1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326

CMPH  C Minimal Perfect Hashing Library

Motivation
==========
A perfect hash function maps a static set of n keys into a set of m integer numbers without collisions, where m is greater than or equal to n. If m is equal to n, the function is called minimal.
Minimal perfect hash functions (concepts.html) are widely used for memory efficient storage and fast retrieval of items from static sets, such as words in natural languages, reserved words in programming languages or interactive systems, universal resource locations (URLs) in Web search engines, or item sets in data mining techniques. Therefore, there are applications for minimal perfect hash functions in information retrieval systems, database systems, language translation systems, electronic commerce systems, compilers, operating systems, among others.
The use of minimal perfect hash functions is, until now, restricted to scenarios where the set of keys being hashed is small, because of the limitations of current algorithms. But in many cases, to deal with huge set of keys is crucial. So, this project gives to the free software community an API that will work with sets in the order of billion of keys.
Probably, the most interesting application for minimal perfect hash functions is its use as an indexing structure for databases. The most popular data structure used as an indexing structure in databases is the B+ tree. In fact, the B+ tree is very used for dynamic applications with frequent insertions and deletions of records. However, for applications with sporadic modifications and a huge number of queries the B+ tree is not the best option, because practical deployments of this structure are extremely complex, and perform poorly with very large sets of keys such as those required for the new frontiers database applications (http://acmqueue.com/modules.php?name=Content&pa=showpage&pid=299).
For example, in the information retrieval field, the work with huge collections is a daily task. The simple assignment of ids to web pages of a collection can be a challenging task. While traditional databases simply cannot handle more traffic once the working set of web page urls does not fit in main memory anymore, minimal perfect hash functions can easily scale to hundred of millions of entries, using stock hardware.
As there are lots of applications for minimal perfect hash functions, it is important to implement memory and time efficient algorithms for constructing such functions. The lack of similar libraries in the free software world has been the main motivation to create the C Minimal Perfect Hashing Library (gperf is a bit different (gperf.html), since it was conceived to create very fast perfect hash functions for small sets of keys and CMPH Library was conceived to create minimal perfect hash functions for very large sets of keys). C Minimal Perfect Hashing Library is a portable LGPLed library to generate and to work with very efficient minimal perfect hash functions.

Description
===========
The CMPH Library encapsulates the newest and more efficient algorithms in an easytouse, productionquality, fast API. The library was designed to work with big entries that cannot fit in the main memory. It has been used successfully for constructing minimal perfect hash functions for sets with more than 100 million of keys, and we intend to expand this number to the order of billion of keys. Although there is a lack of similar libraries, we can point out some of the distinguishable features of the CMPH Library:
 Fast.
 Spaceefficient with main memory usage carefully documented.
 The best modern algorithms are available (or at least scheduled for implementation :)).
 Works with indisk key sets through of using the adapter pattern.
 Serialization of hash functions.
 Portable C code (currently works on GNU/Linux and WIN32 and is reported to work in OpenBSD and Solaris).
 Object oriented implementation.
 Easily extensible.
 Well encapsulated API aiming binary compatibility through releases.
 Free Software.

Supported Algorithms
====================
 CHD Algorithm:
 It is the fastest algorithm to build PHFs and MPHFs in linear time.
 It generates the most compact PHFs and MPHFs we know of.
 It can generate PHFs with a load factor up to 99 %.
 It can be used to generate tperfect hash functions. A tperfect hash function allows at most t collisions in a given bin. It is a wellknown fact that modern memories are organized as blocks which constitute transfer unit. Example of such blocks are cache lines for internal memory or sectors for hard disks. Thus, it can be very useful for devices that carry out I/O operations in blocks.
 It is a two level scheme. It uses a first level hash function to split the key set in buckets of average size determined by a parameter b in the range [1,32]. In the second level it uses displacement values to resolve the collisions that have given rise to the buckets.
 It can generate MPHFs that can be stored in approximately 2.07 bits per key.
 For a load factor equal to the maximum one that is achieved by the BDZ algorithm (81 %), the resulting PHFs are stored in approximately 1.40 bits per key.
 BDZ Algorithm:
 It is very simple and efficient. It outperforms all the ones below.
 It constructs both PHFs and MPHFs in linear time.
 The maximum load factor one can achieve for a PHF is 1/1.23.
 It is based on acyclic random 3graphs. A 3graph is a generalization of a graph where each edge connects 3 vertices instead of only 2.
 The resulting MPHFs are not order preserving.
 The resulting MPHFs can be stored in only (2 + x)cn bits, where c should be larger than or equal to 1.23 and x is a constant larger than 0 (actually, x = 1/b and b is a parameter that should be larger than 2). For c = 1.23 and b = 8, the resulting functions are stored in approximately 2.6 bits per key.
 For its maximum load factor (81 %), the resulting PHFs are stored in approximately 1.95 bits per key.
 BMZ Algorithm:
 Construct MPHFs in linear time.
 It is based on cyclic random graphs. This makes it faster than the CHM algorithm.
 The resulting MPHFs are not order preserving.
 The resulting MPHFs are more compact than the ones generated by the CHM algorithm and can be stored in 4cn bytes, where c is in the range [0.93,1.15].
 BRZ Algorithm:
 A very fast external memory based algorithm for constructing minimal perfect hash functions for sets in the order of billions of keys.
 It works in linear time.
 The resulting MPHFs are not order preserving.
 The resulting MPHFs can be stored using less than 8.0 bits per key.
 CHM Algorithm:
 Construct minimal MPHFs in linear time.
 It is based on acyclic random graphs
 The resulting MPHFs are order preserving.
 The resulting MPHFs are stored in 4cn bytes, where c is greater than 2.
 FCH Algorithm:
 Construct minimal perfect hash functions that require less than 4 bits per key to be stored.
 The resulting MPHFs are very compact and very efficient at evaluation time
 The algorithm is only efficient for small sets.
 It is used as internal algorithm in the BRZ algorithm to efficiently solve larger problems and even so to generate MPHFs that require approximately 4.1 bits per key to be stored. For that, you just need to set the parameters a to brz and c to a value larger than or equal to 2.6.

