.NET Compaction - Add AsChatReducer() extension to expose CompactionStrategy as IChatReducer

dotnet/src/Microsoft.Agents.AI/Compaction/ChatStrategyExtensions.cs

@@ -0,0 +1,59 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Extensions.AI;
+using Microsoft.Shared.DiagnosticIds;
+using Microsoft.Shared.Diagnostics;
+
+namespace Microsoft.Agents.AI.Compaction;
+
+/// <summary>
+/// Provides extension methods for <see cref="CompactionStrategy"/>.
+/// </summary>
+[Experimental(DiagnosticIds.Experiments.AgentsAIExperiments)]
+public static class ChatStrategyExtensions
+{
+    /// <summary>
+    /// Returns an <see cref="IChatReducer"/> that applies this <see cref="CompactionStrategy"/> to reduce a list of messages.
+    /// </summary>
+    /// <param name="strategy">The compaction strategy to wrap as an <see cref="IChatReducer"/>.</param>
+    /// <returns>
+    /// An <see cref="IChatReducer"/> that, on each call to <see cref="IChatReducer.ReduceAsync"/>, builds a
+    /// <see cref="CompactionMessageIndex"/> from the supplied messages and applies the strategy's compaction logic,
+    /// returning the resulting included messages.
+    /// </returns>
+    /// <remarks>
+    /// This allows any <see cref="CompactionStrategy"/> to be used wherever an <see cref="IChatReducer"/> is expected,
+    /// bridging the compaction pipeline into systems bound to the <c>Microsoft.Extensions.AI</c> <see cref="IChatReducer"/> contract.
+    /// </remarks>
+    public static IChatReducer AsChatReducer(this CompactionStrategy strategy)
+    {
+        Throw.IfNull(strategy);
+
+        return new CompactionStrategyChatReducer(strategy);
+    }
+
+    /// <summary>
+    /// An <see cref="IChatReducer"/> adapter that delegates to a <see cref="CompactionStrategy"/>.
+    /// </summary>
+    private sealed class CompactionStrategyChatReducer : IChatReducer
+    {
+        private readonly CompactionStrategy _strategy;
+
+        public CompactionStrategyChatReducer(CompactionStrategy strategy)
+        {
+            this._strategy = strategy;
+        }
+
+        /// <inheritdoc/>
+        public async Task<IEnumerable<ChatMessage>> ReduceAsync(IEnumerable<ChatMessage> messages, CancellationToken cancellationToken = default)
+        {
+            CompactionMessageIndex index = CompactionMessageIndex.Create([.. messages]);
+            await this._strategy.CompactAsync(index, cancellationToken: cancellationToken).ConfigureAwait(false);
+            return index.GetIncludedMessages();
+        }
+    }
+}

dotnet/tests/Microsoft.Agents.AI.UnitTests/Compaction/ChatStrategyExtensionsTests.cs

