Shuffle external mode: Optimization: Don't rewrite unchanged part

run/john.conf

@@ -3971,7 +3971,7 @@ void next()
 
 # Shuffle (try all permutations of) characters in each short input word
 [List.External:Shuffle]
-int max_length, unique, length, base[0x80], id[0x80], jd[0x80];
+int max_length, unique, length, base[0x80], id[0x80], jd[0x80], unseen[0x80];
 
 void init()
 {
@@ -3998,11 +3998,13 @@ void new()
 			}
 		}
 	}
+
+	unseen = 0;
 }
 
 void next()
 {
-	int unseen, c, i, j, k;
+	int prev_unseen, c, i, j, k;
 
 	if ((i = length) <= max_length)
 	while (i--) {
@@ -4011,17 +4013,21 @@ void next()
 			continue;
 		}
 
-		unseen = i = -1;
+		prev_unseen = -1;
+		if (!unseen)
+			unseen = i = -1;
+		else if (i--)
+			prev_unseen = unseen[i];
 		while (++i < length) {
 			/* Skip id[i] previously unseen char indices */
 			j = -1;
 			k = id[i];
 			while (k >= 0)
-				if (unseen & (1 << ++j))
+				if (prev_unseen & (1 << ++j))
 					k--;
 			if (unique) {
 				word[i] = base[j];
-				unseen &= ~(1 << j);
+				unseen[i] = prev_unseen &= ~(1 << j);
 				continue;
 			}
 			/* Same chars must only appear in one order */
@@ -4036,7 +4042,7 @@ void next()
 			if (k < i)
 				break;
 			word[i] = c;
-			unseen &= ~(1 << (jd[i] = j));
+			unseen[i] = prev_unseen &= ~(1 << (jd[i] = j));
 		}
 
 		if (i >= length) /* Not a dupe */