// The idea is that a table with (logically) t buckets is divided // into t/M *groups* of M buckets each. (M is a constant set in // GROUP_SIZE for efficiency.) Each group is stored sparsely. // Thus, inserting into the table causes some array to grow, which is // slow but still constant time. Lookup involves doing a // logical-position-to-sparse-position lookup, which is also slow but // constant time. The larger M is, the slower these operations are // but the less overhead (slightly).
// To store the sparse array, we store a bitmap B, where B[i] = 1 iff // bucket i is non-empty. Then to look up bucket i we really look up // array[# of 1s before i in B]. This is constant time for fixed M.