/* TuringDNA dwell-time heartbeat.
 *
 * Measures how long a user actually spends ON the engine (foreground time),
 * which the server-side run/event timestamps can't capture — they only mark
 * discrete actions, not the reading/thinking gaps between them.
 *
 * Posts { session_id, visible_ms } to /api/ping:
 *   - an initial beat ~3s after load (captures the user_id early, so even a
 *     short visit gets attributed before they leave),
 *   - then every ~30s WHILE the tab is visible (background tabs are skipped —
 *     we only count foreground time), and
 *   - once more on tab-hide / pagehide via navigator.sendBeacon (survives the
 *     tab going away, where a normal fetch would be cancelled).
 *
 * visible_ms is CUMULATIVE foreground time for the session; the server keeps the
 * max, so duplicate or out-of-order beats are harmless. auth.js has already
 * wrapped fetch to attach the JWT on /api/* calls, so periodic beats are
 * attributed to the signed-in user. The sendBeacon fallback is unauthenticated
 * (anonymous), but the server preserves an already-known user_id on that row.
 *
 * Privacy: sends only an opaque random session id + a millisecond duration.
 * No sequence content, no page paths, no identity — matches PRIVACY.md.
 */
(function () {
    'use strict';

    var PING_MS    = 30000;  // periodic cadence while visible
    var INITIAL_MS = 3000;   // first beat — early user_id capture for short visits

    function uuid() {
        try { if (window.crypto && crypto.randomUUID) return crypto.randomUUID(); } catch (e) {}
        return 'xxxxxxxxxxxx4xxxyxxxxxxxxxxxxxxx'.replace(/[xy]/g, function (c) {
            var r = Math.random() * 16 | 0, v = c === 'x' ? r : (r & 0x3 | 0x8);
            return v.toString(16);
        });
    }

    var sessionId = uuid();
    var visibleMs = 0;  // accumulated foreground time from FINISHED segments
    var segStart  = (document.visibilityState === 'visible') ? Date.now() : 0;

    function currentVisibleMs() {
        var ongoing = segStart ? (Date.now() - segStart) : 0;
        return Math.round(visibleMs + ongoing);
    }

    function send(useBeacon) {
        var body = JSON.stringify({ session_id: sessionId, visible_ms: currentVisibleMs() });
        if (useBeacon && navigator.sendBeacon) {
            try {
                navigator.sendBeacon('/api/ping', new Blob([body], { type: 'application/json' }));
                return;
            } catch (e) { /* fall through to fetch */ }
        }
        try {
            fetch('/api/ping', {
                method:  'POST',
                headers: { 'Content-Type': 'application/json' },
                body:    body,
                keepalive: true,
            }).catch(function () {});
        } catch (e) { /* ignore — telemetry is best-effort */ }
    }

    // Close the foreground segment on hide (and flush via beacon); reopen on show.
    document.addEventListener('visibilitychange', function () {
        if (document.visibilityState === 'hidden') {
            if (segStart) { visibleMs += Date.now() - segStart; segStart = 0; }
            send(true);
        } else if (document.visibilityState === 'visible') {
            if (!segStart) segStart = Date.now();
        }
    });

    // pagehide — last reliable chance to record the tail of the session
    // (beforeunload is unreliable on mobile; pagehide fires on bfcache too).
    window.addEventListener('pagehide', function () {
        if (segStart) { visibleMs += Date.now() - segStart; segStart = 0; }
        send(true);
    });

    // Initial beat, then steady cadence. setInterval is throttled in background
    // tabs — fine, since we gate on visibility and only want foreground time.
    setTimeout(function () { if (document.visibilityState === 'visible') send(false); }, INITIAL_MS);
    setInterval(function () { if (document.visibilityState === 'visible') send(false); }, PING_MS);
})();