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icons-hash: misc improvements (#1553)

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* icons-hash: take total probe count into account as well

* icons-hash: use a better PRNG

the older method was using a multiplicative congruential generator (MCG)
which doesn't work too well especially with just 32 bits of state.

change it to a PCG instead with 64 bits of state (and 32 bits of output)
which should give better results.

and since we should get better rng - the search iteration has been
halved as well to save some build time.

* icons-hash: use an xor-rotate hash function

* icons-hash: fix some compiler warnings
This commit is contained in:
N-R-K 2023-01-02 16:53:18 +00:00 committed by GitHub
parent a51437ff16
commit 2000ed5080
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GPG key ID: 4AEE18F83AFDEB23
1 changed files with 54 additions and 33 deletions
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87
src/icons-hash.c
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@ -9,6 +9,7 @@
#include <inttypes.h>
#define GOLDEN_RATIO_32 UINT32_C(2654442313) /* golden ratio for 32bits: (2^32) / 1.61803 */
#define GOLDEN_RATIO_64 UINT64_C(0x9E3793492EEDC3F7)
#define ICONS_TABLE_SIZE 8 /* size in bits. 8 = 256 */
#ifndef TOUPPER
@ -33,9 +34,9 @@
#define ASSERT(X) assert(X)
#define ARRLEN(X) (sizeof(X) / sizeof((X)[0]))
#define MAX(A, B) ((A) > (B) ? (A) : (B))
#define HGEN_ITERARATION (1ul << 14)
#define HGEN_ITERARATION (1ul << 13)
#define ICONS_PROBE_MAX_ALLOWED 6
#define ICONS_MATCH_MAX ((size_t)-1)
#define ICONS_MATCH_MAX (512)
#if 0 /* for logging some interesting info to stderr */
#define log(...) fprintf(stderr, "[INFO]: " __VA_ARGS__)
@ -52,7 +53,7 @@ static uint8_t seen[ARRLEN(table)];
* but ensure they're above 1 and prefer prime numbers.
*/
static uint32_t hash_start = 7;
static uint32_t hash_mul = 251;
static uint32_t hash_mul = 251; /* unused as of now */
/*
* use robin-hood insertion to reduce the max probe length
@ -79,8 +80,9 @@ rh_insert(const struct icon_pair item, uint32_t idx, uint32_t n)
assert(0); /* unreachable */
}
static unsigned int
table_populate(void)
enum { PROBE_MAX, PROBE_TOTAL, PROBE_CNT };
static unsigned int *
table_populate(unsigned int p[static PROBE_CNT])
{
memset(seen, 0x0, sizeof seen);
memset(table, 0x0, sizeof table);
@ -91,10 +93,23 @@ table_populate(void)
rh_insert(icons_ext[i], h, 1);
}
unsigned int max_probe = 0;
for (size_t i = 0; i < ARRLEN(seen); ++i)
max_probe = MAX(max_probe, seen[i]);
return max_probe;
p[PROBE_MAX] = p[PROBE_TOTAL] = 0;
for (size_t i = 0; i < ARRLEN(seen); ++i) {
p[PROBE_MAX] = MAX(p[PROBE_MAX], seen[i]);
p[PROBE_TOTAL] += seen[i];
}
return p;
}
/* permuted congruential generator */
static uint32_t
pcg(uint64_t *state)
{
uint64_t oldstate = *state;
*state *= GOLDEN_RATIO_64;
uint32_t r = (oldstate >> 59);
uint32_t v = (oldstate ^ (oldstate >> 18)) >> 27;
return (v >> (32 - r)) | (v << r);
}
int
@ -104,6 +119,7 @@ main(void)
assert(ICONS_TABLE_SIZE < 16);
assert(1u << ICONS_TABLE_SIZE == ARRLEN(table));
assert((GOLDEN_RATIO_32 & 1) == 1); /* must be odd */
assert((GOLDEN_RATIO_64 & 1) == 1); /* must be odd */
assert(hash_start > 1);
assert(hash_mul > 1);
/* ensure power of 2 hashtable size which allows compiler to optimize
@ -112,41 +128,50 @@ main(void)
assert((ARRLEN(table) & (ARRLEN(table) - 1)) == 0);
unsigned int max_probe = (unsigned)-1;
uint32_t best_hash_start, best_hash_mul;
uint32_t best_hash_start = 0, best_hash_mul = 0, best_total_probe = 9999;
uint64_t hash_start_rng = hash_start, hash_mul_rng = hash_mul;
for (size_t i = 0; i < HGEN_ITERARATION; ++i) {
unsigned z = table_populate();
if (z < max_probe) {
