为什么Google的sparsehash开源库有两个实现:一个是密集哈希表,另一个是稀疏哈希表?
密集哈希表是您普通的教科书哈希表实现。
稀疏哈希表仅存储实际设置的元素,分散在多个数组中。引用自稀疏表实现中的注释:
// 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.
因此,每个元素仅产生1位的开销(在极限情况下)。