};
  BtDb *p = 0;
  bt_db *pBt = 0;
  int rc;
  sqlite4_env *pEnv = sqlite4_env_default();

  if( bClear && zFilename && zFilename[0] ){

    unlink(zFilename);


  }
  
  rc = sqlite4BtNew(pEnv, sizeof(BtDb), &pBt);
  if( rc==SQLITE4_OK ){
    p = (BtDb*)sqlite4BtExtra(pBt);
    p->base.pMethods = &SqlMethods;
    p->pBt = pBt;

  };
  BtDb *p = 0;
  bt_db *pBt = 0;
  int rc;
  sqlite4_env *pEnv = sqlite4_env_default();

  if( bClear && zFilename && zFilename[0] ){
    char *zLog = sqlite3_mprintf("%s-wal", zFilename);
    unlink(zFilename);
    unlink(zLog);
    sqlite3_free(zLog);
  }
  
  rc = sqlite4BtNew(pEnv, sizeof(BtDb), &pBt);
  if( rc==SQLITE4_OK ){
    p = (BtDb*)sqlite4BtExtra(pBt);
    p->base.pMethods = &SqlMethods;
    p->pBt = pBt;

** byte offset 0 and the other at offset <sector-size> (the start of
** the second disk sector in the file, according to the xSectorSize
** VFS method).
*/
struct BtWalHdr {
  u32 iMagic;                     /* Magic number (BT_WAL_MAGIC) */
  u32 iVersion;                   /* File format version */

  u32 nSector;                    /* Sector size when header written */


  u32 iSalt1;                     /* Initial frame cksum-0 value */
  u32 iSalt2;                     /* Initial frame cksum-1 value */
  u32 iFirstFrame;                /* First frame of log (numbered from 1) */

  u32 aCksum[2];                  /* Checksum of all prior fields */
};
................................................................................
** WAL Frame header. All fields are stored in big-endian order.
*/
struct BtFrameHdr {
  u32 pgno;                       /* Page number of this frame */
  u32 ctrl;                       /* Next frame pointer and commit bit */
  u32 aCksum[2];                  /* Frame checksum */
};



/*
** Shared memory header. Shared memory begins with two copies of
** this structure. All fields are stored in machine byte-order.
*/
struct BtShmHdr {
  u32 aLog[6];                    /* First/last frames for each log region */
................................................................................
}

static void btLogZeroSnapshot(BtLog *pLog){
  bt_env *pVfs = pLog->pLock->pVfs;
  memset(&pLog->snapshot, 0, sizeof(BtShmHdr));
  pLog->snapshot.nSector = pVfs->xSectorSize(pLog->pFd);
}

/*




































































































































































































































** Run log recovery. In other words, read the log file from disk and 
** initialize the shared-memory accordingly.
*/
static int btLogRecover(BtLog *pLog){










  return SQLITE4_OK;

































}

/*
** Open the log file for pager pPager. If successful, return the BtLog* 
** handle via output variable *ppLog. If parameter bRecover is true, then
** also run database recovery before returning. In this case, the caller
** has already obtained the required locks.
................................................................................
    sqlite4_free(pEnv, pLog->apShm);
    sqlite4_free(pEnv, pLog);
  }

  return rc;
}

/*
** Return true if log is completely empty (as it is if a file zero bytes
** in size has been opened or created).
*/
static int btLogIsEmpty(BtLog *pLog){
  return (pLog->snapshot.aLog[4]==0);
}

static int btLogWriteData(BtLog *pLog, i64 iOff, u8 *aData, int nData){
  bt_env *pVfs = pLog->pLock->pVfs;
  return pVfs->xWrite(pLog->pFd, iOff, aData, nData);
}

static int btLogWriteHeader(BtLog *pLog, int iHdr, BtWalHdr *pHdr){
  int rc;                         /* Return code */
  i64 iOff;                       /* File offset to write to */
  assert( iHdr==0 || iHdr==1 );

