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improve diff-delta with sparse and/or repetitive data

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It is useless to preserve multiple hash entries for consecutive blocks
with the same hash.  Keeping only the first one will allow for matching
the longest string of identical bytes while subsequent blocks will only
allow for shorter matches.  The backward matching code will match the
end of it as necessary.

This improves both performances (no repeated string compare with long
successions of identical bytes, or even small group of bytes), as well
as compression (less likely to need random hash bucket entry culling),
especially with sparse files.

With well behaved data sets this patch doesn't change much.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>
This commit is contained in:
Nicolas Pitre 2006-05-02 23:31:00 -04:00 committed by Junio C Hamano
parent 2d08e5dd73
commit 06a9f92035
1 changed files with 27 additions and 13 deletions
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40
diff-delta.c
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@ -136,11 +136,12 @@ struct delta_index {
struct delta_index * create_delta_index(const void *buf, unsigned long bufsize)
{
unsigned int i, hsize, hmask, entries, *hash_count;
unsigned int i, hsize, hmask, entries, prev_val, *hash_count;
const unsigned char *data, *buffer = buf;
struct delta_index *index;
struct index_entry *entry, **hash;
void *mem;
unsigned long memsize;
if (!buf || !bufsize)
return NULL;
@ -155,9 +156,10 @@ struct delta_index * create_delta_index(const void *buf, unsigned long bufsize)
hmask = hsize - 1;
/* allocate lookup index */
mem = malloc(sizeof(*index) +
sizeof(*hash) * hsize +
sizeof(*entry) * entries);
memsize = sizeof(*index) +
sizeof(*hash) * hsize +
sizeof(*entry) * entries;
mem = malloc(memsize);
if (!mem)
return NULL;
index = mem;
@ -179,18 +181,26 @@ struct delta_index * create_delta_index(const void *buf, unsigned long bufsize)
}
/* then populate the index */
data = buffer + entries * RABIN_WINDOW - RABIN_WINDOW;
while (data >= buffer) {
prev_val = ~0;
for (data = buffer + entries * RABIN_WINDOW - RABIN_WINDOW;
data >= buffer;
data -= RABIN_WINDOW) {
unsigned int val = 0;
for (i = 1; i <= RABIN_WINDOW; i++)
val = ((val << 8) | data[i]) ^ T[val >> RABIN_SHIFT];
i = val & hmask;
entry->ptr = data + RABIN_WINDOW;
entry->val = val;
entry->next = hash[i];
hash[i] = entry++;
hash_count[i]++;
data -= RABIN_WINDOW;
if (val == prev_val) {
/* keep the lowest of consecutive identical blocks */
entry[-1].ptr = data + RABIN_WINDOW;
} else {
prev_val = val;
i = val & hmask;
entry->ptr = data + RABIN_WINDOW;
entry->val = val;
entry->next = hash[i];
hash[i] = entry++;
hash_count[i]++;
entries--;
}
}
/*
@ -220,6 +230,10 @@ struct delta_index * create_delta_index(const void *buf, unsigned long bufsize)
}
free(hash_count);
/* If we didn't use all hash entries, free the unused memory. */
if (entries)
index = realloc(index, memsize - entries * sizeof(*entry));
return index;
}