Add ability to search for difficulty names using square brackets

osu.Game.Tests/NonVisual/Filtering/FilterQueryParserTest.cs

@@ -2,10 +2,12 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System;
+using System.Linq;
 using NUnit.Framework;
 using osu.Game.Beatmaps;
 using osu.Game.Rulesets.Filter;
 using osu.Game.Screens.Select;
+using osu.Game.Screens.Select.Carousel;
 using osu.Game.Screens.Select.Filter;
 
 namespace osu.Game.Tests.NonVisual.Filtering
@@ -382,6 +384,57 @@ public void TestCustomKeywordIsParsed()
             Assert.AreEqual("unrecognised=keyword", filterCriteria.SearchText.Trim());
         }
 
+        [TestCase("[1]", new[] { 0 })]
+        [TestCase("[1", new[] { 0 })]
+        [TestCase("My[Favourite", new[] { 2 })]
+        [TestCase("My[Favourite]", new[] { 2 })]
+        [TestCase("My[Favourite]Song", new[] { 2 })]
+        [TestCase("Favourite]", new[] { 2 })]
+        [TestCase("[Diff", new[] { 0, 1, 3, 4, 6 })]
+        [TestCase("[Diff]", new[] { 0, 1, 3, 4, 6 })]
+        [TestCase("[Favourite]", new[] { 3 })]
+        [TestCase("Title1 [Diff]", new[] { 0, 1 })]
+        [TestCase("Title1[Diff]", new int[] { })]
+        [TestCase("[diff ]with]", new[] { 4 })]
+        [TestCase("[diff ]with [[ brackets]]]]", new[] { 4 })]
+        [TestCase("[Diff in title]", new int[] { })]
+        [TestCase("[Diff in diff]", new[] { 6 })]
+        [TestCase("diff=Diff", new[] { 0, 1, 3, 4, 6 })]
+        [TestCase("diff=Diff1", new[] { 0 })]
+        [TestCase("diff=\"Diff\"", new[] { 3, 4, 6 })]
+        [TestCase("diff=!\"Diff\"", new int[] { })]
+        public void TestDifficultySearch(string query, int[] expectedBeatmapIndexes)
+        {
+            var carouselBeatmaps = (((string title, string difficultyName)[])new[]
+            {
+                ("Title1", "Diff1"),
+                ("Title1", "Diff2"),
+                ("My[Favourite]Song", "Expert"),
+                ("Title", "My Favourite Diff"),
+                ("Another One", "diff ]with [[ brackets]]]"),
+                ("Diff in title", "a"),
+                ("a", "Diff in diff"),
+            }).Select(info => new CarouselBeatmap(new BeatmapInfo
+            {
+                Metadata = new BeatmapMetadata
+                {
+                    Title = info.title
+                },
+                DifficultyName = info.difficultyName
+            })).ToList();
+
+            var criteria = new FilterCriteria();
+
+            FilterQueryParser.ApplyQueries(criteria, query);
+            carouselBeatmaps.ForEach(b => b.Filter(criteria));
+
+            int[] visibleBeatmaps = carouselBeatmaps
+                                    .Where(b => !b.Filtered.Value)
+                                    .Select(b => carouselBeatmaps.IndexOf(b)).ToArray();
+
+            Assert.That(visibleBeatmaps, Is.EqualTo(expectedBeatmapIndexes));
+        }
+
         private class CustomFilterCriteria : IRulesetFilterCriteria
         {
             public string? CustomValue { get; set; }

osu.Game/Screens/Select/Carousel/CarouselBeatmap.cs

@@ -59,7 +59,7 @@ private bool checkMatch(FilterCriteria criteria)
                      criteria.Artist.Matches(BeatmapInfo.Metadata.ArtistUnicode);
             match &= !criteria.Title.HasFilter || criteria.Title.Matches(BeatmapInfo.Metadata.Title) ||
                      criteria.Title.Matches(BeatmapInfo.Metadata.TitleUnicode);
-
+            match &= !criteria.DifficultyName.HasFilter || criteria.DifficultyName.Matches(BeatmapInfo.DifficultyName);
             match &= !criteria.UserStarDifficulty.HasFilter || criteria.UserStarDifficulty.IsInRange(BeatmapInfo.StarRating);
 
             if (!match) return false;

osu.Game/Screens/Select/FilterCriteria.cs

@@ -38,6 +38,7 @@ public class FilterCriteria
         public OptionalTextFilter Creator;
         public OptionalTextFilter Artist;
         public OptionalTextFilter Title;
+        public OptionalTextFilter DifficultyName;
 
         public OptionalRange<double> UserStarDifficulty = new OptionalRange<double>
         {
@@ -68,8 +69,23 @@ public string SearchText
 
                 string remainingText = value;
 
+                // Match either an open difficulty tag to the end of string,
+                // or match a closed one with a whitespace after it.
+                //
+                // To keep things simple, the closing ']' may be included in the match group,
+                // and is trimmed post-match.
+                foreach (Match quotedSegment in Regex.Matches(value, "(^|\\s)\\[(.*)(\\]\\s|$)"))
+                {
+                    DifficultyName = new OptionalTextFilter
+                    {
+                        SearchTerm = quotedSegment.Groups[2].Value.Trim(']')
+                    };
+
+                    remainingText = remainingText.Replace(quotedSegment.Value, string.Empty);
+                }
+
                 // First handle quoted segments to ensure we keep inline spaces in exact matches.
-                foreach (Match quotedSegment in Regex.Matches(searchText, "(\"[^\"]+\"[!]?)"))
+                foreach (Match quotedSegment in Regex.Matches(value, "(\"[^\"]+\"[!]?)"))
                 {
                     terms.Add(new OptionalTextFilter { SearchTerm = quotedSegment.Value });
                     remainingText = remainingText.Replace(quotedSegment.Value, string.Empty);

osu.Game/Screens/Select/FilterQueryParser.cs

@@ -76,6 +76,9 @@ private static bool tryParseKeywordCriteria(FilterCriteria criteria, string key,
                 case "title":
                     return TryUpdateCriteriaText(ref criteria.Title, op, value);
 
+                case "diff":
+                    return TryUpdateCriteriaText(ref criteria.DifficultyName, op, value);
+
                 default:
                     return criteria.RulesetCriteria?.TryParseCustomKeywordCriteria(key, op, value) ?? false;
             }