Use new API endpoint to do batch lookups

osu.Game/Database/BeatmapLookupCache.cs

@@ -85,7 +85,7 @@ namespace osu.Game.Database
             // Grab at most 50 unique beatmap IDs from the queue.
             lock (taskAssignmentLock)
             {
-                while (pendingBeatmapTasks.Count > 0 && beatmapTasks.Count < 1)
+                while (pendingBeatmapTasks.Count > 0 && beatmapTasks.Count < 50)
                 {
                     (int id, TaskCompletionSource<APIBeatmap> task) next = pendingBeatmapTasks.Dequeue();
 
@@ -103,7 +103,7 @@ namespace osu.Game.Database
             }
 
             // Query the beatmaps.
-            var request = new GetBeatmapRequest(new APIBeatmap { OnlineID = beatmapTasks.Keys.First() });
+            var request = new GetBeatmapsRequest(beatmapTasks.Keys.ToArray());
 
             // rather than queueing, we maintain our own single-threaded request stream.
             // todo: we probably want retry logic here.
@@ -117,16 +117,19 @@ namespace osu.Game.Database
                     createNewTask();
             }
 
-            List<APIBeatmap> foundBeatmaps = new List<APIBeatmap> { request.Response };
+            List<APIBeatmap> foundBeatmaps = request.Response?.Beatmaps;
 
-            foreach (var beatmap in foundBeatmaps)
+            if (foundBeatmaps != null)
             {
-                if (beatmapTasks.TryGetValue(beatmap.OnlineID, out var tasks))
+                foreach (var beatmap in foundBeatmaps)
                 {
-                    foreach (var task in tasks)
+                    if (beatmapTasks.TryGetValue(beatmap.OnlineID, out var tasks))
-                        task.SetResult(beatmap);
+                    {
+                        foreach (var task in tasks)
+                            task.SetResult(beatmap);
 
-                    beatmapTasks.Remove(beatmap.OnlineID);
+                        beatmapTasks.Remove(beatmap.OnlineID);
+                    }
                 }
             }
 

osu.Game/Online/API/Requests/GetBeatmapsRequest.cs

@@ -0,0 +1,24 @@
+// 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 System;
+
+namespace osu.Game.Online.API.Requests
+{
+    public class GetBeatmapsRequest : APIRequest<GetBeatmapsResponse>
+    {
+        private readonly int[] beatmapIds;
+
+        private const int max_ids_per_request = 50;
+
+        public GetBeatmapsRequest(int[] beatmapIds)
+        {
+            if (beatmapIds.Length > max_ids_per_request)
+                throw new ArgumentException($"{nameof(GetBeatmapsRequest)} calls only support up to {max_ids_per_request} IDs at once");
+
+            this.beatmapIds = beatmapIds;
+        }
+
+        protected override string Target => "beatmaps/?ids[]=" + string.Join("&ids[]=", beatmapIds);
+    }
+}

osu.Game/Online/API/Requests/GetBeatmapsResponse.cs

@@ -0,0 +1,15 @@
+// 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 System.Collections.Generic;
+using Newtonsoft.Json;
+using osu.Game.Online.API.Requests.Responses;
+
+namespace osu.Game.Online.API.Requests
+{
+    public class GetBeatmapsResponse : ResponseWithCursor
+    {
+        [JsonProperty("beatmaps")]
+        public List<APIBeatmap> Beatmaps;
+    }
+}