/**
 * KeepItGoing - Offline sync replay
 * Talks to /api/sync/ to pull server changes into IndexedDB and push any
 * locally-queued offline edits back up. Only ever runs from a real,
 * network-loaded page (never from the cached offline app).
 */

const BACKGROUND_SYNC_MIN_INTERVAL_MS = 2 * 60 * 1000;

function getCsrfToken() {
    const el = document.querySelector('[name=csrfmiddlewaretoken]');
    return el ? el.value : null;
}

function stripLocalFields(task) {
    const { _dirty, _pending_conflict_id, ...clean } = task;
    return clean;
}

async function runBackgroundSync() {
    const csrfToken = getCsrfToken();
    if (!csrfToken) {
        return; // Not on an authenticated page (e.g. login/register).
    }

    const deviceId = await ensureDeviceId();
    const lastSyncToken = await getMeta('last_sync_token');
    const dirtyTasks = await getDirtyTasks();

    let response;
    try {
        response = await fetch('/api/sync/', {
            method: 'POST',
            headers: {
                'Content-Type': 'application/json',
                'X-CSRFToken': csrfToken,
            },
            body: JSON.stringify({
                device_id: deviceId,
                last_sync_token: lastSyncToken || null,
                changes: {
                    tasks: dirtyTasks.map(stripLocalFields),
                    tags: [],
                    time_entries: [],
                },
            }),
        });
    } catch (err) {
        return; // Offline or network error - retry next time, nothing to clean up.
    }

    if (!response.ok) {
        return; // Includes 429 throttled - retry next time.
    }

    const data = await response.json();

    const conflictedSyncIds = new Set(
        (data.conflicts || [])
            .map((c) => c.local_data && c.local_data.sync_id)
            .filter(Boolean)
    );

    // Merge server changes first, skipping anything about to be re-asserted
    // by a conflict resolution below (otherwise this would clobber it).
    for (const serverTask of data.server_changes.tasks) {
        if (conflictedSyncIds.has(serverTask.sync_id)) {
            continue;
        }
        await putLocalTask({ ...serverTask, _dirty: false, _pending_conflict_id: null });
    }

    // Clear dirty flags for rows that synced cleanly.
    for (const task of dirtyTasks) {
        if (!conflictedSyncIds.has(task.sync_id)) {
            await putLocalTask({ ...task, _dirty: false, _pending_conflict_id: null });
        }
    }

    // Auto-resolve conflicts as "local wins" - this is a low-frequency-offline
    // client, so conflicts should be rare; a resolution UI is future work.
    for (const conflict of data.conflicts || []) {
        const syncId = conflict.local_data && conflict.local_data.sync_id;
        if (!syncId) {
            continue;
        }
        try {
            const resolveResponse = await fetch(`/api/sync/conflicts/${conflict.id}/resolve/`, {
                method: 'POST',
                headers: {
                    'Content-Type': 'application/json',
                    'X-CSRFToken': csrfToken,
                },
                body: JSON.stringify({ resolution: 'local' }),
            });
            if (!resolveResponse.ok) {
                throw new Error('resolve failed');
            }
            const localTask = dirtyTasks.find((t) => t.sync_id === syncId);
            if (localTask) {
                await putLocalTask({ ...localTask, _dirty: false, _pending_conflict_id: null });
            }
        } catch (err) {
            const localTask = dirtyTasks.find((t) => t.sync_id === syncId);
            if (localTask) {
                await putLocalTask({ ...localTask, _dirty: true, _pending_conflict_id: conflict.id });
            }
        }
    }

    await setMeta('last_sync_token', data.sync_token);
    await setMeta('last_sync_at', new Date().toISOString());
}

async function maybeBackgroundSync() {
    const lastSyncAt = await getMeta('last_sync_at');
    if (lastSyncAt && Date.now() - new Date(lastSyncAt).getTime() < BACKGROUND_SYNC_MIN_INTERVAL_MS) {
        return;
    }
    runBackgroundSync();
}