C++ STL: Order of magnitude faster hash tables with Policy Based Data Structures
Difference between en10 and en11, changed 0 character(s)
 The Policy Hash Table has 20-30% faster linear insertion/deletion, equal performance for random insertion/deletion, and 3-10x increase for writes/reads. However, clear() is slower on a Policy Hash Table with random insertion order.↵

I've often been irritated by how slow unordered_map is in C++. Too often, I have something that runs fast enough in terms of complexity, but the constant factor from unordered_map slows down the solution too much. Yesterday though, after using the useful order statistics tree from https://mirror.codeforces.com/blog/entry/11080, I was curious if there were any other useful data structures hiding in the Policy STL. And lo and behold, I found a hash table.↵

Well, enough backstory, let's look at some numbers.↵
All benchmarks below are compiled with C++14 -O2.↵

unordered map linear insertion:                                 0.816795↵
policy hash table linear insertion:                             0.442094↵

unordered map linear read/write:                                1.91384↵
policy hash table linear read/write:                            0.216669↵

unordered map random insertion:                                 2.98216↵
policy hash table random insertion:                             3.13651↵

unordered map random read/write:                                2.12804↵
policy hash table random read/write:                            0.356518↵

While for linear insertions, the policy hash table gives modest improvements, the policy hash table blows the unordered_map out of the water when it comes to reads/writes. These are order of magnitude improvements that make hash tables usable when they previously weren't.↵
Benchmarks of course, don't always reflect the real world. So here's an example of it allowing a solution to be accepted that TLE's with unordered_map.↵

Example problem (5000 ms time limit): http://mirror.codeforces.com/contest/264/problem/C  ↵
Solution with unordered_map: http://mirror.codeforces.com/contest/264/submission/40542899 (TLE on test case 8)  ↵
Solution with policy hash table directly substituted in: http://mirror.codeforces.com/contest/264/submission/40573491 (TLE on test case 26)  ↵
Solution with unordered_map, rewritten to not use clears:↵
http://mirror.codeforces.com/contest/264/submission/40590437 (TLE on test case 26)↵
Solution with policy hash table and rewritten to not use clears: http://mirror.codeforces.com/contest/264/submission/40574196 (AC with max time of 3180 ms)  ↵

To use this data structure:↵

#include <ext/pb_ds/assoc_container.hpp>↵
using namespace __gnu_pbds;↵
cc_hash_table<int, int> table;↵
From there, the API seems almost exactly the same.↵

Thanks to ~adamant,2018-07-20 for his post that revealed to me the existence of policy data structures, and thanks to ~tonynater,2018-07-20 for the discussion.↵

PS: In other posts for unordered_map, I've seen people claim that reserve and max_load_factor could increase performance drastically. They didn't seem to do much for me. However, if you want to do something similar for these hash tables, use `typedef cc_hash_table<↵
    int, int, hash<int>, equal_to<int>, direct_mask_range_hashing<int>,↵
    hash_standard_resize_policy<hash_exponential_size_policy<>, hash_load_check_resize_trigger<true>, true>>↵
    ht;`, and you should be able to resize and set load factor manually as well.↵

Code for the benchmarks can be found here: https://ideone.com/ZkNMbH↵


  Rev. Lang. By When Δ Comment
en25 English Chilli 2020-04-08 21:39:21 204
en24 English Chilli 2018-12-20 19:59:40 19
en23 English Chilli 2018-08-26 22:29:49 97
en22 English Chilli 2018-07-23 11:48:12 5
en21 English Chilli 2018-07-23 11:26:11 64
en20 English Chilli 2018-07-23 11:23:29 657
en19 English Chilli 2018-07-23 10:23:47 198 Tiny change: ' clears:]( http://cod' -> ' clears:](http://cod'
en18 English Chilli 2018-07-23 09:17:15 390
en17 English Chilli 2018-07-21 23:50:59 504
en16 English Chilli 2018-07-21 05:33:02 10
en15 English Chilli 2018-07-21 05:31:38 4
en14 English Chilli 2018-07-21 05:20:37 2
en13 English Chilli 2018-07-21 05:18:56 5
en12 English Chilli 2018-07-21 04:59:02 842
en11 English Chilli 2018-07-21 01:50:28 0 (published)
en10 English Chilli 2018-07-21 01:08:04 136
en9 English Chilli 2018-07-21 01:04:50 45
en8 English Chilli 2018-07-21 00:42:45 21
en7 English Chilli 2018-07-21 00:41:07 99
en6 English Chilli 2018-07-21 00:40:10 24
en5 English Chilli 2018-07-21 00:37:56 25
en4 English Chilli 2018-07-20 23:34:11 41
en3 English Chilli 2018-07-20 23:21:31 2598 Tiny change: 'n\n\n~~~~~\n\nunordered ' -> 'n\n\n~~~~~unordered '
en2 English Chilli 2018-07-20 23:01:35 4
en1 English Chilli 2018-07-20 23:01:11 1442 Initial revision (saved to drafts)