**
** Set output variable *paHash to point to the start of the hash table
** in the wal-index file. Set *piZero to one less than the frame 
** number of the first frame indexed by this hash table. If a
** slot in the hash table is set to N, it refers to frame number 
** (*piZero+N) in the log.
**
** Finally, set *paPgno such that for all frames F between (*piZero+1) and 
** (*piZero+HASHTABLE_NPAGE), (*paPgno)[F] is the database page number 
** associated with frame F.
*/
static int walHashGet(
  Wal *pWal,                      /* WAL handle */
  int iHash,                      /* Find the iHash'th table */
  volatile ht_slot **paHash,      /* OUT: Pointer to hash index */
  volatile u32 **paPgno,          /* OUT: Pointer to page number array */
  u32 *piZero                     /* OUT: Frame associated with *paPgno[0] */
................................................................................

  if( rc==SQLITE_OK ){
    u32 iZero;
    volatile ht_slot *aHash;

    aHash = (volatile ht_slot *)&aPgno[HASHTABLE_NPAGE];
    if( iHash==0 ){
      aPgno = &aPgno[WALINDEX_HDR_SIZE/sizeof(u32)-1];
      iZero = 0;
    }else{
      iZero = HASHTABLE_NPAGE_ONE + (iHash-1)*HASHTABLE_NPAGE;
      aPgno = &aPgno[-1*iZero-1];
    }
  
    *paPgno = aPgno;
    *paHash = aHash;
    *piZero = iZero;
  }
  return rc;
}

/*
................................................................................
      aHash[i] = 0;
    }
  }
  
  /* Zero the entries in the aPgno array that correspond to frames with
  ** frame numbers greater than pWal->hdr.mxFrame. 
  */
  nByte = ((char *)aHash - (char *)&aPgno[pWal->hdr.mxFrame+1]);
  memset((void *)&aPgno[pWal->hdr.mxFrame+1], 0, nByte);

#ifdef SQLITE_ENABLE_EXPENSIVE_ASSERT
  /* Verify that the every entry in the mapping region is still reachable
  ** via the hash table even after the cleanup.
  */
  if( iLimit ){
    int i;           /* Loop counter */
    int iKey;        /* Hash key */
    for(i=1; i<=iLimit; i++){
      for(iKey=walHash(aPgno[i+iZero]); aHash[iKey]; iKey=walNextHash(iKey)){
        if( aHash[iKey]==i ) break;
      }
      assert( aHash[iKey]==i );
    }
  }
#endif /* SQLITE_ENABLE_EXPENSIVE_ASSERT */
}
................................................................................
    idx = iFrame - iZero;
    assert( idx <= HASHTABLE_NSLOT/2 + 1 );
    
    /* If this is the first entry to be added to this hash-table, zero the
    ** entire hash table and aPgno[] array before proceding. 
    */
    if( idx==1 ){
      int nByte = (u8 *)&aHash[HASHTABLE_NSLOT] - (u8 *)&aPgno[1+iZero];
      memset((void*)&aPgno[1+iZero], 0, nByte);
    }

    /* If the entry in aPgno[] is already set, then the previous writer
    ** must have exited unexpectedly in the middle of a transaction (after
    ** writing one or more dirty pages to the WAL to free up memory). 
    ** Remove the remnants of that writers uncommitted transaction from 
    ** the hash-table before writing any new entries.
    */
    if( aPgno[iFrame] ){
      walCleanupHash(pWal);
      assert( !aPgno[iFrame] );
    }

    /* Write the aPgno[] array entry and the hash-table slot. */
    for(iKey=walHash(iPage); aHash[iKey]; iKey=walNextHash(iKey)){
      assert( nCollide++ < idx );
    }
    aPgno[iFrame] = iPage;
    aHash[iKey] = idx;

#ifdef SQLITE_ENABLE_EXPENSIVE_ASSERT
    /* Verify that the number of entries in the hash table exactly equals
    ** the number of entries in the mapping region.
    */
    {
................................................................................
    ** via the hash table.  This turns out to be a really, really expensive
    ** thing to check, so only do this occasionally - not on every
    ** iteration.
    */
    if( (idx&0x3ff)==0 ){
      int i;           /* Loop counter */
      for(i=1; i<=idx; i++){
        for(iKey=walHash(aPgno[i+iZero]); aHash[iKey]; iKey=walNextHash(iKey)){
          if( aHash[iKey]==i ) break;
        }
        assert( aHash[iKey]==i );
      }
    }
#endif /* SQLITE_ENABLE_EXPENSIVE_ASSERT */
  }
................................................................................
    int rc;

    rc = walHashGet(pWal, i, &aHash, &aPgno, &iZero);
    if( rc!=SQLITE_OK ){
      walIteratorFree(p);
      return rc;
    }

    nEntry = ((i+1)==nSegment)?iLast-iZero:(u32 *)aHash-(u32 *)&aPgno[iZero+1];

    iZero++;
    aPgno += iZero;

    for(j=0; j<nEntry; j++){
      aSpace[j] = j;
    }
    walMergesort((u32 *)aPgno, aTmp, aSpace, &nEntry);
    p->aSegment[i].iZero = iZero;
    p->aSegment[i].nEntry = nEntry;
................................................................................

    rc = walHashGet(pWal, iHash, &aHash, &aPgno, &iZero);
    if( rc!=SQLITE_OK ){
      return rc;
    }
    for(iKey=walHash(pgno); aHash[iKey]; iKey=walNextHash(iKey)){
      u32 iFrame = aHash[iKey] + iZero;
      if( iFrame<=iLast && aPgno[iFrame]==pgno ){
        assert( iFrame>iRead );
        iRead = iFrame;
      }
    }
  }

#ifdef SQLITE_ENABLE_EXPENSIVE_ASSERT

**
** Set output variable *paHash to point to the start of the hash table
** in the wal-index file. Set *piZero to one less than the frame 
** number of the first frame indexed by this hash table. If a
** slot in the hash table is set to N, it refers to frame number 
** (*piZero+N) in the log.
**
** Finally, set *paPgno so that *paPgno[1] is the page number of the
** first frame indexed by the hash table, frame (*piZero+1).

