/*

 ** 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.

 **

 ** $Id$

 */

 #include "sqliteInt.h"

 /*

 ** 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;

   /* All threads share a single random number generator.

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

   */

   static struct {

     unsigned char isInit;          /* True if initialized */

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

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

   } prng;

   /* 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( !prng.isInit ){

     int i;

     char k[256];

     prng.j = 0;

     prng.i = 0;

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

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

       prng.s[i] = i;

     }

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

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

       t = prng.s[prng.j];

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

       prng.s[i] = t;

     }

     prng.isInit = 1;

   }

   /* Generate and return single random byte

   */

   prng.i++;

   t = prng.s[prng.i];

   prng.j += t;

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

   prng.s[prng.j] = t;

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

   return prng.s[t];

 }

 /*

 ** Return N random bytes.

 */

 void sqlite3Randomness(int N, void *pBuf){

   unsigned char *zBuf = pBuf;

   static sqlite3_mutex *mutex = 0;

   if( mutex==0 ){

     mutex = sqlite3_mutex_alloc(SQLITE_MUTEX_STATIC_PRNG);

   }

   sqlite3_mutex_enter(mutex);

   while( N-- ){

     *(zBuf++) = randomByte();

   }

   sqlite3_mutex_leave(mutex);

 }