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copied!<p>I just bought a book "C Interfaces and Implementations". in chapter one , it has implemented a "Atom" structure, sample code as follow:</p> <pre><code>#define NELEMS(x) ((sizeof (x))/(sizeof ((x)[0]))) static struct atom { struct atom *link; int len; char *str; } *buckets[2048]; static unsigned long scatter[] = { 2078917053, 143302914, 1027100827, 1953210302, 755253631, 2002600785, 1405390230, 45248011, 1099951567, 433832350, 2018585307, 438263339, 813528929, 1703199216, 618906479, 573714703, 766270699, 275680090, 1510320440, 1583583926, 1723401032, 1965443329, 1098183682, 1636505764, 980071615, 1011597961, 643279273, 1315461275, 157584038, 1069844923, 471560540, 89017443, 1213147837, 1498661368, 2042227746, 1968401469, 1353778505, 1300134328, 2013649480, 306246424, 1733966678, 1884751139, 744509763, 400011959, 1440466707, 1363416242, 973726663, 59253759, 1639096332, 336563455, 1642837685, 1215013716, 154523136, 593537720, 704035832, 1134594751, 1605135681, 1347315106, 302572379, 1762719719, 269676381, 774132919, 1851737163, 1482824219, 125310639, 1746481261, 1303742040, 1479089144, 899131941, 1169907872, 1785335569, 485614972, 907175364, 382361684, 885626931, 200158423, 1745777927, 1859353594, 259412182, 1237390611, 48433401, 1902249868, 304920680, 202956538, 348303940, 1008956512, 1337551289, 1953439621, 208787970, 1640123668, 1568675693, 478464352, 266772940, 1272929208, 1961288571, 392083579, 871926821, 1117546963, 1871172724, 1771058762, 139971187, 1509024645, 109190086, 1047146551, 1891386329, 994817018, 1247304975, 1489680608, 706686964, 1506717157, 579587572, 755120366, 1261483377, 884508252, 958076904, 1609787317, 1893464764, 148144545, 1415743291, 2102252735, 1788268214, 836935336, 433233439, 2055041154, 2109864544, 247038362, 299641085, 834307717, 1364585325, 23330161, 457882831, 1504556512, 1532354806, 567072918, 404219416, 1276257488, 1561889936, 1651524391, 618454448, 121093252, 1010757900, 1198042020, 876213618, 124757630, 2082550272, 1834290522, 1734544947, 1828531389, 1982435068, 1002804590, 1783300476, 1623219634, 1839739926, 69050267, 1530777140, 1802120822, 316088629, 1830418225, 488944891, 1680673954, 1853748387, 946827723, 1037746818, 1238619545, 1513900641, 1441966234, 367393385, 928306929, 946006977, 985847834, 1049400181, 1956764878, 36406206, 1925613800, 2081522508, 2118956479, 1612420674, 1668583807, 1800004220, 1447372094, 523904750, 1435821048, 923108080, 216161028, 1504871315, 306401572, 2018281851, 1820959944, 2136819798, 359743094, 1354150250, 1843084537, 1306570817, 244413420, 934220434, 672987810, 1686379655, 1301613820, 1601294739, 484902984, 139978006, 503211273, 294184214, 176384212, 281341425, 228223074, 147857043, 1893762099, 1896806882, 1947861263, 1193650546, 273227984, 1236198663, 2116758626, 489389012, 593586330, 275676551, 360187215, 267062626, 265012701, 719930310, 1621212876, 2108097238, 2026501127, 1865626297, 894834024, 552005290, 1404522304, 48964196, 5816381, 1889425288, 188942202, 509027654, 36125855, 365326415, 790369079, 264348929, 513183458, 536647531, 13672163, 313561074, 1730298077, 286900147, 1549759737, 1699573055, 776289160, 2143346068, 1975249606, 1136476375, 262925046, 92778659, 1856406685, 1884137923, 53392249, 1735424165, 1602280572 }; const char *Atom_new(const char *str, int len) { unsigned long h; int i; struct atom *p; assert(str); assert(len >= 0); for (h = 0, i = 0; i < len; i++) h = (h<<1) + scatter[(unsigned char)str[i]]; h &= NELEMS(buckets)-1; for (p = buckets[h]; p; p = p->link) if (len == p->len) { for (i = 0; i < len && p->str[i] == str[i]; ) i++; if (i == len) return p->str; } p = ALLOC(sizeof (*p) + len + 1); p->len = len; p->str = (char *)(p + 1); if (len > 0) memcpy(p->str, str, len); p->str[len] = '\0'; p->link = buckets[h]; buckets[h] = p;//insert atom in front of list return p->str; } </code></pre> <p>at end of chapter , in exercises 3.1, the book's author said "<strong>Most texts recommend using a prime number for the size of buckets. Using a prime and a good hash function usually gives a better distribution of the lengths of the lists hanging off of buckets. Atom uses a power of two, which is sometimes explicitly cited as a bad choice. Write a program to generate or read, say, 10,000 typical strings and measure Atom_new’s speed and the distribution of the lengths of the lists. Then change buckets so that it has 2,039 entries (the largest prime less than 2,048), and repeat the measurements. Does using a prime help? How much does your conclusion depend on your specific machine?</strong>"</p> <p>so I did changed that hash table size to 2039,but it seems a prime number actually made a bad distribution of the lengths of the lists, I have tried 64, 61, 61 actually made a bad distribution too.</p> <p>I am just want to know why a prime table size make a bad distribution, is this because the hash function used with Atom_new a bad hash function?</p> <p>I am using this function to print out the lengths of the atom lists</p> <pre><code>#define B_SIZE 2048 void Atom_print(void) { int i,t; struct atom *atom; for(i= 0;i<B_SIZE;i++) { t = 0; for(atom=buckets[i];atom;atom=atom->link) { ++t; } printf("%d ",t); } } </code></pre>

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