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+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Agents.AI.Compaction;
+using Microsoft.Extensions.AI;
+
+namespace Microsoft.Agents.AI.UnitTests.Compaction;
+
+/// <summary>
+/// Contains tests for the <see cref="ChatStrategyExtensions"/> class.
+/// </summary>
+public class ChatStrategyExtensionsTests
+{
+    [Fact]
+    public void AsChatReducerNullStrategyThrows()
+    {
+        // Act & Assert
+        Assert.Throws<ArgumentNullException>(() => ((CompactionStrategy)null!).AsChatReducer());
+    }
+
+    [Fact]
+    public void AsChatReducerReturnsIChatReducer()
+    {
+        // Arrange
+        ChatReducerCompactionStrategy strategy = new(new IdentityReducer(), CompactionTriggers.Always);
+
+        // Act
+        IChatReducer reducer = strategy.AsChatReducer();
+
+        // Assert
+        Assert.NotNull(reducer);
+    }
+
+    [Fact]
+    public async Task ReduceAsyncReturnsAllMessagesWhenStrategyDoesNotCompactAsync()
+    {
+        // Arrange — trigger never fires, so no compaction occurs
+        ChatReducerCompactionStrategy strategy = new(new IdentityReducer(), CompactionTriggers.Never);
+        IChatReducer reducer = strategy.AsChatReducer();
+
+        List<ChatMessage> messages =
+        [
+            new(ChatRole.User, "Hello"),
+            new(ChatRole.Assistant, "Hi!"),
+        ];
+
+        // Act
+        IEnumerable<ChatMessage> result = await reducer.ReduceAsync(messages, CancellationToken.None);
+
+        // Assert
+        Assert.Equal(messages, result);
+    }
+
+    [Fact]
+    public async Task ReduceAsyncCompactsMessagesWhenStrategyFiresAsync()
+    {
+        // Arrange — reducer keeps only the last message
+        ChatReducerCompactionStrategy strategy = new(
+            new TakeLastReducer(1),
+            CompactionTriggers.Always);
+        IChatReducer reducer = strategy.AsChatReducer();
+
+        List<ChatMessage> messages =
+        [
+            new(ChatRole.User, "First"),
+            new(ChatRole.Assistant, "Response 1"),
+            new(ChatRole.User, "Second"),
+        ];
+
+        // Act
+        IEnumerable<ChatMessage> result = await reducer.ReduceAsync(messages, CancellationToken.None);
+
+        // Assert
+        List<ChatMessage> resultList = [.. result];
+        Assert.Single(resultList);
+        Assert.Equal("Second", resultList[0].Text);
+    }
+
+    [Fact]
+    public async Task ReduceAsyncPassesCancellationTokenToStrategyAsync()
+    {
+        // Arrange
+        using CancellationTokenSource cts = new();
+        CancellationToken capturedToken = default;
+
+        CapturingReducer capturingReducer = new(token => capturedToken = token);
+        ChatReducerCompactionStrategy strategy = new(capturingReducer, CompactionTriggers.Always);
+        IChatReducer reducer = strategy.AsChatReducer();
+
+        List<ChatMessage> messages =
+        [
+            new(ChatRole.User, "Hello"),
+            new(ChatRole.User, "World"),
+        ];
+
+        // Act
+        await reducer.ReduceAsync(messages, cts.Token);
+
+        // Assert
+        Assert.Equal(cts.Token, capturedToken);
+    }
+
+    [Fact]
+    public async Task ReduceAsyncEmptyMessagesReturnsEmptyAsync()
+    {
+        // Arrange
+        ChatReducerCompactionStrategy strategy = new(new IdentityReducer(), CompactionTriggers.Always);
+        IChatReducer reducer = strategy.AsChatReducer();
+
+        // Act
+        IEnumerable<ChatMessage> result = await reducer.ReduceAsync([], CancellationToken.None);
+
+        // Assert
+        Assert.Empty(result);
+    }
+
+    /// <summary>
+    /// An <see cref="IChatReducer"/> that returns messages unchanged.
+    /// </summary>
+    private sealed class IdentityReducer : IChatReducer
+    {
+        public Task<IEnumerable<ChatMessage>> ReduceAsync(IEnumerable<ChatMessage> messages, CancellationToken cancellationToken = default)
+            => Task.FromResult(messages);
+    }
+
+    /// <summary>
+    /// An <see cref="IChatReducer"/> that keeps only the last <c>n</c> messages.
+    /// </summary>
+    private sealed class TakeLastReducer : IChatReducer
+    {
+        private readonly int _count;
+
+        public TakeLastReducer(int count) => this._count = count;
+
+        public Task<IEnumerable<ChatMessage>> ReduceAsync(IEnumerable<ChatMessage> messages, CancellationToken cancellationToken = default)
+            => Task.FromResult(messages.Reverse().Take(this._count));
+    }
+
+    /// <summary>
+    /// An <see cref="IChatReducer"/> that captures the <see cref="CancellationToken"/> passed to <see cref="ReduceAsync"/>.
+    /// </summary>
+    private sealed class CapturingReducer : IChatReducer
+    {
+        private readonly Action<CancellationToken> _capture;
+
+        public CapturingReducer(Action<CancellationToken> capture) => this._capture = capture;
+
+        public Task<IEnumerable<ChatMessage>> ReduceAsync(IEnumerable<ChatMessage> messages, CancellationToken cancellationToken = default)
+        {
+            this._capture(cancellationToken);
+            IEnumerable<ChatMessage> reducedMessages = [messages.Reverse().First()];
+            return Task.FromResult(reducedMessages);
+        }
+    }
+}