+    case TimeUnit::Day:
+        return "day"sv;
+    case TimeUnit::Week:
+        return "week"sv;
+    case TimeUnit::Month:
+        return "month"sv;
+    case TimeUnit::Quarter:
+        return "quarter"sv;
+    case TimeUnit::Year:
+        return "year"sv;
+    default:
+        VERIFY_NOT_REACHED();
+    }
+}
+
+Vector<RelativeTimeFormat> __attribute__((weak)) get_relative_time_format_patterns(StringView, TimeUnit, StringView, Style) { return {}; }
+
+}

Userland/Libraries/LibUnicode/RelativeTimeFormat.h

@@ -0,0 +1,48 @@
+/*
+ * Copyright (c) 2022, Tim Flynn <trflynn89@pm.me>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Optional.h>
+#include <AK/StringView.h>
+#include <AK/Vector.h>
+#include <LibUnicode/Forward.h>
+#include <LibUnicode/Locale.h>
+
+namespace Unicode {
+
+// These are just the subset of fields in the CLDR required for ECMA-402.
+enum class TimeUnit {
+    Second,
+    Minute,
+    Hour,
+    Day,
+    Week,
+    Month,
+    Quarter,
+    Year,
+};
+
+struct RelativeTimeFormat {
+    enum class Plurality {
+        Zero,
+        One,
+        Two,
+        Few,
+        Many,
+        Other,
+    };
+
+    Plurality plurality { Plurality::Other };
+    StringView pattern;
+};
+
+Optional<TimeUnit> time_unit_from_string(StringView time_unit);
+StringView time_unit_to_string(TimeUnit time_unit);
+
+Vector<RelativeTimeFormat> get_relative_time_format_patterns(StringView locale, TimeUnit time_unit, StringView tense_or_number, Style style);
+
+}