  /* Calculate a checksum for the header */
  btLogChecksum(1, (u8*)pHdr, offsetof(BtWalHdr, aCksum), 0, pHdr->aCksum);
................................................................................
  /* Write the object to disk */
  iOff = iHdr * pLog->snapshot.nSector;
  rc = btLogWriteData(pLog, iOff, (u8*)pHdr, sizeof(BtWalHdr));

  return rc;
}

/*
** Locate the iHash'th hash table in shared memory. Return it.
*/
static int btLogFindHash(
  BtLog *pLog,                    /* Log handle */
  int iHash,                      /* Hash table (numbered from 0) to find */
  ht_slot **paHash,               /* OUT: Pointer to hash slots */
  u32 **paPgno,                   /* OUT: Pointer to page number array */
  u32 *piZero                     /* OUT: Frame associated with *paPgno[0] */
){
  int rc;

  rc = btLogMapShm(pLog, iHash);
  if( rc==SQLITE4_OK ){
    u8 *aChunk = pLog->apShm[iHash];
    u32 *aPgno;
    u32 iZero;

    *paHash = (ht_slot*)&aChunk[HASHTABLE_OFFSET_1];
    if( iHash==0 ){
      aPgno = (u32*)&aChunk[sizeof(BtShm)];
      iZero = 1;
    }else{
      aPgno = (u32*)aChunk;
      iZero = 1 + HASHTABLE_NFRAME_ONE + (HASHTABLE_NFRAME * (iHash-1));
    }
    *paPgno = aPgno;
    *piZero = iZero;
  }

  return rc;
}

/*
** Return the index of the hash table that contains the entry for frame
** iFrame. 
*/
static int btLogFrameHash(BtLog *pLog, u32 iFrame){
  if( iFrame<=HASHTABLE_NFRAME_ONE ) return 0;
  return 1 + ((iFrame - HASHTABLE_NFRAME_ONE - 1) / HASHTABLE_NFRAME);
}

/*
** Return a hash key for page number pgno.
*/
static int btLogHashKey(BtLog *pLog, u32 pgno){
  assert( pgno>=1 );
  return ((pgno * HASHTABLE_KEY_MUL) % HASHTABLE_NSLOT);
}

static int btLogHashNext(BtLog *pLog, int iSlot){
  return ((iSlot + 1) % HASHTABLE_NSLOT);
}

static int btLogHashSearch(BtLog *pLog, int iHash, u32 pgno, u32 *piFrame){
  ht_slot *aHash;
  u32 *aPgno;
  u32 iZero;
  int rc;

  rc = btLogFindHash(pLog, iHash, &aHash, &aPgno, &iZero);
................................................................................
    }
  }

  return rc;
}


/*
** Return the offset of frame iFrame within the log file.
*/
static i64 btLogFrameOffset(BtLog *pLog, int pgsz, u32 iFrame){
  return 
      (i64)pLog->snapshot.nSector*2 
    + (i64)(iFrame-1) * (i64)(pgsz + sizeof(BtFrameHdr));
}

/*
** Attempt to read data for page pgno from the log file. If successful,
** the data is written into buffer aData[] (which must be at least as
** large as a database page). In this case SQLITE4_OK is returned.
**
** If the log does not contain any version of page pgno, SQLITE4_NOTFOUND
** is returned and the contents of buffer aData[] are not modified.
................................................................................
    fflush(stderr);
#endif
  }

  return rc;
}

/*
** Add an entry mapping database page pgno to log frame iFrame to the
** the shared hash table. Return SQLITE4_OK if successful, or an SQLite4
** error code if an error occurs.
*/
static int btLogHashInsert(BtLog *pLog, u32 pgno, u32 iFrame){
  int iHash;                      /* Index of hash table to update */
  int rc = SQLITE4_OK;            /* Return code */
  ht_slot *aHash;                 /* Hash slots */
  u32 *aPgno;                     /* Page array for updated hash table */
  u32 iZero;                      /* Zero-offset of updated hash table */

  assert( iFrame>=1 && pgno>=1 );
  assert( iFrame==(pLog->snapshot.aLog[5]+1) );

