/*

 ** 2001 September 15

 **

 ** The author disclaims copyright to this source code.  In place of

 ** a legal notice, here is a blessing:

 **

 **    May you do good and not evil.

 **    May you find forgiveness for yourself and forgive others.

 **    May you share freely, never taking more than you give.

 **

 *************************************************************************

 ** This file contains code to implement a pseudo-random number

 ** generator (PRNG) for SQLite.

 **

 ** Random numbers are used by some of the database backends in order

 ** to generate random integer keys for tables or random filenames.

 **

 ** $Id: random.c,v 1.25 2008/06/19 01:03:18 drh Exp $

 */

 #include "sqliteInt.h"

 /* All threads share a single random number generator.

 ** This structure is the current state of the generator.

 */

 static struct sqlite3PrngType {

   unsigned char isInit;          /* True if initialized */

   unsigned char i, j;            /* State variables */

   unsigned char s[256];          /* State variables */

 } 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.

 */

 static int randomByte(void){

   unsigned char t;

   /* Initialize the state of the random number generator once,

   ** the first time this routine is called.  The seed value does

   ** not need to contain a lot of randomness since we are not

   ** trying to do secure encryption or anything like that...

   **

   ** Nothing in this file or anywhere else in SQLite does any kind of

   ** encryption.  The RC4 algorithm is being used as a PRNG (pseudo-random

   ** number generator) not as an encryption device.

   */

   if( !sqlite3Prng.isInit ){

     int i;

     char k[256];

     sqlite3Prng.j = 0;

     sqlite3Prng.i = 0;

     sqlite3OsRandomness(sqlite3_vfs_find(0), 256, k);

     for(i=0; i<256; i++){

       sqlite3Prng.s[i] = i;

     }

     for(i=0; i<256; i++){

       sqlite3Prng.j += sqlite3Prng.s[i] + k[i];

       t = sqlite3Prng.s[sqlite3Prng.j];

       sqlite3Prng.s[sqlite3Prng.j] = sqlite3Prng.s[i];

       sqlite3Prng.s[i] = t;

     }

     sqlite3Prng.isInit = 1;

   }

   /* Generate and return single random byte

   */

   sqlite3Prng.i++;

   t = sqlite3Prng.s[sqlite3Prng.i];

   sqlite3Prng.j += t;

   sqlite3Prng.s[sqlite3Prng.i] = sqlite3Prng.s[sqlite3Prng.j];

   sqlite3Prng.s[sqlite3Prng.j] = t;

   t += sqlite3Prng.s[sqlite3Prng.i];

   return sqlite3Prng.s[t];

 }

 /*

 ** Return N random bytes.

 */

 void sqlite3_randomness(int N, void *pBuf){

   unsigned char *zBuf = pBuf;

 #ifndef SQLITE_MUTEX_NOOP

   sqlite3_mutex *mutex = sqlite3MutexAlloc(SQLITE_MUTEX_STATIC_PRNG);

 #endif

   sqlite3_mutex_enter(mutex);

   while( N-- ){

     *(zBuf++) = randomByte();

   }

   sqlite3_mutex_leave(mutex);

 }

 #ifndef SQLITE_OMIT_BUILTIN_TEST

 /*

 ** For testing purposes, we sometimes want to preserve the state of

 ** PRNG and restore the PRNG to its saved state at a later time.

 ** The sqlite3_test_control() interface calls these routines to

 ** control the PRNG.

 */

 static struct sqlite3PrngType sqlite3SavedPrng;

 void sqlite3PrngSaveState(void){

   memcpy(&sqlite3SavedPrng, &sqlite3Prng, sizeof(sqlite3Prng));

 }

 void sqlite3PrngRestoreState(void){

   memcpy(&sqlite3Prng, &sqlite3SavedPrng, sizeof(sqlite3Prng));

 }

 void sqlite3PrngResetState(void){

   sqlite3Prng.isInit = 0;

 }

 #endif /* SQLITE_OMIT_BUILTIN_TEST */