Forced moves: find the only way

Most of a Filoop level is solved without guessing. You look for a place where the thread has only one option, draw it, and look again.

What a forced move is

A forced move is a step you don’t have to think about: there is only one place the thread can go. Most levels open with a few of them, and every forced move you make creates new ones.

Look for two kinds:

A button with one free neighbour. Its thread has only one way out, so draw that first step.
An empty cell with exactly two free neighbours. A thread has to pass through it, so it comes in by one and leaves by the other. If those two neighbours are the two buttons of one colour, the cell belongs to that colour.

Two forced moves. The corner touches only the two blue buttons, so blue must go through it. The red button has buttons on three sides, so its only way out is down.

Both moves made. Each one changed the cells around it — that is where to look next.

Watch finished threads too. Once a thread connects its buttons, its cells are a wall for everyone else, and the empty cells along that wall often lose enough options to become forced.

Chains

Forced moves come in chains. When a thread takes a cell, the neighbours of that cell lose an option. A cell that had three free neighbours now has two — and may become forced itself. So after each forced move, check the cells it touched rather than the whole board.

A routine that works:

Scan the border and the buttons for single exits.
Make every forced move you find.
Re-check only the cells next to what you just drew.
When nothing is forced, look for cells that are about to become dead ends — see dead ends and pockets.

When the chain stops

On harder levels the chain breaks, and you face two real options. Don’t guess yet. Play each option in your head for a step or two: often one of them leaves a cell with no way out or cuts a pair of buttons apart. Then the other option is forced. This look-ahead is still a forced move — just a deeper one.

Pick the place for look-ahead carefully: a cell with two options rather than three, or a button with two exits rather than four. Fewer branches means a shorter check.

Filoop’s solver rates levels the same way: the more look-ahead a level needs, the harder it is rated.

Draw forced moves for real. Moves you are only testing — keep them in your head, or draw them and use Undo; it costs nothing.

Practise on the warm-up set — its levels are solved by forced moves alone. Then try today’s daily puzzle. Where to start looking: corners and edges.

Play Filoop Dead ends and pockets →