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redict/zipmap.c

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/* String -> String Map data structure optimized for size.
* This file implements a data structure mapping strings to other strings
* implementing an O(n) lookup data structure designed to be very memory
* efficient.
*
* The Redis Hash type uses this data structure for hashes composed of a small
* number of elements, to switch to an hash table once a given number of
* elements is reached.
*
* Given that many times Redis Hashes are used to represent objects composed
* of few fields, this is a very big win in terms of used memory.
*
* --------------------------------------------------------------------------
*
* Copyright (c) 2009-2010, Salvatore Sanfilippo <antirez at gmail dot com>
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* * Redistributions of source code must retain the above copyright notice,
* this list of conditions and the following disclaimer.
* * Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* * Neither the name of Redis nor the names of its contributors may be used
* to endorse or promote products derived from this software without
* specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*/
/* Memory layout of a zipmap, for the map "foo" => "bar", "hello" => "world":
*
* <zmlen><len>"foo"<len><free>"bar"<len>"hello"<len><free>"world"
*
* <zmlen> is 1 byte length that holds the current size of the zipmap.
* When the zipmap length is greater than or equal to 254, this value
* is not used and the zipmap needs to be traversed to find out the length.
*
* <len> is the length of the following string (key or value).
* <len> lengths are encoded in a single value or in a 5 bytes value.
* If the first byte value (as an unsigned 8 bit value) is between 0 and
* 252, it's a single-byte length. If it is 253 then a four bytes unsigned
* integer follows (in the host byte ordering). A value fo 255 is used to
* signal the end of the hash. The special value 254 is used to mark
* empty space that can be used to add new key/value pairs.
*
* <free> is the number of free unused bytes
* after the string, resulting from modification of values associated to a
* key (for instance if "foo" is set to "bar', and later "foo" will be se to
* "hi", I'll have a free byte to use if the value will enlarge again later,
* or even in order to add a key/value pair if it fits.
*
* <free> is always an unsigned 8 bit number, because if after an
* update operation there are more than a few free bytes, the zipmap will be
* reallocated to make sure it is as small as possible.
*
* The most compact representation of the above two elements hash is actually:
*
* "\x02\x03foo\x03\x00bar\x05hello\x05\x00world\xff"
*
* Note that because keys and values are prefixed length "objects",
* the lookup will take O(N) where N is the number of elements
* in the zipmap and *not* the number of bytes needed to represent the zipmap.
* This lowers the constant times considerably.
*/
#include <stdio.h>
#include <string.h>
#include <assert.h>
#include "zmalloc.h"
#define ZIPMAP_BIGLEN 254
#define ZIPMAP_END 255
/* The following defines the max value for the <free> field described in the
* comments above, that is, the max number of trailing bytes in a value. */
#define ZIPMAP_VALUE_MAX_FREE 4
/* The following macro returns the number of bytes needed to encode the length
* for the integer value _l, that is, 1 byte for lengths < ZIPMAP_BIGLEN and
* 5 bytes for all the other lengths. */
#define ZIPMAP_LEN_BYTES(_l) (((_l) < ZIPMAP_BIGLEN) ? 1 : sizeof(unsigned int)+1)
/* Create a new empty zipmap. */
unsigned char *zipmapNew(void) {
unsigned char *zm = zmalloc(2);
zm[0] = 0; /* Length */
zm[1] = ZIPMAP_END;
return zm;
}
/* Decode the encoded length pointed by 'p' */
static unsigned int zipmapDecodeLength(unsigned char *p) {
unsigned int len = *p;
if (len < ZIPMAP_BIGLEN) return len;
memcpy(&len,p+1,sizeof(unsigned int));
return len;
}
/* Encode the length 'l' writing it in 'p'. If p is NULL it just returns
* the amount of bytes required to encode such a length. */
static unsigned int zipmapEncodeLength(unsigned char *p, unsigned int len) {
if (p == NULL) {
return ZIPMAP_LEN_BYTES(len);
} else {
if (len < ZIPMAP_BIGLEN) {
p[0] = len;
return 1;
} else {
p[0] = ZIPMAP_BIGLEN;
memcpy(p+1,&len,sizeof(len));
return 1+sizeof(len);
}
}
}
/* Search for a matching key, returning a pointer to the entry inside the
* zipmap. Returns NULL if the key is not found.
