## digitalmars.D.learn - Static introspection of suitable hash function depending on type of

- =?UTF-8?B?Tm9yZGzDtnc=?= (41/41) Oct 02 2017 Does anyone have any good reads on the subject of statically

Does anyone have any good reads on the subject of statically choosing suitable hash-functions depending on the type (and in turn size) of the key? I wonder because I'm currently experimenting with a hash set implementation at https://github.com/nordlow/phobos-next/blob/40e2973b74d58470a13a5a6ee0ed9c9a42c6dea1/src/hashset.d and benchmark for different hash-functions for it at https://github.com/nordlow/phobos-next/blob/b8942dc569921b4dadfddbdcdac3a2bb0834a9e0/src/benchmarkAppend.d I'm measuring significant differences in speed depending on the choice of the hash-function: Inserted 1000000 integers in 49 ms, 65 μs, and 9 hnsecs, Checked 1000000 integers in 48 ms, 562 μs, and 2 hnsecs for HashSet!(uint, null, typeidHashOf) Inserted 1000000 integers in 51 ms, 897 μs, and 5 hnsecs, Checked 1000000 integers in 47 ms, 108 μs, and 9 hnsecs for HashSet!(uint, null, hashOf) Inserted 1000000 integers in 60 ms, 641 μs, and 5 hnsecs, Checked 1000000 integers in 70 ms, 664 μs, and 2 hnsecs for HashSet!(uint, null, MurmurHash3!(128u, 64u)) Inserted 1000000 integers in 34 ms, 450 μs, and 5 hnsecs, Checked 1000000 integers in 27 ms, 738 μs, and 8 hnsecs for HashSet!(uint, null, FNV!(64LU, true)) Inserted 1000000 integers in 97 ms, 400 μs, and 6 hnsecs, Checked 1000000 integers in 104 ms, 33 μs, and 1 hnsec for HashSet!(uint, null, XXHash64) integers in 39 ms, 304 μs, and 3 hnsecs for bool[uint] using LDC 1.4.0. A factor 2 for insert() and factor 4 for contains(). The reason is partly because many high-performance hashes, such as XXHash64, have a significant overhead (tens of clock-cycles) because of its super-scalar nature but are fast (~1 clock-cycle per byte) for large keys. The test is dumb for now and is only constructed to benchmark the hash-function. According to a comment at https://stackoverflow.com/questions/46533112/static-introspection-of-suitable-hash-function-depending-on-type-of-hash-key "the C++ standard library already includes hashes for many basic types? (I ask this because if you don't have a special distribution I would assume the standard committee has already made good choices. My answer would be to use those.)" Does Phobos have anything similar today or planned?

Oct 02 2017