diff --git a/includes/srs.php b/includes/srs.php
new file mode 100644
index 0000000..71a0ced
--- /dev/null
+++ b/includes/srs.php
@@ -0,0 +1,34 @@
+<?php
+
+declare(strict_types=1);
+
+function calculateNextInterval(array $word, bool $correct): int
+{
+    $correctCount = $word['correct'] ?? 0;
+    $wrongCount = $word['wrong'] ?? 0;
+
+    if (!$correct) {
+        return 1;
+    }
+
+    // simpele exponential backoff
+    $base = max(1, $correctCount);
+
+    return min(30, (int) pow(2, $base));
+}
+
+function updateWordStats(array &$word, bool $correct): void
+{
+    if ($correct) {
+        $word['correct']++;
+    } else {
+        $word['wrong']++;
+    }
+
+    $days = calculateNextInterval($word, $correct);
+
+    $word['nextReview'] = date(
+        'Y-m-d',
+        strtotime("+{$days} days")
+    );
+}
\ No newline at end of file