Welcome to GridShader
This tool allows you to experiment with a grid-shading puzzle published by GCHQ. Instructions are just below the grid.
How it Works
Grab a horizontal pink block of cells, and move it left or right. The blocks must have one white cell horizontally in between them.
In the 25x25 grid, cells are arranged in contiguous blocks, with the lengths of the blocks being given for each row and column. Each of the contiguous blocks must be separated by at least one empty cell. On this page, the grid is initialised using GCHQ's information about contiguous blocks, row-by-row. The columnar information has not been used to place the blocks onto the grid; instead the blocks have been packed hard to the left. You can select a block and drag it to the left or right (provided of course that that would not result in a collision or overlap with an adjacent block).
To help you to move the blocks and solve the puzzle, the correct sequence of contiguous blocks which should ideally appear in each column has been written onto the middle part of the grid. So for example, in the first column, you should aim to have a block of 7, then 2, followed by 1, 1, and finally another 7. At the bottom of each column, a coloured circular marker indicates your progress. Green is good (but not necessarily correct), red isn't good.
The problem posed by GCHQ also reveals that certain cells are to be shaded in. These are indicated by a small grey square (which will turn purple when covered as required).
The goal is to arrange the blocks so that the columnar information on contiguous blocks matches that given in the original puzzle (and shown in blue numerals within the main body of the grid).
Authors and Contributors
The original puzzle was set in December 2015 by the director of GCHQ, Robert Hannigan.
The program code behind the tool is © 2015, Chris McGinlay, licence under GPL v2. Of course, this excludes the puzzle itself, designed by Robert Hannigan.
The Time Machine GitHub pages theme was designed and developed by @jonrohan.