2 * Bit Array Signatures -- A Dubious Detector of Duplicates
4 * (c) 2002 Martin Mares <mj@ucw.cz>
6 * FIXME: Ref to the original article.
8 * This data structure provides a very compact representation
9 * of a set of strings with insertion and membership search,
10 * but with a certain low probability it cheats by incidentally
11 * reporting a non-member as a member. Generally the larger you
12 * create the structure, the lower this probability is.
14 * How does it work: the structure is just an array of M bits
15 * and each possible element is hashed to a set of (at most) L
16 * bit positions. For each element of the represented set, we
17 * set its L bits to ones and we report as present all elements
18 * whose all L bits ar set.
20 * Analysis: Let's assume N items have already been stored and let A
21 * denote L/M (density of the hash function). The probability that
22 * a fixed bit of the array is set by any of the N items is
23 * 1 - (1-1/M)^(NL) = 1 - ((1-1/M)^M)^NA = approx. 1 - e^-NA.
24 * This is minimized by setting A=(ln 2)/N (try taking derivative).
25 * Given a non-present item, the probability that all of the bits
26 * corresponding to this item are set by the other items (that is,
27 * the structure gives a false answer) is (1-e^-NA)^L = 2^-L.
28 * Hence, if we want to give false answers with probability less
29 * than epsilon, we take L := -log_2 epsilon, M := 1.45*N*L.
31 * Example: For a set of 10^7 items with P[error] < 10^-6, we set
32 * L := 20 and M := 290*10^6 bits = cca 34.5 MB (29 bits per item).
34 * We leave L and an upper bound for N as parameters set during
35 * creation of the structure. Currently, the structure is limited
40 #include "lib/bitsig.h"
46 uns l, m, n, maxn, max_m_mult;
52 bitsig_init(uns perrlog, uns maxn)
58 m = ((u64) maxn * perrlog * 145 + 99) / 100;
59 if (m >= (u64) 1 << 32)
60 die("bitsig_init: bitsig array too large (maximum is 4 Gb)");
61 mbytes = (m + 7) >> 3U;
62 b = xmalloc(sizeof(struct bitsig) + mbytes);
67 b->max_m_mult = (0xffffffff / m) * m;
68 bzero(b->array, mbytes);
69 log(L_DEBUG, "Initialized bitsig array with l=%d, m=%u (%u KB), expecting %d items", b->l, b->m, (mbytes+1023)/1024, maxn);
74 bitsig_hash_init(struct bitsig *b, byte *item)
80 MD5Update(&c, item, strlen(item));
81 MD5Final((byte *) digest, &c);
86 bitsig_hash_bit(struct bitsig *b)
91 b->tmp *= 3006477127U;
93 while (b->tmp >= b->max_m_mult);
98 bitsig_member(struct bitsig *b, byte *item)
102 bitsig_hash_init(b, item);
103 for (i=0; i<b->l; i++)
105 bit = bitsig_hash_bit(b);
106 if (!(b->array[bit >> 3] & (1 << (bit & 7))))
113 bitsig_insert(struct bitsig *b, byte *item)
117 bitsig_hash_init(b, item);
119 for (i=0; i<b->l; i++)
121 bit = bitsig_hash_bit(b);
122 if (!(b->array[bit >> 3] & (1 << (bit & 7))))
125 b->array[bit >> 3] |= (1 << (bit & 7));
128 if (!was && b->n++ == b->maxn+1)
129 log(L_ERROR, "bitsig: Too many items inserted, error rate will be higher than estimated!");
139 struct bitsig *b = bitsig_init(10, 23000);
142 while (fgets(buf, 1024, stdin))
143 printf("%d\n", bitsig_insert(b, buf));