[Rayhem]

Similar story - a PI had written some code to from (row, column) indices of the upper triangle of a matrix (made somewhat tricky by excluding the main diagonal) to a linear index. He used a for loop to start from the beginning and count up for an O(n^2) algorithm - I was able to give him an O(1) constant time formula to do the same thing for a rather dramatic speedup.

[vladf]

I ended up needing this so often for graph processing, and for values which might be inexact if using floating point, that I saved the formula in a blog post. https://vladfeinberg.com/2020/03/07/subset-isomorphism.html

The formula can be "oblivious" to the final size of the matrix too, which is helpful if you're doing some sparse ML training on edges (e.g., GNNs).