#第 07 章:上下文压缩(compaction)

const KEEP_RECENT = 4;

function estimateTokens(systemPrompt: string, messages: Message[]): number {
  let chars = systemPrompt.length;
  for (const m of messages) {
    chars += m.content?.length ?? 0;
    for (const tc of m.tool_calls ?? []) chars += tc.function.arguments.length;
  }
  return Math.ceil(chars / 4);
}

const COMPACT_PROMPT = `You are compacting the conversation history of a coding agent. Summarize the conversation so far, preserving everything the agent needs to continue: the user's goals, decisions made, files read or modified (with paths), commands run and their outcomes, the current state, and agreed next steps. Be concise but complete. Write in the same language as the conversation.`;

async function maybeCompact(systemPrompt: string, session: Session, messages: Message[]): Promise<void> {
  if (estimateTokens(systemPrompt, messages) < CONFIG.compactTokens) return;

  // 切分点向前对齐到一条 user 消息,保证保留部分以 user 开头(协议要求:
  // tool 消息必须紧跟它的 assistant tool_calls,不能孤悬在切分边界上)
  let cut = Math.max(1, messages.length - KEEP_RECENT);
  while (cut < messages.length && messages[cut].role !== "user") cut++;
  if (cut <= 0 || cut >= messages.length - 1) return;

  const oldMessages = messages.slice(0, cut);
  const kept = messages.slice(cut);
  const summary = await chatOnce([
    { role: "system", content: COMPACT_PROMPT },
    { role: "user", content: JSON.stringify(oldMessages, null, 2) },
  ]);

  const firstKeptId = kept[0] ? session.entryIds.get(kept[0]) : undefined;
  session.append({ type: "compaction", summary, compactedCount: oldMessages.length, firstKeptId });

  messages.length = 0;
  messages.push({ role: "user", content: `[Earlier conversation summary]\n${summary}` }, ...kept);
  console.log(
    `\x1b[2m[compacted ${oldMessages.length} older messages; full history stays in the session file]\x1b[0m`,
  );
}