  /* Find the required hash table */
  iHash = btLogFrameHash(pLog, iFrame);
  rc = btLogFindHash(pLog, iHash, &aHash, &aPgno, &iZero);

  /* Update the hash table */
  if( rc==SQLITE4_OK ){
    int iSlot;
    int nCollide = HASHTABLE_NSLOT*2;
    aPgno[iFrame-iZero] = pgno;

    for(iSlot=btLogHashKey(pLog,pgno); ; iSlot=btLogHashNext(pLog, iSlot)){
      if( aHash[iSlot]==0 ){
        aHash[iSlot] = (iFrame-iZero+1);
        break;
      }
      if( (nCollide--)==0 ) return btErrorBkpt(SQLITE4_CORRUPT);
    }
  }

  return rc;
}

/*
** Write a frame to the log file.
*/
int sqlite4BtLogWrite(BtLog *pLog, u32 pgno, u8 *aData, int bCommit){
  const int pgsz = sqlite4BtPagerPagesize((BtPager*)(pLog->pLock));
  int rc = SQLITE4_OK;
  u32 iFrame;                     /* Write this frame (numbered from 1) */
................................................................................
  if( btLogIsEmpty(pLog) ){
    BtWalHdr hdr;
    memset(&hdr, 0, sizeof(BtWalHdr));

    hdr.iMagic = BT_WAL_MAGIC;
    hdr.iVersion = BT_WAL_VERSION;
    hdr.nSector = pLog->snapshot.nSector;

    hdr.iSalt1 = 22;
    hdr.iSalt2 = 23;
    hdr.iFirstFrame = 1;

    rc = btLogWriteHeader(pLog, 0, &hdr);
    if( rc!=SQLITE4_OK ) return rc;



  }

  /* Figure out where exactly to write the new data */
  iFrame = pLog->snapshot.aLog[5] + 1;
  iOff = btLogFrameOffset(pLog, pgsz, iFrame);

  /* Populate the frame header object. */
  memset(&frame, 0, sizeof(frame));
  frame.pgno = pgno;
  frame.ctrl = (bCommit ? 0x80000000 : 0x00000000);
  a = pLog->snapshot.aFrameCksum;
  btLogChecksum(1, (u8*)&frame, offsetof(BtFrameHdr, aCksum), a, frame.aCksum);
  btLogChecksum(1, aData, pgsz, frame.aCksum, frame.aCksum);

#if 0
fprintf(stderr, "writing page %d at log offset %d (frame %d)\n", (int)pgno, (int)iOff, (int)iFrame);
fflush(stderr);

** byte offset 0 and the other at offset <sector-size> (the start of
** the second disk sector in the file, according to the xSectorSize
** VFS method).
*/
struct BtWalHdr {
  u32 iMagic;                     /* Magic number (BT_WAL_MAGIC) */
  u32 iVersion;                   /* File format version */
  u32 iCnt;                       /* 0, 1 or 2 */
  u32 nSector;                    /* Sector size when header written */
  u32 nPgsz;                      /* Database page size in bytes */

  u32 iSalt1;                     /* Initial frame cksum-0 value */
  u32 iSalt2;                     /* Initial frame cksum-1 value */
  u32 iFirstFrame;                /* First frame of log (numbered from 1) */

  u32 aCksum[2];                  /* Checksum of all prior fields */
};
................................................................................
** WAL Frame header. All fields are stored in big-endian order.
*/
struct BtFrameHdr {
  u32 pgno;                       /* Page number of this frame */
  u32 ctrl;                       /* Next frame pointer and commit bit */
  u32 aCksum[2];                  /* Frame checksum */
};

