/**
 * Compaction hook — saves a session summary to memory before context reset.
 *
 * ## When it fires
 *
 * `session_before_compact` — right before Pi compacts the context window.
 * This is triggered automatically when context exceeds the threshold, or
 * manually via `/compact`.
 *
 * ## What it does
 *
 * 1. Receives the compaction preparation data (messages to summarize, etc.)
 * 2. Saves a compact session state summary to memory
 * 3. This summary survives the compaction and is injected on next session start
 *
 * ## Why this matters
 *
 * Without this, compaction effectively "forgets" everything that happened
 * before the cut point. By persisting a summary to Knapsack memory, the
 * model retains awareness of:
 * - What was being worked on
 * - Key decisions made
 * - Files that were modified
 * - Open questions and next steps
 *
 * @module compaction
 */

import type { SessionBeforeCompactEvent } from "@earendil-works/pi-coding-agent";
import type { KnapsackDB } from "../core/database";
import type { KnapsackStore } from "../core/types";

/**
 * Handle the session_before_compact event — persist session state to memory.
 *
 * Creates a "decision"-type memory entry summarizing the session state
 * before compaction occurs.
 *
 * @param event - session_before_compact event from Pi
 * @param db - Knapsack database handle
 * @param store - Knapsack runtime store
 * @returns undefined (does not modify the compaction behavior)
 */
export function compactionHook(
	event: SessionBeforeCompactEvent,
	db: KnapsackDB,
	store: KnapsackStore,
): void {
	const { preparation, reason } = event;

	// Build a compact summary of what's being compacted
	const messageCount = preparation.messagesToSummarize?.length ?? 0;
	const tokenCount = preparation.tokensBefore ?? 0;

	const touched = extractTouchedFiles(preparation.messagesToSummarize);
	const filesLine = touched.length > 0 ? `Files touched: ${touched.slice(0, 8).join(", ")}` : null;

	const summary = [
		`Session compacted (${reason})`,
		`Messages summarized: ${messageCount}`,
		`Tokens before compaction: ${tokenCount}`,
		filesLine,
		preparation.previousSummary ? `Previous summary was carried forward` : null,
	]
		.filter(Boolean)
		.join(" · ");

	db.saveMemory({
		content: summary,
		type: "fact",
		scope: "session",
		importance: 0.6,
		sourceSession: store.sessionId ?? undefined,
	});
}

/**
 * Pull distinct file paths out of the messages being compacted so the
 * summary memory carries at least a hint of what the session touched,
 * not just token and message counts. Caps at a small N to avoid flooding
 * the memory row on a wide-ranging session.
 */
function extractTouchedFiles(messages: unknown): string[] {
	if (!Array.isArray(messages)) return [];
	const paths = new Set<string>();
	const pathRe = /(?:\.\/|[\w./-]+\.[a-z]{1,6})/gi;
	for (const msg of messages) {
		const text = typeof msg === "string" ? msg : JSON.stringify(msg);
		for (const m of text.matchAll(pathRe)) {
			const p = m[0];
			if (!p) continue;
			// Filter out obvious false positives (URLs, version strings).
			if (p.includes("://")) continue;
			if (/^\d+\./.test(p)) continue;
			if (p.length < 3 || p.length > 80) continue;
			paths.add(p);
			if (paths.size >= 16) return Array.from(paths);
		}
	}
	return Array.from(paths);
}