diff --git a/assets/js/offline-engine.js b/assets/js/offline-engine.js
new file mode 100644
index 0000000..cf9305c
--- /dev/null
+++ b/assets/js/offline-engine.js
@@ -0,0 +1,195 @@
+import { getDB } from './db.js';
+
+function getStore(storeName, mode = 'readonly') {
+
+    const db = getDB();
+
+    const tx = db.transaction(storeName, mode);
+
+    return tx.objectStore(storeName);
+}
+
+export async function saveWords(lang, list, words) {
+
+    const store = getStore('words', 'readwrite');
+
+    for (const word of words) {
+
+        store.put({
+            key: `${lang}:${list}:${word.id}`,
+            lang,
+            list,
+            ...word
+        });
+    }
+}
+
+export async function getWords(lang, list) {
+
+    return new Promise((resolve) => {
+
+        const store = getStore('words');
+
+        const req = store.getAll();
+
+        req.onsuccess = () => {
+
+            const result = req.result.filter(
+                w => w.lang === lang && w.list === list
+            );
+
+            resolve(result);
+        };
+    });
+}
+
+export async function getNextWord(lang, list) {
+
+    const words = await getWords(lang, list);
+
+    if (!words.length) {
+        return null;
+    }
+
+    const today = new Date().toISOString().split('T')[0];
+
+    let due = words.filter(word => {
+        return !word.nextReview ||
+            word.nextReview <= today;
+    });
+
+    if (!due.length) {
+        due = words;
+    }
+
+    const random =
+        Math.floor(Math.random() * due.length);
+
+    return due[random];
+}
+
+export async function answerWord(word, answer) {
+
+    const correct =
+        word.answer.trim().toLowerCase() ===
+        answer.trim().toLowerCase();
+
+    const store =
+        getStore('words', 'readwrite');
+
+    const updated = { ...word };
+
+    if (correct) {
+
+        const days =
+            updated.interval
+                ? updated.interval * 2
+                : 1;
+
+        updated.interval = days;
+
+        const d = new Date();
+        d.setDate(d.getDate() + days);
+
+        updated.nextReview =
+            d.toISOString().split('T')[0];
+
+    } else {
+
+        updated.interval = 1;
+
+        const d = new Date();
+        d.setDate(d.getDate() + 1);
+
+        updated.nextReview =
+            d.toISOString().split('T')[0];
+    }
+
+    store.put(updated);
+
+    await updateSessionStats(correct);
+
+    await queueSync({
+        wordId: word.id,
+        lang: word.lang,
+        list: word.list,
+        correct,
+        timestamp: Date.now()
+    });
+
+    return {
+        correct
+    };
+}
+
+async function queueSync(item) {
+
+    const store =
+        getStore('sync_queue', 'readwrite');
+
+    store.add(item);
+}
+
+export async function updateSessionStats(correct) {
+
+    const store =
+        getStore('session', 'readwrite');
+
+    const req =
+        store.get('stats');
+
+    req.onsuccess = () => {
+
+        let stats = req.result;
+
+        if (!stats) {
+            stats = {
+                key: 'stats',
+                correct: 0,
+                wrong: 0
+            };
+        }
+
+        if (correct) {
+            stats.correct++;
+        } else {
+            stats.wrong++;
+        }
+
+        store.put(stats);
+    };
+}
+
+export async function getSessionStats() {
+
+    return new Promise(resolve => {
+
+        const store =
+            getStore('session');
+
+        const req =
+            store.get('stats');
+
+        req.onsuccess = () => {
+
+            resolve(
+                req.result || {
+                    correct: 0,
+                    wrong: 0
+                }
+            );
+        };
+    });
+}
+
+export async function resetSessionStats() {
+
+    const store =
+        getStore('session', 'readwrite');
+
+    store.put({
+        key: 'stats',
+        correct: 0,
+        wrong: 0
+    });
+}
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