[math-fun] Question about XOR

I am sure the next problem has been solved a long time ago, but it is hard to find any references. Does anybody know a solution or references? (We came up with a fast algorithm for it ourselves, but inventing wheel is not always the best thing.) Problem: Given nonnegative integers A, B, C. Find the cardinality of {(X,Y):0 <=X < A,0 <= Y < B, 0 <= (X xor Y) < C} where XOR denotes the bitwise XOR of X and Y. Helger