SQLite

  i64 iWriteOff;                  /* Current write offset within file pTemp1 */
  i64 iReadOff;                   /* Current read offset within file pTemp1 */
  sqlite3_file *pTemp1;           /* PMA file 1 */
  int nPMA;                       /* Number of PMAs stored in pTemp1 */
  SorterRecord *pRecord;          /* Head of in-memory record list */
  int nLimit1;                    /* Minimum PMA size, in bytes */
  int nLimit2;                    /* Maximum PMA size, in bytes */
  char *aSpace;                   /* Space for UnpackRecord() */
  int nSpace;                     /* Size of aSpace in bytes */
};

/*
** The following type is an iterator for a PMA. It caches the current key in 
** variables nKey/aKey. If the iterator is at EOF, pFile==0.
*/
struct VdbeSorterIter {

**
** If the bOmitRowid argument is non-zero, assume both keys end in a rowid
** field. For the purposes of the comparison, ignore it. Also, if bOmitRowid
** is true and key1 contains even a single NULL value, it is considered to
** be less than key2. Even if key2 also contains NULL values.
*/
static int vdbeSorterCompare(
  VdbeCursor *pCsr,               /* Cursor object (for pKeyInfo) */
  int bOmitRowid,                 /* Ignore rowid field at end of keys */
  void *pKey1, int nKey1,         /* Left side of comparison */
  void *pKey2, int nKey2,         /* Right side of comparison */
  int *pRes                       /* OUT: Result of comparison */
){
  KeyInfo *pKeyInfo = pCsr->pKeyInfo;
  VdbeSorter *pSorter = pCsr->pSorter;
  char *aSpace = pSorter->aSpace;
  int nSpace = pSorter->nSpace;
  UnpackedRecord *r2;
  int i;

  if( aSpace==0 ){
    nSpace = ROUND8(sizeof(UnpackedRecord))+(pKeyInfo->nField+1)*sizeof(Mem);
    aSpace = (char *)sqlite3Malloc(nSpace);
    if( aSpace==0 ) return SQLITE_NOMEM;
    pSorter->aSpace = aSpace;
    pSorter->nSpace = nSpace;
  }

  /* This call cannot fail. As the memory is already allocated. */
  r2 = sqlite3VdbeRecordUnpack(pKeyInfo, nKey2, pKey2, aSpace, nSpace);
  assert( r2 && (r2->flags & UNPACKED_NEED_FREE)==0 );

  if( bOmitRowid ){
    for(i=0; i<r2->nField-1; i++){
      if( r2->aMem[i].flags & MEM_Null ){
        *pRes = -1;
        sqlite3VdbeDeleteUnpackedRecord(r2);
        return SQLITE_OK;
      }
    }
    r2->flags |= UNPACKED_PREFIX_MATCH;
    r2->nField--;
    assert( r2->nField>0 );
  }

  *pRes = sqlite3VdbeRecordCompare(nKey1, pKey1, r2);
  /* sqlite3VdbeDeleteUnpackedRecord(r2); */
  return SQLITE_OK;
}

/*
** This function is called to compare two iterator keys when merging 
** multiple b-tree segments. Parameter iOut is the index of the aTree[] 
** value to recalculate.

  if( p1->pFile==0 ){
    iRes = i2;
  }else if( p2->pFile==0 ){
    iRes = i1;
  }else{
    int res;
    int rc = vdbeSorterCompare(
        pCsr, 0, p1->aKey, p1->nKey, p2->aKey, p2->nKey, &res
    );
    if( rc!=SQLITE_OK ) return rc;
    if( res<=0 ){
      iRes = i1;
    }else{
      iRes = i2;
    }

      }
      sqlite3DbFree(db, pSorter->aIter);
    }
    if( pSorter->pTemp1 ){
      sqlite3OsCloseFree(pSorter->pTemp1);
    }
    vdbeSorterRecordFree(db, pSorter->pRecord);
    sqlite3_free(pSorter->aSpace);
    sqlite3DbFree(db, pSorter);
    pCsr->pSorter = 0;
  }
}