*
* If NULL is returned, and totlen is not NULL, it is set to the entire
* size of the zimap, so that the calling function will be able to
* reallocate the original zipmap to make room for more entries. */
static unsigned char *zipmapLookupRaw(unsigned char *zm, unsigned char *key, unsigned int klen, unsigned int *totlen) {
unsigned char *p = zm+1, *k = NULL;
unsigned int l;
while(*p != ZIPMAP_END) {
unsigned char free;
/* Match or skip the key */
l = zipmapDecodeLength(p);
if (k == NULL && l == klen && !memcmp(p+1,key,l)) {
/* Only return when the user doesn't care
* for the total length of the zipmap. */
if (totlen != NULL) {
k = p;
} else {
return p;
}
}
p += zipmapEncodeLength(NULL,l) + l;
/* Skip the value as well */
l = zipmapDecodeLength(p);
p += zipmapEncodeLength(NULL,l);
free = p[0];
p += l+1+free; /* +1 to skip the free byte */
}
if (totlen != NULL) *totlen = (unsigned int)(p-zm)+1;
return k;
}
static unsigned long zipmapRequiredLength(unsigned int klen, unsigned int vlen) {
unsigned int l;
l = klen+vlen+3;
if (klen >= ZIPMAP_BIGLEN) l += 4;
if (vlen >= ZIPMAP_BIGLEN) l += 4;
return l;
}
/* Return the total amount used by a key (encoded length + payload) */
static unsigned int zipmapRawKeyLength(unsigned char *p) {
unsigned int l = zipmapDecodeLength(p);
return zipmapEncodeLength(NULL,l) + l;
}
/* Return the total amount used by a value
* (encoded length + single byte free count + payload) */
static unsigned int zipmapRawValueLength(unsigned char *p) {
unsigned int l = zipmapDecodeLength(p);
unsigned int used;
used = zipmapEncodeLength(NULL,l);
used += p[used] + 1 + l;
return used;
}
/* If 'p' points to a key, this function returns the total amount of
* bytes used to store this entry (entry = key + associated value + trailing
* free space if any). */
static unsigned int zipmapRawEntryLength(unsigned char *p) {
unsigned int l = zipmapRawKeyLength(p);
return l + zipmapRawValueLength(p+l);
}
static inline unsigned char *zipmapResize(unsigned char *zm, unsigned int len) {
zm = zrealloc(zm, len);
zm[len-1] = ZIPMAP_END;
return zm;
}
/* Set key to value, creating the key if it does not already exist.
