Both tools rescue a stuck board, and they do it in opposite ways. Undo rewinds your decisions; shuffle rearranges the tiles. Knowing which fits the situation saves boards.
How shuffle works here
Shuffle takes every remaining tile and redeals it into the remaining positions using the same construction as the original deal — pairs laid into legal free positions in reverse. The result is guaranteed to be solvable from where you stand.
That guarantee matters. A naive shuffle that simply randomises tile positions can produce an unsolvable arrangement, which some implementations do. This one cannot.
How undo works
Undo puts back the last pair you removed, and it has no limit. You can rewind the entire board pair by pair if you want, which means any decision point is recoverable.
Which to use
The rule of thumb: if you know roughly where you went wrong, undo. If you have no idea, shuffle.
- Undo when you recognise the mistake — you split a set of four, or spent a protruding tile, and you can see the moment it happened.
- Undo when you are within ten or fifteen pairs of the error. Rewinding that far is quick.
- Shuffle when the board died gradually and there is no single bad move to rewind to.
- Shuffle when the error was fifty moves ago and rewinding would mean replaying most of the board.
Undo preserves what you learned
After rewinding, you know which route fails. A shuffle throws away your entire reading of the board and you start analysing from scratch.
Shuffles are limited for a reason
Three per board. Unlimited shuffling would make the game trivial — you could simply shuffle every time you stalled and clear any board without thinking. The limit is what makes the sequencing decisions matter.
The practical consequence: treat shuffles as genuinely scarce. Reaching the last twenty tiles with two shuffles unused is a comfortable position; reaching it with none is precarious.
Avoiding the need for either
Both tools are recoveries from a mistake that has already happened. The prevention is watching the pairs counter. When it starts falling steadily rather than fluctuating, the board is tightening, and that is the point to start checking what each removal unlocks.
A worked decision
You are down to eighteen tiles, no pairs available, and you remember taking two 5-of-circles about eight moves ago while the other two were buried. That is a clear undo situation: rewind eight pairs, take a different route through that region, and the shuffle stays in reserve.
If instead you are down to eighteen tiles and cannot identify any specific error, shuffle. Rewinding blind is slower than accepting a fresh arrangement.