Thermo Sudoku
Thermo Sudoku is a variant for those who know the classic by heart. The
base is the same: every row, column and 3×3 block holds the digits 1–9
without repeats. But there are almost no givens on start — instead, the
grid is covered with thermometers. Along each thermometer the digits
strictly increase from the round bulb to the tip of the scale: the next
cell is always larger, and by how much is up to you.
That is the main charm of the genre: a five-cell thermometer with its bulb
at 7 is impossible (7,8,9 and then nothing left to place), so its start is
tightly squeezed. A tip hitting 9 also unfolds the whole chain backwards.
Each such observation is a small victory over the grid.
Thermometers are drawn right on the board: the round bulb and the bending
scale are visible at first glance. Every round is generated on the fly: the
full solution comes first, thermometers are laid only along increasing
paths of that solution, and cells are removed so that the solution stays
unique. Unsolvable grids never happen.
Input is tuned for phones: tap a cell, an on-screen digit pad, erase with a
single button. «Check» highlights mistakes in red, «Hint» places a correct
digit. Three difficulties: «Easy» to get acquainted, «Hard» — when you have
to read the grid almost entirely through the thermometers.