为什么Google sparsehash开源库有两个实现：密集哈希表和稀疏哈希表？
答案 0 :(得分：18)
密集哈希表是普通的教科书哈希表实现。
稀疏哈希表只存储实际已设置的元素，并分成多个数组。引用稀疏表实现中的comments：
// 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).
要知道数组的哪些元素已设置，稀疏表包含位图：
// 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.
这样每个元素只产生1比特的开销（在极限中）。
答案 1 :(得分：3)
sparsehash是一种将密钥映射到值的内存有效方法（每个密钥1-2位）。 Bloom过滤器可以为每个键提供更少的位，但是它们不会将值附加到除外部/可能在内部之外的键，这稍微少于一些信息。