/*

** 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);

}