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split the diff-delta interface
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This patch splits the diff-delta interface into index creation and delta
generation.  A wrapper is provided to preserve the diff-delta() call.

This will allow for an optimization in pack-objects.c where the source
object could be fixed and a full window of objects tentatively tried
against
that same source object without recomputing the source index each time.

This patch only restructure things, plus a couple cleanups for good
measure. There is no performance change yet.

Signed-off-by: Nicolas Pitre <[email protected]>
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Nicolas Pitre authored and Junio C Hamano committed Apr 25, 2006
1 parent 36932ea commit 08abe66
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Showing 3 changed files with 155 additions and 92 deletions.
75 changes: 68 additions & 7 deletions delta.h
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Original file line number Diff line number Diff line change
@@ -1,20 +1,81 @@
#ifndef DELTA_H
#define DELTA_H

/* handling of delta buffers */
extern 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);
extern void *patch_delta(void *src_buf, unsigned long src_size,
void *delta_buf, unsigned long delta_size,
/* opaque object for delta index */
struct delta_index;

/*
* create_delta_index: compute index data from given buffer
*
* This returns a pointer to a struct delta_index that should be passed to
* subsequent create_delta() calls, or to free_delta_index(). A NULL pointer
* is returned on failure. The given buffer must not be freed nor altered
* before free_delta_index() is called. The returned pointer must be freed
* using free_delta_index().
*/
extern struct delta_index *
create_delta_index(const void *buf, unsigned long bufsize);

/*
* free_delta_index: free the index created by create_delta_index()
*/
extern void free_delta_index(struct delta_index *index);

/*
* create_delta: create a delta from given index for the given buffer
*
* This function may be called multiple times with different buffers using
* the same delta_index pointer. If max_delta_size is non-zero and the
* resulting delta is to be larger than max_delta_size then NULL is returned.
* On success, a non-NULL pointer to the buffer with the delta data is
* returned and *delta_size is updated with its size. The returned buffer
* must be freed by the caller.
*/
extern void *
create_delta(const struct delta_index *index,
const void *buf, unsigned long bufsize,
unsigned long *delta_size, unsigned long max_delta_size);

/*
* diff_delta: create a delta from source buffer to target buffer
*
* If max_delta_size is non-zero and the resulting delta is to be larger
* than max_delta_size then NULL is returned. On success, a non-NULL
* pointer to the buffer with the delta data is returned and *delta_size is
* updated with its size. The returned buffer must be freed by the caller.
*/
static inline void *
diff_delta(const void *src_buf, unsigned long src_bufsize,
const void *trg_buf, unsigned long trg_bufsize,
unsigned long *delta_size, unsigned long max_delta_size)
{
struct delta_index *index = create_delta_index(src_buf, src_bufsize);
if (index) {
void *delta = create_delta(index, trg_buf, trg_bufsize,
delta_size, max_delta_size);
free_delta_index(index);
return delta;
}
return NULL;
}

/*
* patch_delta: recreate target buffer given source buffer and delta data
*
* On success, a non-NULL pointer to the target buffer is returned and
* *trg_bufsize is updated with its size. On failure a NULL pointer is
* returned. The returned buffer must be freed by the caller.
*/
extern void *patch_delta(const void *src_buf, unsigned long src_size,
const void *delta_buf, unsigned long delta_size,
unsigned long *dst_size);

/* the smallest possible delta size is 4 bytes */
#define DELTA_SIZE_MIN 4

/*
* This must be called twice on the delta data buffer, first to get the
* expected reference buffer size, and again to get the result buffer size.
* expected source buffer size, and again to get the target buffer size.
*/
static inline unsigned long get_delta_hdr_size(const unsigned char **datap,
const unsigned char *top)
Expand Down
168 changes: 85 additions & 83 deletions diff-delta.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -27,53 +27,70 @@
/* block size: min = 16, max = 64k, power of 2 */
#define BLK_SIZE 16

#define MIN(a, b) ((a) < (b) ? (a) : (b))
/* maximum hash entry list for the same hash bucket */
#define HASH_LIMIT 64

#define GR_PRIME 0x9e370001
#define HASH(v, shift) (((unsigned int)(v) * GR_PRIME) >> (shift))

struct index {
struct index_entry {
const unsigned char *ptr;
unsigned int val;
struct index *next;
struct index_entry *next;
};

static struct index ** delta_index(const unsigned char *buf,
unsigned long bufsize,
unsigned long trg_bufsize,
unsigned int *hash_shift)
struct delta_index {
const void *src_buf;
unsigned long src_size;
unsigned int hash_shift;
struct index_entry *hash[0];
};

struct delta_index * create_delta_index(const void *buf, unsigned long bufsize)
{
unsigned int i, hsize, hshift, hlimit, entries, *hash_count;
const unsigned char *data;
struct index *entry, **hash;
unsigned int i, hsize, hshift, entries, *hash_count;
const unsigned char *data, *buffer = buf;
struct delta_index *index;
struct index_entry *entry, **hash;
void *mem;

if (!buf || !bufsize)
return NULL;

