static struct index ** delta_index(const unsigned char *buf,
unsigned long bufsize,
+ unsigned long trg_bufsize,
unsigned int *hash_shift)
{
- unsigned int hsize, hshift, entries, blksize, i;
+ unsigned int i, hsize, hshift, hlimit, entries, *hash_count;
const unsigned char *data;
struct index *entry, **hash;
void *mem;
/* determine index hash size */
- entries = (bufsize + BLK_SIZE - 1) / BLK_SIZE;
+ entries = bufsize / BLK_SIZE;
hsize = entries / 4;
- for (i = 4; (1 << i) < hsize && i < 16; i++);
+ for (i = 4; (1 << i) < hsize && i < 31; i++);
hsize = 1 << i;
hshift = 32 - i;
*hash_shift = hshift;
entry = mem + hsize * sizeof(*hash);
memset(hash, 0, hsize * sizeof(*hash));
- /* then populate it */
+ /* allocate an array to count hash entries */
+ hash_count = calloc(hsize, sizeof(*hash_count));
+ if (!hash_count) {
+ free(hash);
+ return NULL;
+ }
+
+ /* then populate the index */
data = buf + entries * BLK_SIZE - BLK_SIZE;
- blksize = bufsize - (data - buf);
while (data >= buf) {
- unsigned int val = adler32(0, data, blksize);
+ unsigned int val = adler32(0, data, BLK_SIZE);
i = HASH(val, hshift);
entry->ptr = data;
entry->val = val;
entry->next = hash[i];
hash[i] = entry++;
- blksize = BLK_SIZE;
+ hash_count[i]++;
data -= BLK_SIZE;
}
+ /*
+ * Determine a limit on the number of entries in the same hash
+ * bucket. This guard us against patological data sets causing
+ * really bad hash distribution with most entries in the same hash
+ * bucket that would bring us to O(m*n) computing costs (m and n
+ * corresponding to reference and target buffer sizes).
+ *
+ * The more the target buffer is large, the more it is important to
+ * have small entry lists for each hash buckets. With such a limit
+ * the cost is bounded to something more like O(m+n).
+ */
+ hlimit = (1 << 26) / trg_bufsize;
+ if (hlimit < 4*BLK_SIZE)
+ hlimit = 4*BLK_SIZE;
+
+ /*
+ * Now make sure none of the hash buckets has more entries than
+ * we're willing to test. Otherwise we cull the entry list
+ * uniformly to still preserve a good repartition across
+ * the reference buffer.
+ */
+ for (i = 0; i < hsize; i++) {
+ if (hash_count[i] < hlimit)
+ continue;
+ entry = hash[i];
+ do {
+ struct index *keep = entry;
+ int skip = hash_count[i] / hlimit / 2;
+ do {
+ entry = entry->next;
+ } while(--skip && entry);
+ keep->next = entry;
+ } while(entry);
+ }
+ free(hash_count);
+
return hash;
}
if (!from_size || !to_size)
return NULL;
- hash = delta_index(from_buf, from_size, &hash_shift);
+ hash = delta_index(from_buf, from_size, to_size, &hash_shift);
if (!hash)
return NULL;
while (data < top) {
unsigned int moff = 0, msize = 0;
- unsigned int blksize = MIN(top - data, BLK_SIZE);
- unsigned int val = adler32(0, data, blksize);
- i = HASH(val, hash_shift);
- for (entry = hash[i]; entry; entry = entry->next) {
- const unsigned char *ref = entry->ptr;
- const unsigned char *src = data;
- unsigned int ref_size = ref_top - ref;
- if (entry->val != val)
- continue;
- if (ref_size > top - src)
- ref_size = top - src;
- while (ref_size && *src++ == *ref) {
- ref++;
- ref_size--;
- }
- ref_size = ref - entry->ptr;
- if (ref_size > msize) {
- /* this is our best match so far */
- moff = entry->ptr - ref_data;
- msize = ref_size;
- if (msize >= 0x10000) {
- msize = 0x10000;
+ if (data + BLK_SIZE <= top) {
+ unsigned int val = adler32(0, data, BLK_SIZE);
+ i = HASH(val, hash_shift);
+ for (entry = hash[i]; entry; entry = entry->next) {
+ const unsigned char *ref = entry->ptr;
+ const unsigned char *src = data;
+ unsigned int ref_size = ref_top - ref;
+ if (entry->val != val)
+ continue;
+ if (ref_size > top - src)
+ ref_size = top - src;
+ if (ref_size > 0x10000)
+ ref_size = 0x10000;
+ if (ref_size <= msize)
break;
+ while (ref_size-- && *src++ == *ref)
+ ref++;
+ if (msize < ref - entry->ptr) {
+ /* this is our best match so far */
+ msize = ref - entry->ptr;
+ moff = entry->ptr - ref_data;
}
}
}