First draft of hard link detection via bloom filters.

[adu.git] / bloom.c

diff --git a/bloom.c b/bloom.c
new file mode 100644 (file)
index 0000000..f7a9b1a
--- /dev/null
+++ b/bloom.c
@@ -0,0 +1,183 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <inttypes.h>
+#include <string.h>
+#include <assert.h>
+
+#include "adu.h"
+#include "string.h"
+#include "bloom.h"
+
+#ifdef TEST_BLOOM
+void *adu_calloc(size_t size)
+{
+       void *ret = malloc(size);
+       memset(ret, 0, size);
+       return ret;
+}
+#undef INFO_LOG
+#define INFO_LOG printf
+#endif
+
+static inline uint64_t filter_bits(struct bloom *b)
+{
+       return 1ULL << b->order;
+}
+
+/*
+ * SuperFastHash, by Paul Hsieh.
+ * http://www.azillionmonkeys.com/qed/hash.html
+ */
+
+#if (defined(__GNUC__) && defined(__i386__))
+#define get16bits(d) (*((const uint16_t *) (d)))
+#else
+#define get16bits(d) ((((uint32_t)(((const uint8_t *)(d))[1])) << 8)\
+                       +(uint32_t)(((const uint8_t *)(d))[0]) )
+#endif
+
+
+static uint32_t super_fast_hash(const uint8_t *data, uint32_t len, uint32_t hash)
+{
+       uint32_t tmp;
+       int rem = len & 3;
+
+       len >>= 2;
+
+       for (;len > 0; len--) {
+               hash  += get16bits (data);
+               tmp    = (get16bits (data+2) << 11) ^ hash;
+               hash   = (hash << 16) ^ tmp;
+               data  += 2*sizeof (uint16_t);
+               hash  += hash >> 11;
+       }
+
+       /* Handle end cases */
+       switch (rem) {
+       case 3:
+               hash += get16bits (data);
+               hash ^= hash << 16;
+               hash ^= data[sizeof (uint16_t)] << 18;
+               hash += hash >> 11;
+               break;
+       case 2:
+               hash += get16bits (data);
+               hash ^= hash << 11;
+               hash += hash >> 17;
+               break;
+       case 1:
+               hash += *data;
+               hash ^= hash << 10;
+               hash += hash >> 1;
+       }
+       /* Force "avalanching" of final 127 bits */
+       hash ^= hash << 3;
+       hash += hash >> 5;
+       hash ^= hash << 4;
+       hash += hash >> 17;
+       hash ^= hash << 25;
+       hash += hash >> 6;
+       return hash;
+}
+
+static int test_and_set_bit(uint64_t bitnum, struct bloom *b)
+{
+       uint64_t byte = bitnum / 8;
+       uint8_t bit = 1 << (bitnum % 8);
+       int ret = b->filter[byte] & bit;
+
+       b->filter[byte] |= bit;
+
+       if (!ret)
+               b->num_set_bits++;
+       return ret;
+}
+
+int bloom_test_and_insert(const uint8_t *data, size_t len, struct bloom *b)
+{
+       int i, ret = 0;
+       uint32_t h = 0xb11924e1; /* some arbitrary value */
+
+       for (i = 1; i < b->num_hash_functions; i++) {
+               uint64_t bitnum = super_fast_hash(data, len, h);
+               h = bitnum;
+               if (b->order > 32) {
+                       bitnum = bitnum << 32;
+                       h = super_fast_hash(data, len, h);
+                       bitnum |= h;
+               }
+               bitnum &= filter_bits(b) - 1;
+               ret |= !test_and_set_bit(bitnum, b);
+       }
+       if (ret)
+               b->num_entries++;
+       return !ret;
+}
+
+int bloom_test_and_insert_string(const char *str, struct bloom *b)
+{
+       uint32_t len = strlen(str);
+
+       return bloom_test_and_insert((const uint8_t *)str, len, b);
+}
+
+void bloom_free(struct bloom *b)
+{
+       if (!b)
+               return;
+       free(b->filter);
+       free(b);
+}
+
+int bloom_init(unsigned order, unsigned num_hash_functions,
+               struct bloom **result)
+{
+       struct bloom *b = adu_calloc(sizeof(*b));
+
+       INFO_LOG("allocating bloom filter (order: %u, %u hash functions)\n",
+               order, num_hash_functions);
+       b->order = order;
+       b->num_hash_functions = num_hash_functions;
+       b->filter = adu_calloc(filter_bits(b) / 8);
+       *result = b;
+       return 1;
+}
+
+#ifdef TEST_BLOOM
+
+void add_stdin(struct bloom *b)
+{
+       char buf[255];
+       unsigned false_positives = 0;
+
+       while (fgets(buf, sizeof(buf) - 1, stdin)) {
+               size_t len = strlen(buf);
+               if (!len)
+                       continue;
+               if (buf[len - 1] == '\n')
+                       buf[len - 1] = '\0';
+               if (bloom_test_and_insert_string(buf, b))
+                       false_positives++;
+       }
+       INFO_LOG("filter contains %llu entries\n", b->num_entries);
+       printf("%u possible false positives\n", false_positives);
+}
+
+int main(int argc, char **argv)
+{
+       struct bloom *b;
+
+       if (argc != 3) {
+               printf("usage: $0 j k\n");
+               printf("j: set bloom filter size to m=2^j bits\n");
+               printf("k: # of hash functions to use\n");
+               exit(1);
+       }
+       bloom_init(atoi(argv[1]), atoi(argv[2]), &b);
+       add_stdin(b);
+       INFO_LOG("%u%% of bits are set\n", (unsigned)
+               (b->num_set_bits * 100ULL / filter_bits(b)));
+       return 1;
+}
+
+#endif /* TEST_BLOOM */