/* determine index hash size */
entries = bufsize / BLK_SIZE;
hsize = entries / 4;
for (i = 4; (1 << i) < hsize && i < 31; i++);
hsize = 1 << i;
hshift = 32 - i;
*hash_shift = hshift;

/* allocate lookup index */
mem = malloc(hsize * sizeof(*hash) + entries * sizeof(*entry));
mem = malloc(sizeof(*index) +
sizeof(*hash) * hsize +
sizeof(*entry) * entries);
if (!mem)
return NULL;
index = mem;
mem = index + 1;
hash = mem;
entry = mem + hsize * sizeof(*hash);
mem = hash + hsize;
entry = mem;

index->src_buf = buf;
index->src_size = bufsize;
index->hash_shift = hshift;
memset(hash, 0, hsize * sizeof(*hash));

/* allocate an array to count hash entries */
hash_count = calloc(hsize, sizeof(*hash_count));
if (!hash_count) {
free(hash);
free(index);
return NULL;
}

/* then populate the index */
data = buf + entries * BLK_SIZE - BLK_SIZE;
while (data >= buf) {
data = buffer + entries * BLK_SIZE - BLK_SIZE;
while (data >= buffer) {
unsigned int val = adler32(0, data, BLK_SIZE);
i = HASH(val, hshift);
entry->ptr = data;
Expand All @@ -91,27 +108,18 @@ static struct index ** delta_index(const unsigned char *buf,
* 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
* 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)
if (hash_count[i] < HASH_LIMIT)
continue;
entry = hash[i];
do {
struct index *keep = entry;
int skip = hash_count[i] / hlimit / 2;
struct index_entry *keep = entry;
int skip = hash_count[i] / HASH_LIMIT / 2;
do {
entry = entry->next;
} while(--skip && entry);
Expand All @@ -120,7 +128,12 @@ static struct index ** delta_index(const unsigned char *buf,
}
free(hash_count);

return hash;
return index;
}

void free_delta_index(struct delta_index *index)
{
free(index);
}

/* provide the size of the copy opcode given the block offset and size */
Expand All @@ -131,82 +144,73 @@ static struct index ** delta_index(const unsigned char *buf,
/* the maximum size for any opcode */
#define MAX_OP_SIZE COPYOP_SIZE(0xffffffff, 0xffffffff)

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)
void *
create_delta(const struct delta_index *index,
const void *trg_buf, unsigned long trg_size,
unsigned long *delta_size, unsigned long max_size)
{
unsigned int i, outpos, outsize, hash_shift;
int inscnt;
const unsigned char *ref_data, *ref_top, *data, *top;
unsigned char *out;
struct index *entry, **hash;

if (!from_size || !to_size)
return NULL;
hash = delta_index(from_buf, from_size, to_size, &hash_shift);
if (!hash)
if (!trg_buf || !trg_size)
return NULL;

outpos = 0;
outsize = 8192;
if (max_size && outsize >= max_size)
outsize = max_size + MAX_OP_SIZE + 1;
out = malloc(outsize);
if (!out) {
free(hash);
if (!out)
return NULL;
}

ref_data = from_buf;
ref_top = from_buf + from_size;
data = to_buf;
top = to_buf + to_size;

/* store reference buffer size */
out[outpos++] = from_size;
from_size >>= 7;
while (from_size) {
out[outpos - 1] |= 0x80;
out[outpos++] = from_size;
from_size >>= 7;
i = index->src_size;
while (i >= 0x80) {
out[outpos++] = i | 0x80;
i >>= 7;
}
out[outpos++] = i;

/* store target buffer size */
out[outpos++] = to_size;
to_size >>= 7;
while (to_size) {
out[outpos - 1] |= 0x80;
out[outpos++] = to_size;
to_size >>= 7;
i = trg_size;
while (i >= 0x80) {
out[outpos++] = i | 0x80;
i >>= 7;
}
out[outpos++] = i;

ref_data = index->src_buf;
ref_top = ref_data + index->src_size;
data = trg_buf;
top = trg_buf + trg_size;
hash_shift = index->hash_shift;
inscnt = 0;

while (data < top) {
unsigned int moff = 0, msize = 0;
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;
}
struct index_entry *entry;
unsigned int val = adler32(0, data, BLK_SIZE);
i = HASH(val, hash_shift);
for (entry = index->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;
}
}

Expand Down Expand Up @@ -271,7 +275,6 @@ void *diff_delta(void *from_buf, unsigned long from_size,
out = realloc(out, outsize);
if (!out) {
free(tmp);
free(hash);
return NULL;
}
}
Expand All @@ -280,7 +283,6 @@ void *diff_delta(void *from_buf, unsigned long from_size,
if (inscnt)
out[outpos - inscnt - 1] = inscnt;

free(hash);
*delta_size = outpos;
return out;
}
4 changes: 2 additions & 2 deletions patch-delta.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -13,8 +13,8 @@
#include <string.h>
#include "delta.h"

void *patch_delta(void *src_buf, unsigned long src_size,
void *delta_buf, unsigned long delta_size,
void *patch_delta(const void *src_buf, unsigned long src_size,
const void *delta_buf, unsigned long delta_size,
unsigned long *dst_size)
{
const unsigned char *data, *top;
Expand Down

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