feat(compaction): add staged helpers

src/agents/compaction.test.ts

@@ -0,0 +1,70 @@
+import type { AgentMessage } from "@mariozechner/pi-agent-core";
+import { describe, expect, it } from "vitest";
+
+import {
+  estimateMessagesTokens,
+  pruneHistoryForContextShare,
+  splitMessagesByTokenShare,
+} from "./compaction.js";
+
+function makeMessage(id: number, size: number): AgentMessage {
+  return {
+    role: "user",
+    content: "x".repeat(size),
+    timestamp: id,
+  };
+}
+
+describe("splitMessagesByTokenShare", () => {
+  it("splits messages into two non-empty parts", () => {
+    const messages: AgentMessage[] = [
+      makeMessage(1, 4000),
+      makeMessage(2, 4000),
+      makeMessage(3, 4000),
+      makeMessage(4, 4000),
+    ];
+
+    const parts = splitMessagesByTokenShare(messages, 2);
+    expect(parts.length).toBeGreaterThanOrEqual(2);
+    expect(parts[0]?.length).toBeGreaterThan(0);
+    expect(parts[1]?.length).toBeGreaterThan(0);
+    expect(parts.flat().length).toBe(messages.length);
+  });
+});
+
+describe("pruneHistoryForContextShare", () => {
+  it("drops older chunks until the history budget is met", () => {
+    const messages: AgentMessage[] = [
+      makeMessage(1, 4000),
+      makeMessage(2, 4000),
+      makeMessage(3, 4000),
+      makeMessage(4, 4000),
+    ];
+    const maxContextTokens = 2000; // budget is 1000 tokens (50%)
+    const pruned = pruneHistoryForContextShare({
+      messages,
+      maxContextTokens,
+      maxHistoryShare: 0.5,
+      parts: 2,
+    });
+
+    expect(pruned.droppedChunks).toBeGreaterThan(0);
+    expect(pruned.keptTokens).toBeLessThanOrEqual(Math.floor(maxContextTokens * 0.5));
+    expect(pruned.messages.length).toBeGreaterThan(0);
+  });
+
+  it("keeps history when already within budget", () => {
+    const messages: AgentMessage[] = [makeMessage(1, 1000)];
+    const maxContextTokens = 2000;
+    const pruned = pruneHistoryForContextShare({
+      messages,
+      maxContextTokens,
+      maxHistoryShare: 0.5,
+      parts: 2,
+    });
+
+    expect(pruned.droppedChunks).toBe(0);
+    expect(pruned.messages.length).toBe(messages.length);
+    expect(pruned.keptTokens).toBe(estimateMessagesTokens(messages));
+  });
+});