#define BT_FRAME_COMMIT 0x80000000

/*
** Shared memory header. Shared memory begins with two copies of
** this structure. All fields are stored in machine byte-order.
*/
struct BtShmHdr {
  u32 aLog[6];                    /* First/last frames for each log region */
................................................................................
}

static void btLogZeroSnapshot(BtLog *pLog){
  bt_env *pVfs = pLog->pLock->pVfs;
  memset(&pLog->snapshot, 0, sizeof(BtShmHdr));
  pLog->snapshot.nSector = pVfs->xSectorSize(pLog->pFd);
}

/*
** Return the offset of frame iFrame within the log file.
*/
static i64 btLogFrameOffset(BtLog *pLog, int pgsz, u32 iFrame){
  return 
      (i64)pLog->snapshot.nSector*2 
    + (i64)(iFrame-1) * (i64)(pgsz + sizeof(BtFrameHdr));
}

static int btLogWriteData(BtLog *pLog, i64 iOff, u8 *aData, int nData){
  bt_env *pVfs = pLog->pLock->pVfs;
  return pVfs->xWrite(pLog->pFd, iOff, aData, nData);
}

static int btLogReadData(BtLog *pLog, i64 iOff, u8 *aData, int nData){
  bt_env *pVfs = pLog->pLock->pVfs;
  return pVfs->xRead(pLog->pFd, iOff, aData, nData);
}

static int btLogReadHeader(BtLog *pLog, int iOff, BtWalHdr *pHdr){
  int rc = btLogReadData(pLog, (i64)iOff, (u8*)pHdr, sizeof(BtWalHdr));
  if( rc==SQLITE4_OK ){
    u32 aCksum[2];
    btLogChecksum(1, (u8*)pHdr, offsetof(BtWalHdr, aCksum), 0, aCksum);
    if( pHdr->iMagic!=BT_WAL_MAGIC 
     || aCksum[0]!=pHdr->aCksum[0] 
     || aCksum[1]!=pHdr->aCksum[1] 
    ){
      rc = SQLITE4_NOTFOUND;
    }
  }
  return rc;
}

/*
** This function is used as part of recovery. It reads the contents of
** the log file from disk and invokes the xFrame callback for each valid
** frame in the file.
*/
static int btLogTraverse(
  BtLog *pLog,                    /* Log module handle */
  BtWalHdr *pHdr,                 /* Log header read from file */
  int(*xFrame)(BtLog*, void*, u32, BtFrameHdr*), /* Frame callback */
  void *pCtx                      /* Passed as second argument to xFrame */
){
  sqlite4_env *pEnv = pLog->pLock->pEnv;
  const int pgsz = pHdr->nPgsz;
  u32 iFrame = pHdr->iFirstFrame;
  u32 aCksum[2];
  BtFrameHdr fhdr;                /* Frame header */
  u8 *aBuf;                       /* Buffer for frame data */
  int rc = SQLITE4_OK;

  aCksum[0] = pHdr->iSalt1;
  aCksum[1] = pHdr->iSalt2;

  aBuf = sqlite4_malloc(pEnv, pgsz);
  if( aBuf==0 ){
    rc = SQLITE4_NOMEM;
  }

  while( rc==SQLITE4_OK ){
    i64 iOff;
    iOff = btLogFrameOffset(pLog, pgsz, iFrame);

    rc = btLogReadData(pLog, iOff, (u8*)&fhdr, sizeof(BtFrameHdr));
    if( rc==SQLITE4_OK ){
      rc = btLogReadData(pLog, iOff+sizeof(BtFrameHdr), aBuf, pgsz);
    }
    if( rc==SQLITE4_OK ){
      btLogChecksum(1, (u8*)&fhdr, offsetof(BtFrameHdr,aCksum), aCksum, aCksum);
      btLogChecksum(1, aBuf, pgsz, aCksum, aCksum);
      if( aCksum[0]!=fhdr.aCksum[0] || aCksum[1]!=fhdr.aCksum[1] ) break;
    }
    if( rc==SQLITE4_OK ){
      rc = xFrame(pLog, pCtx, iFrame, &fhdr);
    }

