SlitherlinkFree, no sign-up

Draw lines along the edges between the dots to form a single closed loop. The loop may not cross itself or branch. A cell with a number has exactly that many of its four sides on the loop; cells without numbers can have any number of sides used.

Tap or click an edge (the gap between two dots) to cycle: line → × (a note that no line goes there) → empty. The × marks are just notes for you and aren't part of the answer check. Undo takes back your last change. With a keyboard, move the cursor with the arrow keys and press Space or Enter.

Start by marking × on all four sides of every 0. Two 3s side by side always use the edge between them and their two outer edges. A 3 in a corner uses both corner edges, and a 1 in a corner uses neither. Every dot has either 0 or 2 lines, so a dead end must always continue somewhere.

Yes. Each puzzle is checked by a program to have exactly one solution before it's used, and only the numbers needed to pin down the loop are kept.

By the time from your first move to completing the loop, fastest first. You only need a nickname, up to 30 submissions per size a day. The finished board is checked against the rules on the server.