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Refactor the sqlite3_randomness() implementation for improved performance.

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FossilOrigin-Name: 4144dffb57b5ed791d7a6d2f26fab5e7dc77fbd1
This commit is contained in:
drh
2013-08-21 22:09:25 +00:00
parent b49bc86a1a
commit cf5ff12105
3 changed files with 21 additions and 44 deletions
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51
src/random.c
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@@ -28,24 +28,11 @@ static SQLITE_WSD struct sqlite3PrngType {
} sqlite3Prng;
/*
** Get a single 8-bit random value from the RC4 PRNG. The Mutex
** must be held while executing this routine.
**
** Why not just use a library random generator like lrand48() for this?
** Because the OP_NewRowid opcode in the VDBE depends on having a very
** good source of random numbers. The lrand48() library function may
** well be good enough. But maybe not. Or maybe lrand48() has some
** subtle problems on some systems that could cause problems. It is hard
** to know. To minimize the risk of problems due to bad lrand48()
** implementations, SQLite uses this random number generator based
** on RC4, which we know works very well.
**
** (Later): Actually, OP_NewRowid does not depend on a good source of
** randomness any more. But we will leave this code in all the same.
** Return N random bytes.
*/
static u8 randomByte(void){
void sqlite3_randomness(int N, void *pBuf){
unsigned char t;
unsigned char *zBuf = pBuf;
/* The "wsdPrng" macro will resolve to the pseudo-random number generator
** state vector. If writable static data is unsupported on the target,
@@ -60,6 +47,10 @@ static u8 randomByte(void){
# define wsdPrng sqlite3Prng
#endif
#if SQLITE_THREADSAFE
sqlite3_mutex *mutex = sqlite3MutexAlloc(SQLITE_MUTEX_STATIC_PRNG);
sqlite3_mutex_enter(mutex);
#endif
/* Initialize the state of the random number generator once,
** the first time this routine is called. The seed value does
@@ -88,28 +79,14 @@ static u8 randomByte(void){
wsdPrng.isInit = 1;
}
/* Generate and return single random byte
*/
wsdPrng.i++;
t = wsdPrng.s[wsdPrng.i];
wsdPrng.j += t;
wsdPrng.s[wsdPrng.i] = wsdPrng.s[wsdPrng.j];
wsdPrng.s[wsdPrng.j] = t;
t += wsdPrng.s[wsdPrng.i];
return wsdPrng.s[t];
}
/*
** Return N random bytes.
*/
void sqlite3_randomness(int N, void *pBuf){
unsigned char *zBuf = pBuf;
#if SQLITE_THREADSAFE
sqlite3_mutex *mutex = sqlite3MutexAlloc(SQLITE_MUTEX_STATIC_PRNG);
#endif
sqlite3_mutex_enter(mutex);
while( N-- ){
*(zBuf++) = randomByte();
wsdPrng.i++;
t = wsdPrng.s[wsdPrng.i];
wsdPrng.j += t;
wsdPrng.s[wsdPrng.i] = wsdPrng.s[wsdPrng.j];
wsdPrng.s[wsdPrng.j] = t;
t += wsdPrng.s[wsdPrng.i];
*(zBuf++) = wsdPrng.s[t];
}
sqlite3_mutex_leave(mutex);
}