    iFrame++;
  }

  return rc;
}

/*
** Locate the iHash'th hash table in shared memory. Return it.
*/
static int btLogFindHash(
  BtLog *pLog,                    /* Log handle */
  int iHash,                      /* Hash table (numbered from 0) to find */
  ht_slot **paHash,               /* OUT: Pointer to hash slots */
  u32 **paPgno,                   /* OUT: Pointer to page number array */
  u32 *piZero                     /* OUT: Frame associated with *paPgno[0] */
){
  int rc;

  rc = btLogMapShm(pLog, iHash);
  if( rc==SQLITE4_OK ){
    u8 *aChunk = pLog->apShm[iHash];
    u32 *aPgno;
    u32 iZero;

    *paHash = (ht_slot*)&aChunk[HASHTABLE_OFFSET_1];
    if( iHash==0 ){
      aPgno = (u32*)&aChunk[sizeof(BtShm)];
      iZero = 1;
    }else{
      aPgno = (u32*)aChunk;
      iZero = 1 + HASHTABLE_NFRAME_ONE + (HASHTABLE_NFRAME * (iHash-1));
    }
    *paPgno = aPgno;
    *piZero = iZero;
  }

  return rc;
}


/*
** Return the index of the hash table that contains the entry for frame
** iFrame. 
*/
static int btLogFrameHash(BtLog *pLog, u32 iFrame){
  if( iFrame<=HASHTABLE_NFRAME_ONE ) return 0;
  return 1 + ((iFrame - HASHTABLE_NFRAME_ONE - 1) / HASHTABLE_NFRAME);
}

/*
** Return a hash key for page number pgno.
*/
static int btLogHashKey(BtLog *pLog, u32 pgno){
  assert( pgno>=1 );
  return ((pgno * HASHTABLE_KEY_MUL) % HASHTABLE_NSLOT);
}

static int btLogHashNext(BtLog *pLog, int iSlot){
  return ((iSlot + 1) % HASHTABLE_NSLOT);
}

/*
** Add an entry mapping database page pgno to log frame iFrame to the
** the shared hash table. Return SQLITE4_OK if successful, or an SQLite4
** error code if an error occurs.
*/
static int btLogHashInsert(BtLog *pLog, u32 pgno, u32 iFrame){
  int iHash;                      /* Index of hash table to update */
  int rc = SQLITE4_OK;            /* Return code */
  ht_slot *aHash;                 /* Hash slots */
  u32 *aPgno;                     /* Page array for updated hash table */
  u32 iZero;                      /* Zero-offset of updated hash table */

  assert( iFrame>=1 && pgno>=1 );

  /* Find the required hash table */
  iHash = btLogFrameHash(pLog, iFrame);
  rc = btLogFindHash(pLog, iHash, &aHash, &aPgno, &iZero);

  /* Update the hash table */
  if( rc==SQLITE4_OK ){
    int iSlot;
    int nCollide = HASHTABLE_NSLOT*2;
    aPgno[iFrame-iZero] = pgno;

    for(iSlot=btLogHashKey(pLog,pgno); ; iSlot=btLogHashNext(pLog, iSlot)){
      if( aHash[iSlot]==0 ){
        aHash[iSlot] = (iFrame-iZero+1);
        break;
      }
      if( (nCollide--)==0 ) return btErrorBkpt(SQLITE4_CORRUPT);
    }
  }