*/
static int walHashGet(
  Wal *pWal,                      /* WAL handle */
  int iHash,                      /* Find the iHash'th table */
  volatile ht_slot **paHash,      /* OUT: Pointer to hash index */
  volatile u32 **paPgno,          /* OUT: Pointer to page number array */
  u32 *piZero                     /* OUT: Frame associated with *paPgno[0] */
................................................................................

  if( rc==SQLITE_OK ){
    u32 iZero;
    volatile ht_slot *aHash;

    aHash = (volatile ht_slot *)&aPgno[HASHTABLE_NPAGE];
    if( iHash==0 ){
      aPgno = &aPgno[WALINDEX_HDR_SIZE/sizeof(u32)];
      iZero = 0;
    }else{
      iZero = HASHTABLE_NPAGE_ONE + (iHash-1)*HASHTABLE_NPAGE;

    }
  
    *paPgno = &aPgno[-1];
    *paHash = aHash;
    *piZero = iZero;
  }
  return rc;
}

/*
................................................................................
      aHash[i] = 0;
    }
  }
  
  /* Zero the entries in the aPgno array that correspond to frames with
  ** frame numbers greater than pWal->hdr.mxFrame. 
  */
  nByte = ((char *)aHash - (char *)&aPgno[iLimit+1]);
  memset((void *)&aPgno[iLimit+1], 0, nByte);

#ifdef SQLITE_ENABLE_EXPENSIVE_ASSERT
  /* Verify that the every entry in the mapping region is still reachable
  ** via the hash table even after the cleanup.
  */
  if( iLimit ){
    int i;           /* Loop counter */
    int iKey;        /* Hash key */
    for(i=1; i<=iLimit; i++){
      for(iKey=walHash(aPgno[i]); aHash[iKey]; iKey=walNextHash(iKey)){
        if( aHash[iKey]==i ) break;
      }
      assert( aHash[iKey]==i );
    }
  }
#endif /* SQLITE_ENABLE_EXPENSIVE_ASSERT */
}
................................................................................
    idx = iFrame - iZero;
    assert( idx <= HASHTABLE_NSLOT/2 + 1 );
    
    /* If this is the first entry to be added to this hash-table, zero the
    ** entire hash table and aPgno[] array before proceding. 
    */
    if( idx==1 ){
      int nByte = (u8 *)&aHash[HASHTABLE_NSLOT] - (u8 *)&aPgno[1];
      memset((void*)&aPgno[1], 0, nByte);
    }

    /* If the entry in aPgno[] is already set, then the previous writer
    ** must have exited unexpectedly in the middle of a transaction (after
    ** writing one or more dirty pages to the WAL to free up memory). 
    ** Remove the remnants of that writers uncommitted transaction from 
    ** the hash-table before writing any new entries.
    */
    if( aPgno[idx] ){
      walCleanupHash(pWal);
      assert( !aPgno[idx] );
    }

    /* Write the aPgno[] array entry and the hash-table slot. */
    for(iKey=walHash(iPage); aHash[iKey]; iKey=walNextHash(iKey)){
      assert( nCollide++ < idx );
    }
    aPgno[idx] = iPage;
    aHash[iKey] = idx;

#ifdef SQLITE_ENABLE_EXPENSIVE_ASSERT
    /* Verify that the number of entries in the hash table exactly equals
    ** the number of entries in the mapping region.
    */
    {
................................................................................
    ** via the hash table.  This turns out to be a really, really expensive
    ** thing to check, so only do this occasionally - not on every
    ** iteration.
    */
    if( (idx&0x3ff)==0 ){
      int i;           /* Loop counter */
      for(i=1; i<=idx; i++){
        for(iKey=walHash(aPgno[i]); aHash[iKey]; iKey=walNextHash(iKey)){
          if( aHash[iKey]==i ) break;
        }
        assert( aHash[iKey]==i );
      }
    }
#endif /* SQLITE_ENABLE_EXPENSIVE_ASSERT */
  }
................................................................................
    int rc;

    rc = walHashGet(pWal, i, &aHash, &aPgno, &iZero);
    if( rc!=SQLITE_OK ){
      walIteratorFree(p);
      return rc;
    }
    aPgno++;
    nEntry = ((i+1)==nSegment)?iLast-iZero:(u32 *)aHash-(u32 *)aPgno;

    iZero++;


    for(j=0; j<nEntry; j++){
      aSpace[j] = j;
    }
    walMergesort((u32 *)aPgno, aTmp, aSpace, &nEntry);
    p->aSegment[i].iZero = iZero;
    p->aSegment[i].nEntry = nEntry;
................................................................................

    rc = walHashGet(pWal, iHash, &aHash, &aPgno, &iZero);
    if( rc!=SQLITE_OK ){
      return rc;
    }
    for(iKey=walHash(pgno); aHash[iKey]; iKey=walNextHash(iKey)){
      u32 iFrame = aHash[iKey] + iZero;
      if( iFrame<=iLast && aPgno[aHash[iKey]]==pgno ){
        assert( iFrame>iRead );
        iRead = iFrame;
      }
    }
  }

#ifdef SQLITE_ENABLE_EXPENSIVE_ASSERT