GCHQ Director's Challenge Christmas Card Puzzle
This is the grid shading puzzle that was printed on Christmas cards sent out for the 2015 season by Britain's intelligence agency, GCHQ.
I've created this site as a way to play the puzzle without pen and paper.
For explanation and rules, check out the original puzzle press release at
GHCQ's home page
Version specific instructions:
Tap a grid cell to toggle its shading.
Use a long-tap to toggle a cell's lock. This allows you to lock in cells that you're certain about, so that you don't accidentally change them.
When you've met the requirements for a row or column, the label will turn green. When all rows and columns are green, you've won!