LocalHtmlDataTest/indexeddb/src/sync-engine.ts

import type { SyncEvent, IndexDefinition, OpenOptions } from './types.js';
import type { IDBStore } from './idb-store.js';
import type { FolderStore } from './folder-store.js';
import type { Emitter } from './emitter.js';
import { canonicalJson, sha256Hex, eventFilename } from './utils.js';

/**
 * Knows about every registered collection's index definitions so
 * it can maintain indexes when replaying events during sync.
 */
export class SyncEngine {
  private clientId: string;
  private conflictResolver?: OpenOptions['conflictResolver'];
  private collectionIndexes = new Map<string, IndexDefinition[]>();
  private syncing = false;

  constructor(
    private idb: IDBStore,
    private folder: FolderStore,
    private emitter: Emitter,
    clientId: string,
    conflictResolver?: OpenOptions['conflictResolver'],
  ) {
    this.clientId = clientId;
    this.conflictResolver = conflictResolver;
  }

  // ── Collection index registration ──────────────────────────

  registerIndexes(store: string, indexes: IndexDefinition[]): void {
    this.collectionIndexes.set(store, indexes);
  }

  getIndexes(store: string): IndexDefinition[] {
    return this.collectionIndexes.get(store) ?? [];
  }

  // ── Local writes (write to IDB + folder) ───────────────────

  async writeKV(
    key: string,
    value: unknown,
    ts: number,
    rev: number,
  ): Promise<void> {
    const event: SyncEvent = {
      type: 'put',
      store: 'kv',
      key,
      ts,
      clientId: this.clientId,
      data: value,
      rev,
    };
    await this.persistEvent(event);
  }

  async deleteKV(key: string, ts: number, rev: number): Promise<void> {
    const event: SyncEvent = {
      type: 'delete',
      store: 'kv',
      key,
      ts,
      clientId: this.clientId,
      rev,
    };
    await this.persistEvent(event);
  }

  async writeDoc(
    store: string,
    id: string,
    data: unknown,
    ts: number,
    rev: number,
  ): Promise<void> {
    const event: SyncEvent = {
      type: 'put',
      store,
      key: id,
      id,
      ts,
      clientId: this.clientId,
      data,
      rev,
    };
    await this.persistEvent(event);
  }

  async deleteDocEvent(
    store: string,
    id: string,
    ts: number,
    rev: number,
  ): Promise<void> {
    const event: SyncEvent = {
      type: 'delete',
      store,
      key: id,
      id,
      ts,
      clientId: this.clientId,
      rev,
    };
    await this.persistEvent(event);
  }

  // ── Core persist: IDB + folder ─────────────────────────────

  private async persistEvent(event: SyncEvent): Promise<void> {
    const canonical = canonicalJson(event);
    const hash = await sha256Hex(canonical);
    const filename = eventFilename(event.ts, hash);

    // Write to IDB first (fast path)
    await this.applyEvent(event);
    await this.idb.markEventApplied(filename);

    // Then persist to folder (if available)
    if (this.folder.hasHandle && (await this.folder.hasPermission())) {
      try {
        await this.folder.writeEvent(filename, event);
      } catch (err) {
        this.emitter.emit('folder:lost-permission', err);
      }
    }

    this.emitter.emit('change', {
      type: event.type,
      store: event.store,
      key: event.key,
      id: event.id,
      data: event.data,
    });
  }

  // ── Sync: import folder events into IDB ────────────────────

  async sync(): Promise<void> {
    if (this.syncing) return;
    if (!this.folder.hasHandle) return;

    const hasAccess = await this.folder.hasPermission();
    if (!hasAccess) {
      this.emitter.emit('folder:lost-permission');
      return;
    }

    this.syncing = true;
    this.emitter.emit('sync:start');

    try {
      const appliedSet = await this.idb.getAppliedSet();
      const filenames = await this.folder.scanEventFilenames();
      let importCount = 0;

      for (const name of filenames) {
        if (appliedSet.has(name)) continue;

        const event = await this.folder.readEventFile(name);
        if (!event) continue;

        const hadConflict = await this.applyEventWithConflictCheck(event);
        await this.idb.markEventApplied(name);
        importCount++;

        if (hadConflict) {
          this.emitter.emit('conflict', {
            filename: name,
            event,
          });
        }

        this.emitter.emit('change', {
          type: event.type,
          store: event.store,
          key: event.key,
          id: event.id,
          data: event.data,
          source: 'sync',
        });
      }

      this.emitter.emit('sync:end', { imported: importCount });
    } catch (err) {
      this.emitter.emit('sync:end', { error: err });
    } finally {
      this.syncing = false;
    }
  }

  // ── Apply a single event to IDB ────────────────────────────

  private async applyEvent(event: SyncEvent): Promise<void> {
    if (event.store === 'kv') {
      if (event.type === 'put') {
        await this.idb.putKV({
          key: event.key,
          value: event.data,
          ts: event.ts,
          rev: event.rev ?? 0,
        });
      } else {
        await this.idb.deleteKV(event.key);
      }
    } else {
      const indexes = this.getIndexes(event.store);
      if (event.type === 'put') {
        await this.idb.putDoc(
          {
            store: event.store,
            id: event.id ?? event.key,
            data: event.data,
            ts: event.ts,
            rev: event.rev ?? 0,
          },
          indexes,
        );
      } else {
        await this.idb.deleteDoc(
          event.store,
          event.id ?? event.key,
          event.ts,
          event.rev ?? 0,
          indexes,
        );
      }
    }
  }

  /**
   * Apply an incoming event, checking for conflicts using LWW.
   * Returns true if a conflict was detected (even if resolved).
   */
  private async applyEventWithConflictCheck(
    event: SyncEvent,
  ): Promise<boolean> {
    let hadConflict = false;

    if (event.store === 'kv') {
      const existing = await this.idb.getKV(event.key);
      if (existing) {
        if (event.ts > existing.ts) {
          // incoming wins
          hadConflict = true;
        } else if (event.ts === existing.ts) {
          // tie-break: use custom resolver or skip
          hadConflict = true;
          if (this.conflictResolver) {
            const resolved = this.conflictResolver(existing.value, event.data);
            event = { ...event, data: resolved };
          }
          // With equal ts, still apply (deterministic: both sides converge)
        } else {
          // existing is newer — skip
          return true; // was a conflict but existing wins
        }
      }
    } else {
      const existing = await this.idb.getRawDoc(
        event.store,
        event.id ?? event.key,
      );
      if (existing && !existing.deleted) {
        if (event.ts > existing.ts) {
          hadConflict = true;
        } else if (event.ts === existing.ts) {
          hadConflict = true;
          if (this.conflictResolver) {
            const resolved = this.conflictResolver(existing.data, event.data);
            event = { ...event, data: resolved };
          }
        } else {
          return true;
        }
      }
    }

    await this.applyEvent(event);
    return hadConflict;
  }
}