  return rc;
}

/*
** Remove everything following frame iFrame from the iHash'th hash table.
*/
static int btLogHashRollback(BtLog *pLog, int iHash, u32 iFrame){
  const int nPgno = (iHash==0 ? HASHTABLE_NFRAME_ONE : HASHTABLE_NFRAME);
  ht_slot *aHash;                 /* Hash slots */
  u32 *aPgno;                     /* Page array for updated hash table */
  u32 iZero;                      /* Zero-offset of updated hash table */
  int rc;

  rc = btLogFindHash(pLog, iHash, &aHash, &aPgno, &iZero);
  if( rc==SQLITE4_OK ){
    int i;
    ht_slot iMax;
    iMax = (iFrame - iZero) + 1;

    for(i=0; i<HASHTABLE_NSLOT; i++){
      if( aHash[i]>iMax ) aHash[i] = 0;
    }
    memset(&aPgno[iMax], 0, (nPgno-iMax)*sizeof(u32));
  }

  return rc;
}


/*
** Return true if log is completely empty (as it is if a file zero bytes
** in size has been opened or created).
*/
static int btLogIsEmpty(BtLog *pLog){
  return (pLog->snapshot.aLog[4]==0);
}

static int btLogRecoverFrame(
  BtLog *pLog,                    /* Log module handle */
  void *pCtx,                     /* woints to type u32 - pgno of last commit*/
  u32 iFrame,                     /* Frame number */
  BtFrameHdr *pHdr                /* Frame header */
){
  btLogHashInsert(pLog, pHdr->pgno, iFrame);
  if( pHdr->ctrl & BT_FRAME_COMMIT ){
    *(u32*)pCtx = iFrame;
    memcpy(pLog->snapshot.aFrameCksum, pHdr->aCksum, sizeof(pHdr->aCksum));
  }

#if 0
  fprintf(stderr, "recovered frame=%d pgno=%d\n", iFrame, pHdr->pgno);
  fflush(stderr);
#endif
  return 0;
}

/*
** Run log recovery. In other words, read the log file from disk and 
** initialize the shared-memory accordingly.
*/
static int btLogRecover(BtLog *pLog){
  bt_env *pVfs = pLog->pLock->pVfs;
  i64 nByte = 0;                  /* Size of log file on disk */
  int rc;                         /* Return code */
  BtWalHdr *pHdr = 0;

  /* Read a log file header from the start of the file. */
  rc = pVfs->xSize(pLog->pFd, &nByte);
  if( rc==SQLITE4_OK && nByte>0 ){
    BtWalHdr hdr1;
    rc = btLogReadHeader(pLog, 0, &hdr1);
    if( rc==SQLITE4_OK ){
      BtWalHdr hdr2;
      rc = btLogReadHeader(pLog, hdr1.nSector, &hdr2);
      if( rc==SQLITE4_NOTFOUND ){
        pHdr = &hdr1;
      }else if( rc==SQLITE4_OK ){
        int aGreater[3] = {1, 2, 0};
        pHdr = ((hdr2.iCnt==aGreater[hdr1.iCnt]) ? &hdr2 : &hdr1);
      }
    }else if( rc==SQLITE4_NOTFOUND ){
      int iOff;
      for(iOff=256; iOff<=65536 && rc==SQLITE4_NOTFOUND; iOff=iOff*2){
        rc = btLogReadHeader(pLog, iOff, &hdr1);
      }
      if( rc==SQLITE4_OK ){
        pHdr = &hdr1;
      }
    }
    if( rc==SQLITE4_NOTFOUND ) rc = SQLITE4_OK;
  }

