Implement pagination in HashSearcher (#160)

- Update DecomposedQuery to include Limit and Offset
- Update HashSearcher.Search to apply Skip/Take logic
- Ensure stable sorting by ID when paginating
- Enable and implement skipped test in HashSearcherTests
- Add new test for Offset logic

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: google-labs-jules[bot]

Octans.Core/Querying/HashSearcher.cs

@@ -32,18 +32,49 @@ public async Task<HashSet<HashItem>> Search(DecomposedQuery request, Cancellatio
     {
         if (request.IsEmpty() || request.SystemPredicates.OfType<EverythingPredicate>().Any())
         {
-            var allHashes = await context.Hashes.ToListAsync(cancellationToken);
+            var query = context.Hashes.AsQueryable();
+
+            if (request.Offset > 0)
+            {
+                query = query.OrderBy(h => h.Id).Skip(request.Offset);
+            }
+
+            if (request.Limit.HasValue)
+            {
+                if (request.Offset == 0)
+                {
+                    query = query.OrderBy(h => h.Id);
+                }
+                query = query.Take(request.Limit.Value);
+            }
+
+            var allHashes = await query.ToListAsync(cancellationToken);
             return allHashes.ToHashSet();
         }
 
         var matchingIds = await GetMatchingTagIds(request, cancellationToken);
 
-        var hashes = await context.Mappings
+        var hashesQuery = context.Mappings
             .Where(m => matchingIds.Contains(m.Tag.Id))
             .Include(m => m.Hash)
             .Select(m => m.Hash)
-            .Distinct()
-            .ToListAsync(cancellationToken);
+            .Distinct();
+
+        if (request.Offset > 0)
+        {
+            hashesQuery = hashesQuery.OrderBy(h => h.Id).Skip(request.Offset);
+        }
+
+        if (request.Limit.HasValue)
+        {
+            if (request.Offset == 0)
+            {
+                hashesQuery = hashesQuery.OrderBy(h => h.Id);
+            }
+            hashesQuery = hashesQuery.Take(request.Limit.Value);
+        }
+
+        var hashes = await hashesQuery.ToListAsync(cancellationToken);
 
         return hashes.ToHashSet();
     }

Octans.Core/Querying/QueryTagConverter.cs

@@ -92,6 +92,9 @@ public class DecomposedQuery
     public HashSet<string> WildcardDoublesToInclude { get; init; } = [];
     public HashSet<string> WildcardDoublesToExclude { get; init; } = [];
 
+    public int? Limit { get; init; }
+    public int Offset { get; init; }
+
     public bool IsEmpty()
     {
         var hasContent = TagsToInclude.Any() || WildcardNamespacesToInclude.Any() || WildcardSubtagsToInclude.Any() ||

Octans.Tests/Querying/HashSearcherTests.cs

@@ -148,10 +148,39 @@ public async Task CountAsync_ReturnsAll_WhenEmptyQuery()
         count.Should().Be(total);
     }
 
-    [Fact(Skip = "Not implemented yet")]
-    public void ReturnsOnlyNHashes_WhenALimitOfNIsSpecified()
+    [Fact]
+    public async Task ReturnsOnlyNHashes_WhenALimitOfNIsSpecified()
+    {
+        await SeedData();
+        var request = new DecomposedQuery
+        {
+            Limit = 2
+        };
+
+        var results = await _sut.Search(request);
+
+        results.Should().HaveCount(2);
+    }
+
+    [Fact]
+    public async Task SkipsNHashes_WhenAnOffsetOfNIsSpecified()
     {
-        throw new NotImplementedException();
+        await SeedData();
+
+        // Get all items ordered by ID to simulate the default sort order in Search
+        var allItems = await _db.Hashes.OrderBy(h => h.Id).ToListAsync();
+
+        var expected = allItems.Skip(2).Take(1).Single();
+
+        var request = new DecomposedQuery
+        {
+            Offset = 2,
+            Limit = 1
+        };
+
+        var results = await _sut.Search(request);
+
+        results.Single().Id.Should().Be(expected.Id);
     }
 
     private async Task SeedData()

TASKS.md

@@ -25,7 +25,7 @@ Network.
     - **Task:** Implement the actual logic to fetch data from different downloaders (Galleries/Queries) periodically.
 
 - [ ] **Search & Querying**
-    - **Task:** Implement search limits/pagination in `HashSearcher` (currently skipped test in `HashSearcherTests`).
+    - [x] **Task:** Implement search limits/pagination in `HashSearcher` (currently skipped test in `HashSearcherTests`).
     - **Task:** Implement parsing for other `system:` predicates in `QueryParser` (e.g. `system:inbox`, `system:archive`, etc.). Only `system:everything` is currently supported.
 
 ## 2. User Interface (Client)