Dead ends and pockets
Most mistakes in Filoop are not wrong threads but empty cells that nobody can reach any more. Learn to see them the moment they appear.
Two ways out for every cell
When a level is solved, every cell is covered. A cell in the middle of a thread touches two cells of its own colour — the one before it and the one after it. So an empty cell that a thread still has to pass through needs at least two ways out.
Count free neighbours, and count the open end of an unfinished thread next to the cell as one more way in. If an empty cell is down to one, a thread could enter it but never leave. That is a dead end, and the move that made it has to be undone.
Pockets
A pocket is a closed group of empty cells, walled in by threads, buttons and the edge of the board. A pocket is fine only if some thread can fill it. A thread can get in through a button on its border or through the open end of an unfinished thread. And a thread that gets in must also get out — unless its own second button is inside.
If no thread can get in, the pocket stays empty, and the level can’t be finished.
A one-cell pocket is the strictest case. A thread can’t end on an empty cell, so it has to pass straight through: an open end or button on one side and the same colour on the other. If the cell is surrounded by finished threads, it is lost.
Pockets tend to appear in two places: in a corner, when a thread turns away from the edge one cell too early, and between two long threads running side by side.
Don’t split a pair
The same check works for buttons. If a thread puts two buttons of one colour on different sides of a wall, that colour can never connect. Before you draw a long straight line through the middle, look at what ends up on each side of it — more on that in long threads and short threads.
A quick check after every long move
- Look at the cells next to the thread you just drew.
- An empty cell with only one way in? Undo.
- A closed area? Make sure some thread can get in — and out.
- A pair split in two? Undo.
Drag a thread over another one and the other thread is cut; pull back and it returns. Undo goes back step by step. Testing an idea costs nothing.
Pockets are most dangerous on boards with few long threads — try the set with 3–5 threads. The basics are in corners and edges.