News for version 2.0
====================
Cleaned up most warnings for the c code.
Experimental C++ interface (enablecxxmph) implementing the BDZ algorithm in
a convenient interface, which serves as the basis
for dropin replacements for std::unordered_map, sparsehash::sparse_hash_map
and sparsehash::dense_hash_map. Potentially faster lookup time at the expense
of insertion time. See cxxmpph/mph_map.h and cxxmph/mph_index.h for details.
News for version 1.1
====================
Fixed a bug in the chd_pc algorithm and reorganized tests.
News for version 1.0
====================
This is a bugfix only version, after which a revamp of the cmph code and
algorithms will be done.
News for version 0.9
====================
 The CHD algorithm (chd.html), which is an algorithm that can be tuned to generate MPHFs that require approximately 2.07 bits per key to be stored. The algorithm outperforms the BDZ algorithm (bdz.html) and therefore is the fastest one available in the literature for sets that can be treated in internal memory.
 The CHD_PH algorithm (chd.html), which is an algorithm to generate PHFs with load factor up to 99 %. It is actually the CHD algorithm without the ranking step. If we set the load factor to 81 %, which is the maximum that can be obtained with the BDZ algorithm (bdz.html), the resulting functions can be stored in 1.40 bits per key. The space requirement increases with the load factor.
 All reported bugs and suggestions have been corrected and included as well.
News for version 0.8
====================
 An algorithm to generate MPHFs that require around 2.6 bits per key to be stored (bdz.html), which is referred to as BDZ algorithm. The algorithm is the fastest one available in the literature for sets that can be treated in internal memory.
 An algorithm to generate PHFs with range m = cn, for c > 1.22 (bdz.html), which is referred to as BDZ_PH algorithm. It is actually the BDZ algorithm without the ranking step. The resulting functions can be stored in 1.95 bits per key for c = 1.23 and are considerably faster than the MPHFs generated by the BDZ algorithm.
 An adapter to support a vector of struct as the source of keys has been added.
 An API to support the ability of packing a perfect hash function into a preallocated contiguous memory space. The computation of a packed function is still faster and can be easily mmapped.
 The hash functions djb2, fnv and sdbm were removed because they do not use random seeds and therefore are not useful for MPHFs algorithms.
 All reported bugs and suggestions have been corrected and included as well.
News log (newslog.html)

Examples
========
Using cmph is quite simple. Take a look.
#include <cmph.h>
#include <string.h>
// Create minimal perfect hash function from inmemory vector
int main(int argc, char **argv)
{
// Creating a filled vector
unsigned int i = 0;
const char *vector[] = {"aaaaaaaaaa", "bbbbbbbbbb", "cccccccccc", "dddddddddd", "eeeeeeeeee",
"ffffffffff", "gggggggggg", "hhhhhhhhhh", "iiiiiiiiii", "jjjjjjjjjj"};
unsigned int nkeys = 10;
FILE* mphf_fd = fopen("temp.mph", "w");
// Source of keys
cmph_io_adapter_t *source = cmph_io_vector_adapter((char **)vector, nkeys);
//Create minimal perfect hash function using the brz algorithm.
cmph_config_t *config = cmph_config_new(source);
cmph_config_set_algo(config, CMPH_BRZ);
cmph_config_set_mphf_fd(config, mphf_fd);
cmph_t *hash = cmph_new(config);
cmph_config_destroy(config);
cmph_dump(hash, mphf_fd);
cmph_destroy(hash);
fclose(mphf_fd);
//Find key
mphf_fd = fopen("temp.mph", "r");
hash = cmph_load(mphf_fd);
while (i < nkeys) {
const char *key = vector[i];
unsigned int id = cmph_search(hash, key, (cmph_uint32)strlen(key));
fprintf(stderr, "key:%s  hash:%u\n", key, id);
i++;
}
//Destroy hash
cmph_destroy(hash);
cmph_io_vector_adapter_destroy(source);
fclose(mphf_fd);
return 0;
}
Download vector_adapter_ex1.c (examples/vector_adapter_ex1.c). This example does not work in versions below 0.6. You need to update the sources from GIT to make it work.