* If 'update' is not NULL, *update is set to 1 if the key was
* already preset, otherwise to 0. */
unsigned char *zipmapSet(unsigned char *zm, unsigned char *key, unsigned int klen, unsigned char *val, unsigned int vlen, int *update) {
unsigned int zmlen, offset;
unsigned int freelen, reqlen = zipmapRequiredLength(klen,vlen);
unsigned int empty, vempty;
unsigned char *p;
freelen = reqlen;
if (update) *update = 0;
p = zipmapLookupRaw(zm,key,klen,&zmlen);
if (p == NULL) {
/* Key not found: enlarge */
zm = zipmapResize(zm, zmlen+reqlen);
p = zm+zmlen-1;
zmlen = zmlen+reqlen;
/* Increase zipmap length (this is an insert) */
if (zm[0] < ZIPMAP_BIGLEN) zm[0]++;
} else {
/* Key found. Is there enough space for the new value? */
/* Compute the total length: */
if (update) *update = 1;
freelen = zipmapRawEntryLength(p);
if (freelen < reqlen) {
/* Store the offset of this key within the current zipmap, so
* it can be resized. Then, move the tail backwards so this
* pair fits at the current position. */
offset = p-zm;
zm = zipmapResize(zm, zmlen-freelen+reqlen);
p = zm+offset;
/* The +1 in the number of bytes to be moved is caused by the
* end-of-zipmap byte. Note: the *original* zmlen is used. */
memmove(p+reqlen, p+freelen, zmlen-(offset+freelen+1));
zmlen = zmlen-freelen+reqlen;
freelen = reqlen;
}
}
/* We now have a suitable block where the key/value entry can
* be written. If there is too much free space, move the tail
* of the zipmap a few bytes to the front and shrink the zipmap,
* as we want zipmaps to be very space efficient. */
empty = freelen-reqlen;
if (empty >= ZIPMAP_VALUE_MAX_FREE) {
/* First, move the tail <empty> bytes to the front, then resize
* the zipmap to be <empty> bytes smaller. */
offset = p-zm;
memmove(p+reqlen, p+freelen, zmlen-(offset+freelen+1));
zmlen -= empty;
zm = zipmapResize(zm, zmlen);
p = zm+offset;
vempty = 0;
} else {
vempty = empty;
}
/* Just write the key + value and we are done. */
/* Key: */
p += zipmapEncodeLength(p,klen);
memcpy(p,key,klen);
p += klen;
/* Value: */
p += zipmapEncodeLength(p,vlen);
*p++ = vempty;
memcpy(p,val,vlen);
return zm;
}
/* Remove the specified key. If 'deleted' is not NULL the pointed integer is
* set to 0 if the key was not found, to 1 if it was found and deleted. */
unsigned char *zipmapDel(unsigned char *zm, unsigned char *key, unsigned int klen, int *deleted) {
unsigned int zmlen, freelen;
unsigned char *p = zipmapLookupRaw(zm,key,klen,&zmlen);
if (p) {
freelen = zipmapRawEntryLength(p);
memmove(p, p+freelen, zmlen-((p-zm)+freelen+1));
zm = zipmapResize(zm, zmlen-freelen);
/* Decrease zipmap length */
if (zm[0] < ZIPMAP_BIGLEN) zm[0]--;
if (deleted) *deleted = 1;
} else {
if (deleted) *deleted = 0;
}
return zm;
}
/* Call it before to iterate trought elements via zipmapNext() */
unsigned char *zipmapRewind(unsigned char *zm) {
return zm+1;
}
/* This function is used to iterate through all the zipmap elements.
* In the first call the first argument is the pointer to the zipmap + 1.
* In the next calls what zipmapNext returns is used as first argument.
* Example:
*
* unsigned char *i = zipmapRewind(my_zipmap);
* while((i = zipmapNext(i,&key,&klen,&value,&vlen)) != NULL) {
* printf("%d bytes key at $p\n", klen, key);
* printf("%d bytes value at $p\n", vlen, value);
* }
*/
unsigned char *zipmapNext(unsigned char *zm, unsigned char **key, unsigned int *klen, unsigned char **value, unsigned int *vlen) {
if (zm[0] == ZIPMAP_END) return NULL;
if (key) {
*key = zm;
*klen = zipmapDecodeLength(zm);
*key += ZIPMAP_LEN_BYTES(*klen);
}
zm += zipmapRawKeyLength(zm);
if (value) {
*value = zm+1;
*vlen = zipmapDecodeLength(zm);
*value += ZIPMAP_LEN_BYTES(*vlen);
}
zm += zipmapRawValueLength(zm);
return zm;
}
/* Search a key and retrieve the pointer and len of the associated value.
