CII-原子

<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 <string.h>
#include <stdlib.h>
#include "atom.h"

#define NELEMS(x) ((sizeof (x)) / (sizeof ((x)[0])))

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
};

static struct atom {
	struct atom *link;
	int len;
	char *str;
} *buckets[2048];

const char *Atom_string(const char *str)
{
	return Atom_new(str, strlen(str));
}

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

	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) //推断是否已经存在
		if (len == p->len) {
			for ( i =0; i < len && p->str[i] == str[i];)
				i++;
			if (i == len)
				return p->str;
		}

	p = malloc (sizeof (*p) + len + 1); // sizeof(*p) = 12字节 (4 + 4 + 4)
	p->len = len;
	p->str = (char *)(p + 1);
	if(len > 0)
		memcpy(p->str, str, len);
	p->str[len] = '';
	p->link = buckets[h];
	buckets[h] = p;

	return p->str;
}

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

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

	for (i = 0; i < NELEMS(buckets); i++) //遍历整个hash表
		for(p = buckets[i]; p; p = p->link)
			if(p->str == str) //仅仅需比較内存地址
				return p->len;
	return 0;
}

将str[i]转换为无符号字符能够避免C语言有关“普通”字符的二义性：字符能够是有符号或者无符号的，假设不转换，在使用带符号字符的机器上，超过127的str[i]值将产生负的索引值。