## ✅ Overview of New Features:

### 1. ✅ **Multi-tab sync**

* Already discussed; based on `storage` events.

### 2. ✅ **`_rev` field** (like CouchDB/PouchDB)

* Every document has a `_rev` field like `"1-uuid"`, `"2-uuid"`, etc.
* On `update`, we compare the provided `_rev` against the current one:

  * If it's **older or equal** → **conflict**.
  * If **newer** → update is accepted.
* New `_rev` generated with incremented prefix and new UUID suffix.

### 3. ✅ **Optional Revision Log**

* Config option: `enableRevisions: true`
* If enabled, we store prior versions (and conflicts) in-memory in a `_revisions` map.

---

## 📦 Final Version of `RxCollection`

```js
import { BehaviorSubject } from 'rxjs';
import { map } from 'rxjs/operators';

function generateUUID() {
  return crypto.randomUUID();
}

function nextRevision(currentRev = '0-') {
  const [revNum] = currentRev.split('-');
  const nextNum = parseInt(revNum, 10) + 1;
  return `${nextNum}-${generateUUID()}`;
}

class RxCollection {
  constructor(name, schema, options = {}) {
    this.name = name;
    this.schema = schema;
    this.options = options;
    this._docs$ = new BehaviorSubject([]);
    this._idToStorageKey = new Map();
    this._revisions = new Map(); // optional revision log
    this._loadAllDocs();
    window.addEventListener('storage', this._handleStorageChange.bind(this));
  }

  _buildStorageKey(doc) {
    return `${this.name}:${doc.name}:${doc._id}`;
  }

  _extractIdFromKey(key) {
    const parts = key.split(':');
    return parts[2]; // uuid
  }

  _loadAllDocs() {
    const allKeys = Object.keys(localStorage);
    const docs = [];

    for (const key of allKeys) {
      if (key.startsWith(`${this.name}:`)) {
        const doc = JSON.parse(localStorage.getItem(key));
        docs.push(doc);
        this._idToStorageKey.set(doc._id, key);
      }
    }

    this._docs$.next(docs);
  }

  _handleStorageChange(event) {
    if (!event.key || !event.key.startsWith(`${this.name}:`)) return;

    const id = this._extractIdFromKey(event.key);
    const docs = this._docs$.getValue();
    const index = docs.findIndex(d => d._id === id);

    if (event.newValue === null) {
      if (index !== -1) {
        docs.splice(index, 1);
        this._idToStorageKey.delete(id);
        this._docs$.next([...docs]);
      }
    } else {
      const doc = JSON.parse(event.newValue);
      this._idToStorageKey.set(id, event.key);
      if (index !== -1) docs[index] = doc;
      else docs.push(doc);
      this._docs$.next([...docs]);
    }
  }

  async insert(doc) {
    const _id = generateUUID();
    const _rev = nextRevision();
    const newDoc = { _id, _rev, ...doc };
    const key = this._buildStorageKey(newDoc);

    localStorage.setItem(key, JSON.stringify(newDoc));
    this._idToStorageKey.set(_id, key);
    const docs = this._docs$.getValue();
    this._docs$.next([...docs, newDoc]);
    return newDoc;
  }

  async has(id) {
    return this._idToStorageKey.has(id);
  }

  async delete(id) {
    const key = this._idToStorageKey.get(id);
    if (!key) return false;

    localStorage.removeItem(key);
    this._idToStorageKey.delete(id);

    const docs = this._docs$.getValue().filter(d => d._id !== id);
    this._docs$.next(docs);
    return true;
  }

  async update(id, partial) {
    const key = this._idToStorageKey.get(id);
    if (!key) return null;

    const currentDoc = JSON.parse(localStorage.getItem(key));
    const incomingRev = partial._rev || '0-';
    const [incomingRevNum] = incomingRev.split('-');
    const [currentRevNum] = currentDoc._rev.split('-');

    if (parseInt(incomingRevNum, 10) <= parseInt(currentRevNum, 10)) {
      // Conflict
      if (this.options.enableRevisions) {
        if (!this._revisions.has(id)) this._revisions.set(id, []);
        this._revisions.get(id).push({ conflict: true, doc: partial });
      }
      return null;
    }

    const updated = {
      ...currentDoc,
      ...partial,
      _rev: nextRevision(currentDoc._rev)
    };

    // If name changes, we must update storage key
    let newKey = key;
    if (partial.name && partial.name !== currentDoc.name) {
      newKey = this._buildStorageKey(updated);
      localStorage.removeItem(key);
      this._idToStorageKey.set(id, newKey);
    }

    if (this.options.enableRevisions) {
      if (!this._revisions.has(id)) this._revisions.set(id, []);
      this._revisions.get(id).push({ ...currentDoc });
    }

    localStorage.setItem(newKey, JSON.stringify(updated));
    const docs = this._docs$.getValue().map(d => (d._id === id ? updated : d));
    this._docs$.next(docs);
    return updated;
  }

  find({ selector }) {
    const filterFn = doc =>
      Object.entries(selector).every(([key, condition]) => {
        if ('$gt' in condition) return doc[key] > condition['$gt'];
        if ('$lt' in condition) return doc[key] < condition['$lt'];
        return doc[key] === condition;
      });

    return {
      exec: () => Promise.resolve(this._docs$.getValue().filter(filterFn)),
      $: this._docs$.asObservable().pipe(map(docs => docs.filter(filterFn)))
    };
  }

  getRevisionHistory(id) {
    return this._revisions.get(id) || [];
  }
}
```

---

### 📦 Updated `RxDatabase`

```js
class RxDatabase {
  constructor(name) {
    this.name = name;
    this.collections = {};
  }

  async addCollections(collectionsObj) {
    for (const [name, config] of Object.entries(collectionsObj)) {
      const { schema, options } = config;
      this.collections[name] = new RxCollection(name, schema, options);
    }
  }
}
```

---

### 🧪 Usage

```js
const db = await createRxDatabase({ name: 'heroesdb' });

await db.addCollections({
  heroes: {
    schema: {
      properties: { name: { type: 'string' }, healthpoints: { type: 'number' } },
      required: ['name', 'healthpoints']
    },
    options: { enableRevisions: true }
  }
});

// Insert
const hero = await db.heroes.insert({ name: 'alice', healthpoints: 100 });

// Update with correct _rev
const updated = await db.heroes.update(hero._id, {
  healthpoints: 90,
  _rev: hero._rev
});

// Conflict (same _rev)
const conflict = await db.heroes.update(hero._id, {
  healthpoints: 85,
  _rev: hero._rev
});
console.log('Conflict?', conflict === null); // true

// View history
const history = db.heroes.getRevisionHistory(hero._id);
console.log('Revision history:', history);
```

---

### ✅ Summary

| Feature              | Supported?    |
| -------------------- | ------------- |
| Per-doc localStorage | ✅            |
| UUID-based `_id`     | ✅            |
| `_rev` tracking      | ✅            |
| Revision conflict    | ✅            |
| Revision log         | ✅ (optional) |
| Multi-tab sync       | ✅            |