  /* If a header was successfully read from the file, scan the log file
  ** and populate the shared-memory hash tables.  */
  if( pHdr ){
    u32 iCommit = 0;
    rc = btLogTraverse(pLog, pHdr, btLogRecoverFrame, (void*)&iCommit);
    if( rc==SQLITE4_OK && iCommit>0 ){
      pLog->snapshot.aLog[4] = 1;
      pLog->snapshot.aLog[5] = iCommit;
      rc = btLogHashRollback(pLog, btLogFrameHash(pLog, iCommit), iCommit);
    }
  }

  return rc;
}

/*
** Open the log file for pager pPager. If successful, return the BtLog* 
** handle via output variable *ppLog. If parameter bRecover is true, then
** also run database recovery before returning. In this case, the caller
** has already obtained the required locks.
................................................................................
    sqlite4_free(pEnv, pLog->apShm);
    sqlite4_free(pEnv, pLog);
  }

  return rc;
}














static int btLogWriteHeader(BtLog *pLog, int iHdr, BtWalHdr *pHdr){
  int rc;                         /* Return code */
  i64 iOff;                       /* File offset to write to */
  assert( iHdr==0 || iHdr==1 );

  /* Calculate a checksum for the header */
  btLogChecksum(1, (u8*)pHdr, offsetof(BtWalHdr, aCksum), 0, pHdr->aCksum);
................................................................................
  /* Write the object to disk */
  iOff = iHdr * pLog->snapshot.nSector;
  rc = btLogWriteData(pLog, iOff, (u8*)pHdr, sizeof(BtWalHdr));

  return rc;
}























































static int btLogHashSearch(BtLog *pLog, int iHash, u32 pgno, u32 *piFrame){
  ht_slot *aHash;
  u32 *aPgno;
  u32 iZero;
  int rc;

  rc = btLogFindHash(pLog, iHash, &aHash, &aPgno, &iZero);
................................................................................
    }
  }

  return rc;
}











/*
** Attempt to read data for page pgno from the log file. If successful,
** the data is written into buffer aData[] (which must be at least as
** large as a database page). In this case SQLITE4_OK is returned.
**
** If the log does not contain any version of page pgno, SQLITE4_NOTFOUND
** is returned and the contents of buffer aData[] are not modified.
................................................................................
    fflush(stderr);
#endif
  }

  return rc;
}






































/*
** Write a frame to the log file.
*/
int sqlite4BtLogWrite(BtLog *pLog, u32 pgno, u8 *aData, int bCommit){
  const int pgsz = sqlite4BtPagerPagesize((BtPager*)(pLog->pLock));
  int rc = SQLITE4_OK;
  u32 iFrame;                     /* Write this frame (numbered from 1) */
................................................................................
  if( btLogIsEmpty(pLog) ){
    BtWalHdr hdr;
    memset(&hdr, 0, sizeof(BtWalHdr));

    hdr.iMagic = BT_WAL_MAGIC;
    hdr.iVersion = BT_WAL_VERSION;
    hdr.nSector = pLog->snapshot.nSector;
    hdr.nPgsz = pgsz;
    hdr.iSalt1 = 22;
    hdr.iSalt2 = 23;
    hdr.iFirstFrame = 1;

    rc = btLogWriteHeader(pLog, 0, &hdr);
    if( rc!=SQLITE4_OK ) return rc;

    pLog->snapshot.aFrameCksum[0] = hdr.iSalt1;
    pLog->snapshot.aFrameCksum[1] = hdr.iSalt2;
  }

  /* Figure out where exactly to write the new data */
  iFrame = pLog->snapshot.aLog[5] + 1;
  iOff = btLogFrameOffset(pLog, pgsz, iFrame);

  /* Populate the frame header object. */
  memset(&frame, 0, sizeof(frame));
  frame.pgno = pgno;
  frame.ctrl = (bCommit ? BT_FRAME_COMMIT : 0);
  a = pLog->snapshot.aFrameCksum;
  btLogChecksum(1, (u8*)&frame, offsetof(BtFrameHdr, aCksum), a, frame.aCksum);
  btLogChecksum(1, aData, pgsz, frame.aCksum, frame.aCksum);

#if 0
fprintf(stderr, "writing page %d at log offset %d (frame %d)\n", (int)pgno, (int)iOff, (int)iFrame);
fflush(stderr);