/*
** Allocate space for a file-handle and open a temporary file. If successful,

/*
** Attemp to merge the two sorted lists p1 and p2 into a single list. If no
** error occurs set *ppOut to the head of the new list and return SQLITE_OK.
*/
static int vdbeSorterMerge(
  sqlite3 *db,                    /* Database handle */
  VdbeCursor *pCsr,               /* For pKeyInfo */
  SorterRecord *p1,               /* First list to merge */
  SorterRecord *p2,               /* Second list to merge */
  SorterRecord **ppOut            /* OUT: Head of merged list */
){
  int rc = SQLITE_OK;
  SorterRecord *pFinal = 0;
  SorterRecord **pp = &pFinal;

  while( p1 || p2 ){
    if( p1==0 ){
      *pp = p2;
      p2 = 0;
    }else if( p2==0 ){
      *pp = p1;
      p1 = 0;
    }else{
      int res;
      rc = vdbeSorterCompare(
          pCsr, 0, p1->pVal, p1->nVal, p2->pVal, p2->nVal, &res
      );
      if( rc!=SQLITE_OK ){
        vdbeSorterRecordFree(db, p1);
        vdbeSorterRecordFree(db, p2);
        vdbeSorterRecordFree(db, pFinal);
        pFinal = 0;
        break;

*/
static int vdbeSorterSort(sqlite3 *db, VdbeCursor *pCsr){
  int rc = SQLITE_OK;
  int i;
  SorterRecord **aSlot;
  SorterRecord *p;
  VdbeSorter *pSorter = pCsr->pSorter;


  aSlot = (SorterRecord **)sqlite3MallocZero(64 * sizeof(SorterRecord *));
  if( !aSlot ){
    return SQLITE_NOMEM;
  }

  p = pSorter->pRecord;
  while( p ){
    SorterRecord *pNext = p->pNext;
    p->pNext = 0;
    for(i=0; rc==SQLITE_OK && aSlot[i]; i++){
      rc = vdbeSorterMerge(db, pCsr, p, aSlot[i], &p);
      aSlot[i] = 0;
    }
    if( rc!=SQLITE_OK ){
      vdbeSorterRecordFree(db, pNext);
      break;
    }
    aSlot[i] = p;
    p = pNext;
  }

  p = 0;
  for(i=0; i<64; i++){
    if( rc==SQLITE_OK ){
      rc = vdbeSorterMerge(db, pCsr, p, aSlot[i], &p);
    }else{
      vdbeSorterRecordFree(db, aSlot[i]);
    }
  }
  pSorter->pRecord = p;

#if 0
  {
    SorterRecord *pTmp1 = 0;
    SorterRecord *pTmp2;
    for(pTmp2=pSorter->pRecord; pTmp2 && rc==SQLITE_OK; pTmp2=pTmp2->pNext){
      if( pTmp1 ){
        int res;
        rc = vdbeSorterCompare(pCsr, 
            0, pTmp1->pVal, pTmp1->nVal, pTmp2->pVal, pTmp2->nVal, &res
        );
        assert( rc!=SQLITE_OK || res<0 );
      }
      pTmp1 = pTmp2;
    }
  }

  int *pRes                       /* OUT: Result of comparison */
){
  int rc;
  VdbeSorter *pSorter = pCsr->pSorter;
  void *pKey; int nKey;           /* Sorter key to compare pVal with */

  pKey = vdbeSorterRowkey(pSorter, &nKey);
  rc = vdbeSorterCompare(pCsr, 1, pVal->z, pVal->n, pKey, nKey, pRes);
  assert( rc!=SQLITE_OK || *pRes<=0 );
  return rc;
}

#endif /* #ifndef SQLITE_OMIT_MERGE_SORT */