initial commit

ContextCompactor.cs

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+using Microsoft.Extensions.AI;
+using System.Text.RegularExpressions;
+using Spectre.Console;
+
+namespace AnchorCli;
+
+/// <summary>
+/// Compacts the chat history when context usage grows too large.
+/// Asks the same model to summarize the conversation, then replaces
+/// the history with [system prompt, summary, last N user/assistant turns].
+/// </summary>
+internal sealed partial class ContextCompactor(IChatClient client)
+{
+    /// <summary>Number of recent user+assistant turn pairs to keep verbatim.</summary>
+    private const int KeepRecentTurns = 2;
+
+    /// <summary>Minimum result length to consider for compaction.</summary>
+    private const int MinResultLength = 300;
+
+    /// <summary>Matches hashline-encoded output: "lineNumber:hash|content"</summary>
+    private static readonly Regex HashlinePattern =
+        MyRegex();
+
+    private readonly IChatClient _client = client;
+
+    /// <summary>
+    /// Compacts large hashline-encoded tool results from previous turns.
+    /// This is the cheapest and most impactful optimization — no LLM call needed.
+    /// A 300-line ReadFile result (~10K tokens) becomes a one-line note (~20 tokens).
+    /// </summary>
+    /// <param name="history">The chat history to compact in-place.</param>
+    /// <param name="currentTurnStartIndex">
+    /// Index of the first message added during the current turn.
+    /// Messages before this index are from previous turns and eligible for compaction.
+    /// </param>
+    /// <returns>Number of tool results that were compacted.</returns>
+    public static int CompactStaleToolResults(List<ChatMessage> history, int currentTurnStartIndex)
+    {
+        int compacted = 0;
+
+        for (int i = 0; i < currentTurnStartIndex && i < history.Count; i++)
+        {
+            var msg = history[i];
+
+            foreach (var content in msg.Contents)
+            {
+                if (content is FunctionResultContent frc &&
+                    frc.Result is string resultStr &&
+                    resultStr.Length >= MinResultLength &&
+                    HashlinePattern.IsMatch(resultStr))
+                {
+                    // Count lines and replace with compact summary
+                    int lineCount = resultStr.Count(c => c == '\n');
+                    frc.Result = $"[File content: {lineCount} lines — already consumed. Re-read the file if you need fresh anchors.]";
+                    compacted++;
+                }
+            }
+        }
+
+        return compacted;
+    }
+
+    /// <summary>
+    /// Compacts the history in-place via LLM summarization. Returns true if compaction was performed.
+    /// </summary>
+    public async Task<bool> TryCompactAsync(
+        List<ChatMessage> history,
+        CancellationToken ct = default)
+    {
+        // Need at least: system + some conversation to compact
+        if (history.Count < 5)
+            return false;
+
+        // Split: system prompt (index 0) | middle (compactable) | tail (keep verbatim)
+        var systemMsg = history[0];
+
+        // Find the cut point: keep the last N user+assistant pairs
+        int keepFromIndex = FindKeepIndex(history);
+        if (keepFromIndex <= 1)
+            return false; // Nothing to compact
+
+        // Extract the middle section to summarize
+        var toSummarize = history.Skip(1).Take(keepFromIndex - 1).ToList();
+        var tail = history.Skip(keepFromIndex).ToList();
+
+        // Build a summarization prompt
+        var summaryMessages = new List<ChatMessage>
+        {
+            new(ChatRole.System, """
+                You are a conversation summarizer. Summarize the following coding conversation
+                between a user and an AI assistant. Focus on:
+                - What files were read, created, or modified (and their paths)
+                - What changes were made and why
+                - Any decisions or preferences expressed by the user
+                - Current state of the work (what's done, what's pending)
+                Be concise but preserve all actionable context. Output a single summary paragraph.
+                Do NOT include any hashline anchors (lineNumber:hash|) in your summary.
+            """)
+        };
+        summaryMessages.AddRange(toSummarize);
+        summaryMessages.Add(new(ChatRole.User,
+            "Summarize the above conversation concisely, preserving all important context for continuing the work."));
+
+        string summary;
+        try
+        {
+            var response = await _client.GetResponseAsync(summaryMessages, cancellationToken: ct);
+            summary = response.Text ?? "(summary unavailable)";
+        }
+        catch
+        {
+            // If summarization fails, don't compact
+            return false;
+        }
+
+        // Rebuild history: system + summary + recent turns
+        history.Clear();
+        history.Add(systemMsg);
+        history.Add(new(ChatRole.User,
+            "[CONTEXT COMPACTED — The following is a summary of the earlier conversation]\n" + summary));
+        history.Add(new(ChatRole.Assistant,
+            "Understood, I have the context from our earlier conversation. I'll continue from where we left off."));
+        history.AddRange(tail);
+
+        return true;
+    }
+
+    /// <summary>
+    /// Finds the index from which to keep messages verbatim (the last N turn pairs).
+    /// </summary>
+    private static int FindKeepIndex(List<ChatMessage> history)
+    {
+        int pairsFound = 0;
+        int idx = history.Count - 1;
+
+        while (idx > 0 && pairsFound < KeepRecentTurns)
+        {
+            // Walk backwards looking for user+assistant pairs
+            if (history[idx].Role == ChatRole.Assistant && idx > 1 &&
+                history[idx - 1].Role == ChatRole.User)
+            {
+                pairsFound++;
+                idx -= 2;
+            }
+            else
+            {
+                idx--;
+            }
+        }
+
+        // idx+1 is the first message to keep
+        return Math.Max(1, idx + 1);
+    }
+
+    [GeneratedRegex(@"^\d+:[0-9a-fA-F]{2}\|", RegexOptions.Multiline | RegexOptions.Compiled)]
+    private static partial Regex MyRegex();
+}