#include <cmph.h>
#include <stdio.h>
#include <string.h>
// Create minimal perfect hash function from indisk keys using BDZ algorithm
int main(int argc, char **argv)
{
//Open file with newline separated list of keys
FILE * keys_fd = fopen("keys.txt", "r");
cmph_t *hash = NULL;
if (keys_fd == NULL)
{
fprintf(stderr, "File \"keys.txt\" not found\n");
exit(1);
}
// Source of keys
cmph_io_adapter_t *source = cmph_io_nlfile_adapter(keys_fd);
cmph_config_t *config = cmph_config_new(source);
cmph_config_set_algo(config, CMPH_BDZ);
hash = cmph_new(config);
cmph_config_destroy(config);
//Find key
const char *key = "jjjjjjjjjj";
unsigned int id = cmph_search(hash, key, (cmph_uint32)strlen(key));
fprintf(stderr, "Id:%u\n", id);
//Destroy hash
cmph_destroy(hash);
cmph_io_nlfile_adapter_destroy(source);
fclose(keys_fd);
return 0;
}
Download file_adapter_ex2.c (examples/file_adapter_ex2.c) and keys.txt (examples/keys.txt). This example does not work in versions below 0.8. You need to update the sources from GIT to make it work.
Click here to see more examples (examples.html)

The cmph application
====================
cmph is the name of both the library and the utility
application that comes with this package. You can use the cmph
application for constructing minimal perfect hash functions from the command line.
The cmph utility
comes with a number of flags, but it is very simple to create and to query
minimal perfect hash functions:
$ # Using the chm algorithm (default one) for constructing a mphf for keys in file keys_file
$ ./cmph g keys_file
$ # Query id of keys in the file keys_query
$ ./cmph m keys_file.mph keys_query
The additional options let you set most of the parameters you have
available through the C API. Below you can see the full help message for the
utility.
usage: cmph [v] [h] [V] [k nkeys] [f hash_function] [g [c algorithm_dependent_value][s seed] ]
[a algorithm] [M memory_in_MB] [b algorithm_dependent_value] [t keys_per_bin] [d tmp_dir]
[m file.mph] keysfile
Minimum perfect hashing tool
h print this help message
c c value determines:
* the number of vertices in the graph for the algorithms BMZ and CHM
* the number of bits per key required in the FCH algorithm
* the load factor in the CHD_PH algorithm
a algorithm  valid values are
* bmz
* bmz8
* chm
* brz
* fch
* bdz
* bdz_ph
* chd_ph
* chd
f hash function (may be used multiple times)  valid values are
* jenkins
V print version number and exit
v increase verbosity (may be used multiple times)
k number of keys
g generation mode
s random seed
m minimum perfect hash function file
M main memory availability (in MB) used in BRZ algorithm
d temporary directory used in BRZ algorithm
b the meaning of this parameter depends on the algorithm selected in the a option:
* For BRZ it is used to make the maximal number of keys in a bucket lower than 256.
In this case its value should be an integer in the range [64,175]. Default is 128.
* For BDZ it is used to determine the size of some precomputed rank
information and its value should be an integer in the range [3,10]. Default
is 7. The larger is this value, the more compact are the resulting functions
and the slower are them at evaluation time.
* For CHD and CHD_PH it is used to set the average number of keys per bucket
and its value should be an integer in the range [1,32]. Default is 4. The
larger is this value, the slower is the construction of the functions.
This parameter has no effect for other algorithms.
t set the number of keys per bin for a tperfect hashing function. A tperfect
hash function allows at most t collisions in a given bin. This parameter applies
only to the CHD and CHD_PH algorithms. Its value should be an integer in the
range [1,128]. Defaul is 1
keysfile line separated file with keys
Additional Documentation
========================
FAQ (faq.html)
Downloads
=========
Use the project page at sourceforge: http://sf.net/projects/cmph
License Stuff
=============
Code is under the LGPL and the MPL 1.1.

Enjoy!
Davi de Castro Reis (davi@users.sourceforge.net)
Djamel Belazzougui (db8192@users.sourceforge.net)
Fabiano Cupertino Botelho (fc_botelho@users.sourceforge.net)
Nivio Ziviani (nivio@dcc.ufmg.br)
Last Updated: Fri Jun 6 17:16:57 2014
