/**
 * 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(record) {
    const { _dirty, _pending_conflict_id, ...clean } = record;
    return clean;
}

// server_changes.time_entries[] uses a field named "task" holding the task's
// sync_id, but *creating* a new entry requires "task_sync_id" instead
// (sync/views.py process_time_entry_changes). Normalize to task_sync_id
// locally so the rest of the app only ever deals with one field name.
function timeEntryToWire(entry) {
    return stripLocalFields(entry);
}

function timeEntryFromWire(serverEntry) {
    const { task, ...rest } = serverEntry;
    // duration_seconds isn't part of the sync wire format at all (a
    // pre-existing gap in TimeEntrySyncSerializer) - compute it locally so
    // the offline app's time totals are correct regardless of where an
    // entry came from.
    const durationSeconds = rest.ended_at
        ? Math.round((new Date(rest.ended_at) - new Date(rest.started_at)) / 1000)
        : null;
    return { ...rest, task_sync_id: task, duration_seconds: durationSeconds };
}

// Shared merge-then-auto-resolve logic for one entity type's slice of a
// sync response: merge server rows (skipping anything about to be
// re-asserted by a conflict resolution), clear dirty flags for rows that
// synced cleanly, then resolve each conflict as "local wins".
async function mergeAndResolveEntity({ entityType, dirtyLocal, serverRows, conflicts, csrfToken, putLocal, fromWire }) {
    const normalize = fromWire || ((r) => r);
    const relevantConflicts = conflicts.filter((c) => c.entity_type === entityType);
    const conflictedIds = new Set(
        relevantConflicts.map((c) => c.local_data && c.local_data.sync_id).filter(Boolean)
    );

    for (const serverRow of serverRows) {
        const normalized = normalize(serverRow);
        if (conflictedIds.has(normalized.sync_id)) {
            continue;
        }
        await putLocal({ ...normalized, _dirty: false, _pending_conflict_id: null });
    }

    for (const row of dirtyLocal) {
        if (!conflictedIds.has(row.sync_id)) {
            await putLocal({ ...row, _dirty: false, _pending_conflict_id: null });
        }
    }

    for (const conflict of relevantConflicts) {
        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 localRow = dirtyLocal.find((r) => r.sync_id === syncId);
            if (localRow) {
                await putLocal({ ...localRow, _dirty: false, _pending_conflict_id: null });
            }
        } catch (err) {
            const localRow = dirtyLocal.find((r) => r.sync_id === syncId);
            if (localRow) {
                await putLocal({ ...localRow, _dirty: true, _pending_conflict_id: conflict.id });
            }
        }
    }
}

// Returns true if locally-queued offline changes were successfully pushed up
// this call - meaning the current (server-rendered) page was rendered before
// those changes existed and is now stale, so the caller should refresh it.
// Returns false if there was nothing to push, or the sync didn't complete.
async function runBackgroundSync() {
    const csrfToken = getCsrfToken();
    if (!csrfToken) {
        return false; // Not on an authenticated page (e.g. login/register).
    }

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

    const dirtyTasks = await getDirtyTasks();
    const dirtyTags = await getDirtyTags();
    const dirtyTimeEntries = await getDirtyTimeEntries();
    const hadPendingChanges = dirtyTasks.length + dirtyTags.length + dirtyTimeEntries.length > 0;

    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: dirtyTags.map(stripLocalFields),
                    time_entries: dirtyTimeEntries.map(timeEntryToWire),
                },
            }),
        });
    } catch (err) {
        return false; // Offline or network error - retry next time, nothing to clean up.
    }

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

    const data = await response.json();
    const conflicts = data.conflicts || [];

    await mergeAndResolveEntity({
        entityType: 'task', dirtyLocal: dirtyTasks, serverRows: data.server_changes.tasks,
        conflicts, csrfToken, putLocal: putLocalTask,
    });
    await mergeAndResolveEntity({
        entityType: 'tag', dirtyLocal: dirtyTags, serverRows: data.server_changes.tags,
        conflicts, csrfToken, putLocal: putLocalTag,
    });
    await mergeAndResolveEntity({
        entityType: 'time_entry', dirtyLocal: dirtyTimeEntries, serverRows: data.server_changes.time_entries,
        conflicts, csrfToken, putLocal: putLocalTimeEntry, fromWire: timeEntryFromWire,
    });

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

    return hadPendingChanges;
}

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