Building 120 puzzles that can all be solved

BlockGrid's Puzzles mode gives you a board with some blocks already on it and a handful of pieces. Your job is to clear every block using exactly those pieces.

There's one thing a puzzle like that must never do: be impossible. If you spend twenty minutes on a board that can't be cleared, you don't blame the puzzle. You blame yourself, and then you stop playing. So I set myself a rule: every puzzle has to be proven solvable before it ships.

Making puzzles backwards

Designing 120 of these by hand would take months and still leave room for mistakes. Instead, each puzzle is built backwards, starting from the end.

1. Start with an empty board, which is how a puzzle finishes.
2. Choose the lines that the last piece will clear. Take that piece's cells out of those lines. What's left of the lines is the board just before the final move.
3. Do the same again for the piece before that, and the one before that.

When you run out of pieces, you've got the starting board. Because it was made by undoing real moves, you know there's at least one way through.

Then proving it forwards

Building backwards guarantees a solution in theory. To be sure, a solver then plays every puzzle forwards through the game's actual rules: pieces dealt three at a time, any order within each set of three, line clears exactly as they happen in the game. It searches until it finds a winning line and records it.

All 120 puzzles pass. If I ever change a rule in the game, the solver runs again and tells me if anything broke.

Grading the difficulty

The same solver also measures how forgiving each puzzle is: from the starting position, how many first moves still lead to a win? A puzzle where almost any opening works is gentle. One where only a single line survives is hard. The puzzles are sorted into six chapters of twenty, easiest first, so the difficulty rises steadily rather than in random jumps.

Why bother

It would have been quicker to make puzzles at random and hope. But BlockGrid is meant to be calm, and nothing undermines calm like the nagging suspicion that a puzzle is broken. When you're stuck on puzzle 87, you can be certain there's an answer.

Puzzles 1 to 10 are free to play. The rest come with BlockGrid's one-time unlock, along with Zen mode, the 10×10 Big Board and four extra themes. Read more about BlockGrid.