max_probe = z;
unsigned *p = table_populate((unsigned [PROBE_CNT]){0});
if (p[PROBE_MAX] < max_probe ||
(p[PROBE_MAX] == max_probe && p[PROBE_TOTAL] < best_total_probe))
{
max_probe = p[PROBE_MAX];
best_total_probe = p[PROBE_TOTAL];
best_hash_start = hash_start;
best_hash_mul = hash_mul;
}
hash_start *= GOLDEN_RATIO_32;
hash_mul *= GOLDEN_RATIO_32;
hash_start = pcg(&hash_start_rng);
hash_mul = pcg(&hash_mul_rng);
}
assert(max_probe < ICONS_PROBE_MAX_ALLOWED);
hash_start = best_hash_start;
hash_mul = best_hash_mul;
{
unsigned tmp = table_populate();
assert(tmp == max_probe);
unsigned *p = table_populate((unsigned [PROBE_CNT]){0});
assert(p[PROBE_MAX] == max_probe);
assert(p[PROBE_TOTAL] == best_total_probe);
}
/* sanity check */
double nitems = 0;
unsigned int total_probe = 0;
for (size_t i = 0; i < ARRLEN(icons_ext); ++i) {
if (icons_ext[i].icon[0] == 0)
continue;
uint32_t found = 0, h = icon_ext_hash(icons_ext[i].match);
for (uint32_t k = 0; k < max_probe; ++k) {
uint32_t z = (h + k) % ARRLEN(table);
++total_probe;
if (table[z].match && strcasecmp(icons_ext[i].match, table[z].match) == 0) {
found = 1;
break;
}
}
assert(found);
++nitems;
}
assert(total_probe == best_total_probe);
size_t match_max = 0, icon_max = 0;
for (size_t i = 0; i < ARRLEN(icons_name); ++i) {
@ -160,6 +185,7 @@ main(void)
icon_max = MAX(icon_max, strlen(dir_icon.icon) + 1);
icon_max = MAX(icon_max, strlen(exec_icon.icon) + 1);
icon_max = MAX(icon_max, strlen(file_icon.icon) + 1);
assert(icon_max < ICONS_MATCH_MAX);
const char *uniq[ARRLEN(icons_ext)] = {0};
size_t uniq_head = 0;
@ -183,6 +209,7 @@ main(void)
log("load-factor: %.2f (%u/%zu)\n", (nitems * 100.0) / (double)ARRLEN(table),
(unsigned int)nitems, ARRLEN(table));
log("max_probe : %6u\n", max_probe);
log("total_probe: %6u\n", total_probe);
log("uniq icons : %6zu\n", uniq_head);
log("no-compact : %6zu bytes\n", ARRLEN(table) * icon_max);
log("compaction : %6zu bytes\n", uniq_head * icon_max + ARRLEN(table));
@ -216,13 +243,13 @@ main(void)
for (size_t i = 0; i < ARRLEN(table); ++i) {
if (table[i].icon == NULL || table[i].icon[0] == '\0') /* skip empty entries */
continue;
int k;
size_t k;
for (k = 0; k < uniq_head; ++k) {
if (strcasecmp(table[i].icon, uniq[k]) == 0)
break;
}
assert(k < uniq_head);
printf("\t[%3zu] = {\"%s\", %d, %hhu },\n",
printf("\t[%3zu] = {\"%s\", %zu, %hhu },\n",
i, table[i].match, k, table[i].color);
}
printf("};\n\n");
@ -239,30 +266,24 @@ static uint32_t
icon_ext_hash(const char *str)
{
uint32_t i, hash = hash_start;
const unsigned int z = (sizeof hash * CHAR_BIT) - ICONS_TABLE_SIZE;
enum { wsz = sizeof hash * CHAR_BIT, z = wsz - ICONS_TABLE_SIZE, r = 5 };
/* FNV style xor-mul hashing. Some other hashing which gives good results:
* Jenkin's one-at-a-time: https://en.wikipedia.org/wiki/Jenkins_hash_function#one_at_a_time
* xor-rotate: ((hash >> (32 - 5)) | (hash << 5)) ^ TOUPPER((unsigned char)str[i]);
/* just an xor-rotate hash. in general, this is a horrible hash
* function but for our specific input it works fine while being
* computationally cheap.
*/
for (i = 0; i < ICONS_MATCH_MAX && str[i] != '\0'; ++i) {
hash ^= TOUPPER((unsigned char)str[i]);
hash *= hash_mul;
hash = (hash >> (wsz - r)) | (hash << r);
}
/* due to the multiply, the entropy of our hash is hidden in the high
* bits. so we take the high bits as our map into the table.
*/
#if 0
/* enable this part if the hash function is to be changed to a non-multiplying one.
* gives better distribution than modulo: https://probablydance.com/2018/06/16/
*/
/* finalizer: https://probablydance.com/2018/06/16 */
hash ^= (hash >> z);
hash *= GOLDEN_RATIO_32;
#endif
hash >>= z;
hash >>= z;
ASSERT(hash < ARRLEN(table));
return hash;
}
#endif