C.Interface.And.Implementations—Atom的实现

An  atom is a pointer to a unique, immutable sequence of zero or more arbitrary bytes. Most atoms are pointers to null-terminated strings, but a pointer to any sequence of bytes can be an atom. There is only a single occurrence of any atom, which is why it’s called an atom. Two atoms are identical if they point to the same location. Comparing two byte sequences for equality by simply comparing pointers is one of the advantages of atoms. Another advantage is that using atoms saves space because there’s only one occurrence of each sequence.

Sedgewick (1990) and Knuth (1973b) describe hashing in detail and give guidelines for writing good hash functions. 

===================================atom.h========================================

#ifndef ATOM_INCLUDED
#define ATOM_INCLUDED

extern       int   Atom_length(const char *str);
extern const char *Atom_new   (const char *str, int len);
extern const char *Atom_string(const char *str);
extern const char *Atom_int   (long n);

#endif

===================================atom.c=========================================

#include "atom.h"
#include <string.h>
#include "assert.h"
#include <limits.h>
#include "mem.h"

#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_string(const char *str){
    assert(str);
    return Atom_new(str, strlen(str));
}

const char *Atom_int(long n){
    char str[43];
    char *s = str + sizeof str;
    unsigned long m;

    if(n == LONG_MIN)
        m = LONG_MAX + 1UL;
    else if (n < 0)
        m = -n;
    else
        m = n;
    do
        *--s = m%10 + '0';
    while((m /= 10) > 0);
    if(n < 0)
        *--s = '-';
    return Atom_new(s, (str + sizeof str) - s);
}

const char* Atom_new(const char *str, int len){
    unsigned long h;
    int i;
    struct atom *p;

    assert(str);
    assert(len >= 0);
    //hash str[0..len-1] to h
    for(h = 0, i = 0; i < len; ++i)
        h = (h<<1) + scatter[(unsigned char)str[i]];
    h = %= NELEMS(buckets);
    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;
        }
    }
    //allocate a new entry
    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;
    return p->str;
}

int Atom_length(const char *str){
    struct atom *p;
    int i;

    assert(str);
    for(i = 0; i < NELEMS(buckets); ++i){
        for(p = buckets[i]; p; p =p->link){
            if(p->str == str)
                return p->len;
        }
    }

    assert(0);
    return 0;
}