ISAAC is a pseudorandom number generator designed by Bob Jenkins to be cryptographically secure. It uses an array of 256 4-byte integers called mm as internal state, writing the results to another 256-integer array, from which they are read one at a time until it's empty, at which point they are recomputed. The computation consists of altering mm[i] with mm[i^128], two elements of mm found by indirection, an accumulator, and a counter, for all values of i from 0 to 256.
See http://www.burtleburtle.net/bob/rand/isaacafa.html for more explanation and code in various languages.
