Add ruleset interface for extending filter criteria

osu.Game/Rulesets/Filter/IRulesetFilterCriteria.cs

@@ -0,0 +1,44 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using osu.Game.Beatmaps;
+using osu.Game.Screens.Select;
+using osu.Game.Screens.Select.Filter;
+
+namespace osu.Game.Rulesets.Filter
+{
+    /// <summary>
+    /// Allows for extending the beatmap filtering capabilities of song select (as implemented in <see cref="FilterCriteria"/>)
+    /// with ruleset-specific criteria.
+    /// </summary>
+    public interface IRulesetFilterCriteria
+    {
+        /// <summary>
+        /// Checks whether the supplied <paramref name="beatmap"/> satisfies ruleset-specific custom criteria,
+        /// in addition to the ones mandated by song select.
+        /// </summary>
+        /// <param name="beatmap">The beatmap to test the criteria against.</param>
+        /// <returns>
+        /// <c>true</c> if the beatmap matches the ruleset-specific custom filtering criteria,
+        /// <c>false</c> otherwise.
+        /// </returns>
+        bool Matches(BeatmapInfo beatmap);
+
+        /// <summary>
+        /// Attempts to parse a single custom keyword criterion, given by the user via the song select search box.
+        /// The format of the criterion is:
+        /// <code>
+        /// {key}{op}{value}
+        /// </code>
+        /// </summary>
+        /// <param name="key">The key (name) of the criterion.</param>
+        /// <param name="op">The operator in the criterion.</param>
+        /// <param name="value">The value of the criterion.</param>
+        /// <returns>
+        /// <c>true</c> if the keyword criterion is valid, <c>false</c> if it has been ignored.
+        /// Valid criteria are stripped from <see cref="FilterCriteria.SearchText"/>,
+        /// while ignored criteria are included in <see cref="FilterCriteria.SearchText"/>.
+        /// </returns>
+        bool TryParseCustomKeywordCriteria(string key, Operator op, string value);
+    }
+}