Avoid state sharing

README.md

@@ -95,6 +95,10 @@ This library is also used in production by [KnowledgePicker](https://knowledgepi
 
 As mentioned [above](#how-to-use), only subset of functionality is implemented now, but all contributions are welcome. Feel free to open [issues](https://github.com/knowledgepicker/word-cloud/issues) and [pull requests](https://github.com/knowledgepicker/word-cloud/pulls).
 
+### Testing
+
+Tests are currently only supported on Linux, because they are snapshot tests (generating a word cloud image and comparing it byte-by-byte with a snapshot) and more work is needed to ensure this is cross-platform (e.g., use exactly the same font). On Windows, tests can be run in WSL (Visual Studio supports this directly). Tests are also automatically run in GitHub Actions.
+
 ### Release process
 
 After pushing a tag, GitHub workflow `release.yml` is triggered which builds and publishes the NuGet package.

src/KnowledgePicker.WordCloud/Drawing/IGraphicEngine.cs

@@ -35,5 +35,7 @@ public interface IGraphicEngine : IDisposable
     public interface IGraphicEngine<TBitmap> : IGraphicEngine
     {
         TBitmap Bitmap { get; }
+
+        IGraphicEngine<TBitmap> Clone();
     }
 }

src/KnowledgePicker.WordCloud/Drawing/SkGraphicEngine.cs

@@ -13,6 +13,16 @@ public sealed class SkGraphicEngine : IGraphicEngine<SKBitmap>
         private readonly SKPaint textPaint;
         private readonly WordCloudInput wordCloud;
 
+        private SkGraphicEngine(ISizer sizer, WordCloudInput wordCloud,
+            SKPaint textPaint)
+        {
+            Sizer = sizer;
+            this.wordCloud = wordCloud;
+            this.textPaint = textPaint;
+            Bitmap = new SKBitmap(wordCloud.Width, wordCloud.Height);
+            canvas = new SKCanvas(Bitmap);
+        }
+
         public SkGraphicEngine(ISizer sizer, WordCloudInput wordCloud,
             SKTypeface? font = null, bool antialias = true)
         {
@@ -52,6 +62,11 @@ public void Draw(PointD location, RectangleD measured, string text, int count, s
                 (float)(location.Y - measured.Top), textPaint);
         }
 
+        public IGraphicEngine<SKBitmap> Clone()
+        {
+            return new SkGraphicEngine(Sizer, wordCloud, textPaint);
+        }
+
         public void Dispose()
         {
             textPaint.Dispose();

src/KnowledgePicker.WordCloud/Layouts/BaseLayout.cs

@@ -39,6 +39,8 @@ public IEnumerable<LayoutItem> GetWordsInArea(RectangleD area)
             return QuadTree.Query(area);
         }
 
+        public abstract ILayout Clone();
+
         protected bool IsInsideSurface(RectangleD targetRectangle)
         {
             return IsInside(Surface, targetRectangle);

src/KnowledgePicker.WordCloud/Layouts/ILayout.cs

@@ -11,5 +11,6 @@ public interface ILayout
     {
         int Arrange(IEnumerable<WordCloudEntry> entries, IGraphicEngine engine);
         IEnumerable<LayoutItem> GetWordsInArea(RectangleD area);
+        ILayout Clone();
     }
 }

src/KnowledgePicker.WordCloud/Layouts/SpiralLayout.cs

@@ -40,6 +40,11 @@ public override bool TryFindFreeRectangle(SizeD size, out RectangleD foundRectan
             return false;
         }
 
+        public override ILayout Clone()
+        {
+            return new SpiralLayout(WordCloud);
+        }
+
         private static double GetPseudoRandomStartAngle(SizeD size)
         {
             return size.Height * size.Width;

src/KnowledgePicker.WordCloud/Primitives/LayoutItem.cs

@@ -3,7 +3,7 @@ namespace KnowledgePicker.WordCloud.Primitives
     /// <summary>
     /// Word arranged somewhere in word cloud.
     /// </summary>
-    public class LayoutItem
+    public record LayoutItem
     {
         public LayoutItem(WordCloudEntry entry, PointD location, RectangleD measured)
         {

src/KnowledgePicker.WordCloud/WordCloudGenerator.cs

@@ -41,13 +41,19 @@ public WordCloudGenerator(WordCloudInput wordCloud,
         private T Process<T>(
             Func<IGraphicEngine<TBitmap>, IEnumerable<LayoutItem>, T> handler)
         {
+            // Ensure state is not shared.
+            // TODO: We should instead use factory pattern.
+            // But that would be a big change in usage of this class.
+            var localEngine = engine.Clone();
+            var localLayout = layout.Clone();
+
             // Arrange word cloud.
             var size = new SizeD(wordCloud.Width, wordCloud.Height);
-            layout.Arrange(wordCloud.Entries, engine);
+            localLayout.Arrange(wordCloud.Entries, localEngine);
 
             // Process results.
             var area = new RectangleD(new PointD(0, 0), size);
-            return handler(engine, layout.GetWordsInArea(area));
+            return handler(localEngine, localLayout.GetWordsInArea(area));
         }
 
         public IEnumerable<(LayoutItem Item, double FontSize)> Arrange()

test/KnowledgePicker.WordCloud.Tests/WordCloudGeneratorTests.cs

@@ -0,0 +1,39 @@
+using KnowledgePicker.WordCloud.Drawing;
+using KnowledgePicker.WordCloud.Layouts;
+using KnowledgePicker.WordCloud.Primitives;
+using KnowledgePicker.WordCloud.Sizers;
+using SkiaSharp;
+
+namespace KnowledgePicker.WordCloud.Tests;
+
+public class WordCloudGeneratorTests
+{
+    [Fact] // https://github.com/knowledgepicker/word-cloud/issues/17
+    public void DoesNotShareState()
+    {
+        // Arrange.
+        var wordCloud = new WordCloudInput(new[]
+        {
+            new WordCloudEntry("a", 1),
+            new WordCloudEntry("b", 1),
+        })
+        {
+            Width = 1024,
+            Height = 256,
+            MinFontSize = 8,
+            MaxFontSize = 32
+        };
+        var sizer = new LogSizer(wordCloud);
+        using var engine = new SkGraphicEngine(sizer, wordCloud);
+        var layout = new SpiralLayout(wordCloud);
+        var wcg = new WordCloudGenerator<SKBitmap>(wordCloud, engine, layout);
+
+        // Act.
+        var result1 = wcg.Arrange().ToArray();
+        var result2 = wcg.Arrange().ToArray();
+
+        // Assert.
+        Assert.Equal(result1.AsEnumerable(), result2);
+        Assert.Equal(2, result2.Length);
+    }
+}