Create a grid of 50x50. When you click on a cell, all values in the cells in the same row and column are increased by 1 or, if a cell was empty, it will get a value of 1.

After each change a cell will briefly turn yellow.

If 5 consecutive numbers in the Fibonacci sequence are next to each other, these cells will briefly turn green and will be cleared.

The solution is implemented as a .NET Core project with an MVC-style single-page app at the front, a Web API and a Domain library in the back-end. It's certainly the case that this could all be implemented in JS however this is not my strength (especially testing JavaScript) so I have elected to structure the solution in this way.

Domain logic is contained within the Domain project. The Grid type is the location of the main state storage for each Grid, for identifying sequences and raising events.

Instead of working out each Fibonacci number as square values change, I have chosen to store the Fibonacci index of each value in an integer array. This is a trade-off between memory and performance; doubling the memory usage of each Grid in order to save time working out the Fibonacci number index of each square as it is clicked.

The Fibonacci method used differs slightly from the cardinal Fibonacci sequence because the zero and first 1 digit of the sequence are ignored. This is because values in the grid squares are compared by value rather than by ordinal position in the sequence. Thus, the Fibonacci sequence which is normally { 0, 1, 1, 2, 3, 5 ... } is actually considered as { 1, 2, 3, 5 ... }. The zero is ignored as this is used to denote that a square is empty.

With each click, the whole grid (each row and column and in each direction) is examined to see whether it contains a sequence. This is not efficient and can be optimised by only examining the rows and columns that have been changed by a given click. This will require no change to the tests because the aggregate result will be the same.