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Added two new encodings to ziplist.c

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1) One integer "immediate" encoding that can encode from 0 to 12 in the
encoding byte itself.
2) One 8 bit signed integer encoding that can encode 8 bit signed small
integers in a single byte.

The idea is to exploit all the not used bits we have around in a
backward compatible way.
This commit is contained in:
antirez 2012-04-24 16:54:07 +02:00
parent 053d56a1fa
commit a76b9063d0
1 changed files with 46 additions and 19 deletions
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65
src/ziplist.c
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@ -52,14 +52,21 @@
* String value with length less than or equal to 16383 bytes (14 bits).
* |10______|qqqqqqqq|rrrrrrrr|ssssssss|tttttttt| - 5 bytes
* String value with length greater than or equal to 16384 bytes.
* |1100____| - 1 byte
* |11000000| - 1 byte
* Integer encoded as int16_t (2 bytes).
* |1101____| - 1 byte
* |11010000| - 1 byte
* Integer encoded as int32_t (4 bytes).
* |1110____| - 1 byte
* |11100000| - 1 byte
* Integer encoded as int64_t (8 bytes).
* |1111____| - 1 byte
* |11110000| - 1 byte
* Integer encoded as 24 bit signed (3 bytes).
* |11111110| - 1 byte
* Integer encoded as 8 bit signed (1 byte).
* |1111xxxx| - (with xxxx between 0000 and 1101) immediate 4 bit integer.
* Unsigned integer from 0 to 12. The encoded value is actually from
* 1 to 13 because 0000 and 1111 can not be used, so 1 should be
* subtracted from the encoded 4 bit value to obtain the right value.
* |11111111| - End of ziplist.
*
* All the integers are represented in little endian byte order.
*/
@ -79,8 +86,8 @@
#define ZIP_BIGLEN 254
/* Different encoding/length possibilities */
#define ZIP_STR_MASK (0xc0)
#define ZIP_INT_MASK (0x30)
#define ZIP_STR_MASK 0xc0
#define ZIP_INT_MASK 0x30
#define ZIP_STR_06B (0 << 6)
#define ZIP_STR_14B (1 << 6)
#define ZIP_STR_32B (2 << 6)
@ -88,6 +95,12 @@
#define ZIP_INT_32B (0xc0 | 1<<4)
#define ZIP_INT_64B (0xc0 | 2<<4)
#define ZIP_INT_24B (0xc0 | 3<<4)
#define ZIP_INT_8B 0xfe
/* 4 bit integer immediate encoding */
#define ZIP_INT_IMM_MASK 0x0f
#define ZIP_INT_IMM_MIN 0xf1 /* 11110001 */
#define ZIP_INT_IMM_MAX 0xfd /* 11111101 */
#define ZIP_INT_IMM_VAL(v) (v & ZIP_INT_IMM_MASK)
#define INT24_MAX 0x7fffff
#define INT24_MIN (-INT24_MAX - 1)
@ -119,21 +132,22 @@ typedef struct zlentry {
unsigned char *p;
} zlentry;
#define ZIP_ENTRY_ENCODING(ptr, encoding) do { \
(encoding) = (ptr[0]) & (ZIP_STR_MASK | ZIP_INT_MASK); \
if (((encoding) & ZIP_STR_MASK) < ZIP_STR_MASK) { \
/* String encoding: 2 MSBs */ \
(encoding) &= ZIP_STR_MASK; \
} \
/* Extract the encoding from the byte pointed by 'ptr' and set it into
* 'encoding'. */
#define ZIP_ENTRY_ENCODING(ptr, encoding) do { \
(encoding) = (ptr[0]); \
if ((encoding) < ZIP_STR_MASK) (encoding) &= ZIP_STR_MASK; \
} while(0)
/* Return bytes needed to store integer encoded by 'encoding' */
static unsigned int zipIntSize(unsigned char encoding) {
switch(encoding) {
case ZIP_INT_16B: return sizeof(int16_t);
case ZIP_INT_24B: return sizeof(int32_t)-sizeof(int8_t);
case ZIP_INT_32B: return sizeof(int32_t);
case ZIP_INT_64B: return sizeof(int64_t);
case ZIP_INT_8B: return 1;
case ZIP_INT_16B: return 2;
case ZIP_INT_24B: return 3;
case ZIP_INT_32B: return 4;
case ZIP_INT_64B: return 8;
default: return 0; /* 4 bit immediate */
}
assert(NULL);
return 0;
@ -278,7 +292,11 @@ static int zipTryEncoding(unsigned char *entry, unsigned int entrylen, long long
if (string2ll((char*)entry,entrylen,&value)) {
/* Great, the string can be encoded. Check what's the smallest
* of our encoding types that can hold this value. */
if (value >= INT16_MIN && value <= INT16_MAX) {
if (value >= 0 && value <= 12) {
*encoding = ZIP_INT_IMM_MIN+value;
} else if (value >= INT8_MIN && value <= INT8_MAX) {
*encoding = ZIP_INT_8B;
} else if (value >= INT16_MIN && value <= INT16_MAX) {
*encoding = ZIP_INT_16B;
} else if (value >= INT24_MIN && value <= INT24_MAX) {
*encoding = ZIP_INT_24B;
@ -298,7 +316,9 @@ static void zipSaveInteger(unsigned char *p, int64_t value, unsigned char encodi
int16_t i16;
int32_t i32;
int64_t i64;
if (encoding == ZIP_INT_16B) {
if (encoding == ZIP_INT_8B) {
((char*)p)[0] = (char)value;
} else if (encoding == ZIP_INT_16B) {
i16 = value;
memcpy(p,&i16,sizeof(i16));
memrev16ifbe(p);
@ -314,6 +334,8 @@ static void zipSaveInteger(unsigned char *p, int64_t value, unsigned char encodi
i64 = value;
memcpy(p,&i64,sizeof(i64));
memrev64ifbe(p);
} else if (encoding >= ZIP_INT_IMM_MIN && encoding <= ZIP_INT_IMM_MAX) {
/* Nothing to do, the value is stored in the encoding itself. */
} else {
assert(NULL);
}
@ -324,7 +346,10 @@ static int64_t zipLoadInteger(unsigned char *p, unsigned char encoding) {
int16_t i16;
int32_t i32;
int64_t i64, ret = 0;
if (encoding == ZIP_INT_16B) {
printf("%02x\n", encoding);
if (encoding == ZIP_INT_8B) {
ret = ((char*)p)[0];
} else if (encoding == ZIP_INT_16B) {
memcpy(&i16,p,sizeof(i16));
memrev16ifbe(&i16);
ret = i16;
@ -341,6 +366,8 @@ static int64_t zipLoadInteger(unsigned char *p, unsigned char encoding) {
memcpy(&i64,p,sizeof(i64));
memrev64ifbe(&i64);
ret = i64;
} else if (encoding >= ZIP_INT_IMM_MIN && encoding <= ZIP_INT_IMM_MAX) {
ret = (encoding & ZIP_INT_IMM_MASK)-1;
} else {
assert(NULL);
}