#include <stdlib.h>
#include <string.h>
-#include <zlib.h>
#include "delta.h"
-/* block size: min = 16, max = 64k, power of 2 */
-#define BLK_SIZE 16
-
-#define MIN(a, b) ((a) < (b) ? (a) : (b))
-
-#define GR_PRIME 0x9e370001
-#define HASH(v, shift) (((unsigned int)(v) * GR_PRIME) >> (shift))
-
struct index {
const unsigned char *ptr;
- unsigned int val;
struct index *next;
};
static struct index ** delta_index(const unsigned char *buf,
unsigned long bufsize,
- unsigned int *hash_shift)
+ unsigned long trg_bufsize)
{
- unsigned int hsize, hshift, entries, blksize, i;
+ unsigned long hsize;
+ unsigned int i, hshift, hlimit, *hash_count;
const unsigned char *data;
struct index *entry, **hash;
void *mem;
/* determine index hash size */
- entries = (bufsize + BLK_SIZE - 1) / BLK_SIZE;
- hsize = entries / 4;
- for (i = 4; (1 << i) < hsize && i < 16; i++);
+ hsize = bufsize / 4;
+ for (i = 8; (1 << i) < hsize && i < 24; i += 2);
hsize = 1 << i;
- hshift = 32 - i;
- *hash_shift = hshift;
+ hshift = (i - 8) / 2;
- /* allocate lookup index */
- mem = malloc(hsize * sizeof(*hash) + entries * sizeof(*entry));
+ /*
+ * Allocate lookup index. Note the first hash pointer
+ * is used to store the hash shift value.
+ */
+ mem = malloc((1 + hsize) * sizeof(*hash) + bufsize * sizeof(*entry));
if (!mem)
return NULL;
hash = mem;
- entry = mem + hsize * sizeof(*hash);
+ *hash++ = (void *)hshift;
+ entry = mem + (1 + hsize) * sizeof(*hash);
memset(hash, 0, hsize * sizeof(*hash));
- /* then populate it */
- data = buf + entries * BLK_SIZE - BLK_SIZE;
- blksize = bufsize - (data - buf);
- while (data >= buf) {
- unsigned int val = adler32(0, data, blksize);
- i = HASH(val, hshift);
- entry->ptr = data;
- entry->val = val;
+ /* 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 + bufsize - 2;
+ while (data > buf) {
+ entry->ptr = --data;
+ i = data[0] ^ ((data[1] ^ (data[2] << hshift)) << hshift);
entry->next = hash[i];
hash[i] = entry++;
- blksize = BLK_SIZE;
- data -= BLK_SIZE;
+ hash_count[i]++;
}
- return hash;
+ /*
+ * 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 < 16)
+ hlimit = 16;
+
+ /*
+ * Now make sure none of the hash buckets has more entries than
+ * we're willing to test. Otherwise we cull the entry list to
+ * limit identical three byte prefixes to still preserve a good
+ * repartition across the reference buffer.
+ */
+ for (i = 0; i < hsize; i++) {
+ struct index **list, *bucket, *remaining;
+ int cnt;
+ if (hash_count[i] < hlimit)
+ continue;
+
+ bucket = NULL;
+ list = &bucket;
+ remaining = hash[i];
+ cnt = 0;
+ while (cnt < hlimit && remaining) {
+ struct index *this = remaining, *that;
+ remaining = remaining->next;
+ for (that = bucket; that; that = that->next) {
+ if (!memcmp(that->ptr, this->ptr, 3))
+ break;
+ }
+ if (that)
+ continue; /* discard */
+ cnt++;
+ *list = this;
+ list = &(this->next);
+ this->next = NULL;
+ }
+ hash[i] = bucket;
+ }
+ free(hash_count);
+
+ return hash-1;
}
/* provide the size of the copy opcode given the block offset and size */
void *diff_delta(void *from_buf, unsigned long from_size,
void *to_buf, unsigned long to_size,
unsigned long *delta_size,
- unsigned long max_size)
+ unsigned long max_size,
+ void **from_index)
{
unsigned int i, outpos, outsize, inscnt, hash_shift;
const unsigned char *ref_data, *ref_top, *data, *top;
if (!from_size || !to_size)
return NULL;
- hash = delta_index(from_buf, from_size, &hash_shift);
- if (!hash)
- return NULL;
+ if (from_index && *from_index) {
+ hash = *from_index;
+ } else {
+ hash = delta_index(from_buf, from_size, to_size);
+ if (!hash)
+ return NULL;
+ if (from_index)
+ *from_index = hash;
+ }
+ hash_shift = (unsigned int)(*hash++);
outpos = 0;
outsize = 8192;
outsize = max_size + MAX_OP_SIZE + 1;
out = malloc(outsize);
if (!out) {
- free(hash);
+ if (!from_index)
+ free(hash-1);
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 + 3 <= top) {
+ i = data[0] ^ ((data[1] ^ (data[2] << hash_shift)) << 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 (ref_size > top - src)
+ ref_size = top - src;
+ if (ref_size > 0x10000)
+ ref_size = 0x10000;
+ if (ref_size <= msize)
break;
+ if (*ref != *src)
+ continue;
+ while (ref_size-- && *++src == *++ref);
+ if (msize < ref - entry->ptr) {
+ /* this is our best match so far */
+ msize = ref - entry->ptr;
+ moff = entry->ptr - ref_data;
}
}
}
out = realloc(out, outsize);
if (!out) {
free(tmp);
- free(hash);
+ if (!from_index)
+ free(hash-1);
return NULL;
}
}
if (inscnt)
out[outpos - inscnt - 1] = inscnt;
- free(hash);
+ if (!from_index)
+ free(hash-1);
*delta_size = outpos;
return out;
}
projects / git.git / blobdiff