* If the key is found the function returns 1, otherwise 0. */
int zipmapGet(unsigned char *zm, unsigned char *key, unsigned int klen, unsigned char **value, unsigned int *vlen) {
unsigned char *p;
if ((p = zipmapLookupRaw(zm,key,klen,NULL)) == NULL) return 0;
p += zipmapRawKeyLength(p);
*vlen = zipmapDecodeLength(p);
*value = p + ZIPMAP_LEN_BYTES(*vlen) + 1;
return 1;
}
/* Return 1 if the key exists, otherwise 0 is returned. */
int zipmapExists(unsigned char *zm, unsigned char *key, unsigned int klen) {
return zipmapLookupRaw(zm,key,klen,NULL) != NULL;
}
/* Return the number of entries inside a zipmap */
unsigned int zipmapLen(unsigned char *zm) {
unsigned int len = 0;
if (zm[0] < ZIPMAP_BIGLEN) {
len = zm[0];
} else {
unsigned char *p = zipmapRewind(zm);
while((p = zipmapNext(p,NULL,NULL,NULL,NULL)) != NULL) len++;
/* Re-store length if small enough */
if (len < ZIPMAP_BIGLEN) zm[0] = len;
}
return len;
}
void zipmapRepr(unsigned char *p) {
unsigned int l;
printf("{status %u}",*p++);
while(1) {
if (p[0] == ZIPMAP_END) {
printf("{end}");
break;
} else {
unsigned char e;
l = zipmapDecodeLength(p);
printf("{key %u}",l);
p += zipmapEncodeLength(NULL,l);
fwrite(p,l,1,stdout);
p += l;
l = zipmapDecodeLength(p);
printf("{value %u}",l);
p += zipmapEncodeLength(NULL,l);
e = *p++;
fwrite(p,l,1,stdout);
p += l+e;
if (e) {
printf("[");
while(e--) printf(".");
printf("]");
}
}
}
printf("\n");
}
#ifdef ZIPMAP_TEST_MAIN
int main(void) {
unsigned char *zm;
zm = zipmapNew();
zm = zipmapSet(zm,(unsigned char*) "name",4, (unsigned char*) "foo",3,NULL);
zm = zipmapSet(zm,(unsigned char*) "surname",7, (unsigned char*) "foo",3,NULL);
zm = zipmapSet(zm,(unsigned char*) "age",3, (unsigned char*) "foo",3,NULL);
zipmapRepr(zm);
exit(1);
zm = zipmapSet(zm,(unsigned char*) "hello",5, (unsigned char*) "world!",6,NULL);
zm = zipmapSet(zm,(unsigned char*) "foo",3, (unsigned char*) "bar",3,NULL);
zm = zipmapSet(zm,(unsigned char*) "foo",3, (unsigned char*) "!",1,NULL);
zipmapRepr(zm);
zm = zipmapSet(zm,(unsigned char*) "foo",3, (unsigned char*) "12345",5,NULL);
zipmapRepr(zm);
zm = zipmapSet(zm,(unsigned char*) "new",3, (unsigned char*) "xx",2,NULL);
zm = zipmapSet(zm,(unsigned char*) "noval",5, (unsigned char*) "",0,NULL);
zipmapRepr(zm);
zm = zipmapDel(zm,(unsigned char*) "new",3,NULL);
zipmapRepr(zm);
printf("\nPerform a direct lookup:\n");
{
unsigned char *value;
unsigned int vlen;
if (zipmapGet(zm,(unsigned char*) "foo",3,&value,&vlen)) {
printf(" foo is associated to the %d bytes value: %.*s\n",
vlen, vlen, value);
}
}
printf("\nIterate trought elements:\n");
{
unsigned char *i = zipmapRewind(zm);
unsigned char *key, *value;
unsigned int klen, vlen;
while((i = zipmapNext(i,&key,&klen,&value,&vlen)) != NULL) {
printf(" %d:%.*s => %d:%.*s\n", klen, klen, key, vlen, vlen, value);
}
}
return 0;
}
#endif
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