SQLite

**
** In other words, each time we advance to the next sorter element, log2(N)
** key comparison operations are required, where N is the number of segments
** being merged (rounded up to the next power of 2).
*/
struct VdbeSorter {
  int nWorking;                   /* Start a new b-tree after this many pages */
  int nAlloc;                     /* Allocated size of aIter[] and aTree[] */
  int nTree;                      /* Used size of aTree/aIter (power of 2) */
  VdbeSorterIter *aIter;          /* Array of iterators to merge */
  int *aTree;                     /* Current state of incremental merge */

  i64 iWriteOff;                  /* Current write offset within file pTemp1 */
  sqlite3_file *pTemp1;           /* PMA file 1 */
  i64 *aOffset;                   /* Array of PMA offsets for file 1 */

  u8 *aKey;                       /* Pointer to current key */
};

/* Minimum allowable value for the VdbeSorter.nWorking variable */
#define SORTER_MIN_SEGMENT_SIZE 10

/* Maximum number of segments to merge in a single go */
#define SORTER_MAX_MERGE_COUNT 2

/*
** Append integer iOff to the VdbeSorter.aOffset[] array of the sorter object
** passed as the second argument. SQLITE_NOMEM is returned if an OOM error
** is encountered, or SQLITE_OK if no error occurs.
**
** TODO: The aOffset[] array may grow indefinitely. Fix this.
*/
static int vdbeSorterAppendOffset(sqlite3 *db, VdbeSorter *p, i64 iOff){
  int *aNew;                      /* New VdbeSorter.aRoot[] array */
  p->aOffset = sqlite3DbReallocOrFree(
      db, p->aOffset, (p->nOffset+1)*sizeof(i64)
  );
  if( !p->aOffset ) return SQLITE_NOMEM;
  p->aOffset[p->nOffset++] = iOff;
  return SQLITE_OK;
}

** Free any cursor components allocated by sqlite3VdbeSorterXXX routines.
*/
void sqlite3VdbeSorterClose(sqlite3 *db, VdbeCursor *pCsr){
  VdbeSorter *pSorter = pCsr->pSorter;
  if( pSorter ){
    if( pSorter->aIter ){
      int i;
      for(i=0; i<pSorter->nAlloc; i++){
        vdbeSorterIterZero(db, &pSorter->aIter[i]);
      }
      sqlite3DbFree(db, pSorter->aIter);
      sqlite3DbFree(db, pSorter->aTree);
    }
    if( pSorter->pTemp1 ){
      sqlite3OsCloseFree(pSorter->pTemp1);
    }
    sqlite3DbFree(db, pSorter->aOffset);
    sqlite3DbFree(db, pSorter);
    pCsr->pSorter = 0;

        rc = sqlite3BtreeCursor(pCsr->pBt, iRoot, 1, pCsr->pKeyInfo, p);
      }
    }
  }
  return rc;
}

/*
** Extend the pSorter->aIter[] and pSorter->aTree[] arrays using DbRealloc().
** Return SQLITE_OK if successful, or SQLITE_NOMEM otherwise.
*/
static int vdbeSorterGrowArrays(sqlite3* db, VdbeSorter *pSorter){
  int *aTree;                     /* New aTree[] allocation */
  VdbeSorterIter *aIter;          /* New aIter[] allocation */
  int nOld = pSorter->nAlloc;     /* Current size of arrays */
  int nNew = (nOld?nOld*2:4);     /* Size of arrays after reallocation */

  /* Realloc aTree[]. */
  aTree = sqlite3DbRealloc(db, pSorter->aTree, sizeof(int)*nNew);
  if( !aTree ) return SQLITE_NOMEM;
  memset(&aTree[nOld], 0, (nNew-nOld) * sizeof(int));
  pSorter->aTree = aTree;

  /* Realloc aIter[]. */
  aIter = sqlite3DbRealloc(db, pSorter->aIter, sizeof(VdbeSorterIter)*nNew);
  if( !aIter ) return SQLITE_NOMEM;
  memset(&aIter[nOld], 0, (nNew-nOld) * sizeof(VdbeSorterIter));
  pSorter->aIter = aIter;

  /* Set VdbeSorter.nAlloc to the new size of the arrays and return OK. */
  pSorter->nAlloc = nNew;
  return SQLITE_OK;
}

/*
** Helper function for sqlite3VdbeSorterRewind().
*/
static int vdbeSorterInitMerge(
  sqlite3 *db,
  VdbeCursor *pCsr,
  int iFirst,
  int *piNext
){
  Pager *pPager = sqlite3BtreePager(pCsr->pBt);
  VdbeSorter *pSorter = pCsr->pSorter;
  int rc = SQLITE_OK;
  int i;
  int nMaxRef = (pSorter->nWorking * 9/10);
  int N = 2;


  assert( iFirst<pSorter->nOffset );














  /* Initialize as many iterators as possible. */
  for(i=iFirst; 
      rc==SQLITE_OK && i<pSorter->nOffset && (i-iFirst)<SORTER_MAX_MERGE_COUNT; 
      i++
  ){
    int iIter = i - iFirst;

    assert( iIter<=pSorter->nAlloc );
    if( iIter==pSorter->nAlloc ){
      rc = vdbeSorterGrowArrays(db, pSorter);
    }

    if( rc==SQLITE_OK ){
      VdbeSorterIter *pIter = &pSorter->aIter[iIter];
      i64 iStart = pSorter->aOffset[i];
      i64 iEof;
      if( i==(pSorter->nOffset-1) ){
        iEof = pSorter->iWriteOff;
      }else{
        iEof = pSorter->aOffset[i+1];
      }
      rc = vdbeSorterIterInit(db, pSorter->pTemp1, iStart, iEof, pIter);
      if( i>iFirst+1 ){
        int nRef = (i-iFirst)*10;
        if( nRef>=nMaxRef ){
          i++;
          break;
        }
      }
    }
  }
  *piNext = i;

  assert( i>iFirst );
  while( (i-iFirst)>N ) N += N;
  pSorter->nTree = N;

  /* Populate the aTree[] array. */
  for(i=N-1; rc==SQLITE_OK && i>0; i--){
    rc = vdbeSorterDoCompare(pCsr, i);
  }

  if( rc!=SQLITE_OK ) return rc;
  if( pSorter->nOffset==0 ){
    *pbEof = 1;
    return SQLITE_OK;
  }

  while( rc==SQLITE_OK ){
    int iRoot = 0;
    int iNext = 0;                /* Index of next segment to open */
    int iNew = 0;                 /* Index of new, merged, PMA */

    do {

      /* This call configures iterators for merging. */
      rc = vdbeSorterInitMerge(db, pCsr, iNext, &iNext);
      assert( iNext>0 );
      assert( rc!=SQLITE_OK || pSorter->aIter[ pSorter->aTree[1] ].pFile );

      if( rc==SQLITE_OK && (iRoot>0 || iNext<pSorter->nOffset) ){
        int pgno;
        int bEof = 0;

        if( pTemp2==0 ){
          rc = vdbeSorterOpenTempFile(db, &pTemp2);
        }
        if( rc==SQLITE_OK ){
          pSorter->aOffset[iRoot] = iWrite2;
        }

        while( rc==SQLITE_OK && bEof==0 ){
          int nByte;
          VdbeSorterIter *pIter = &pSorter->aIter[ pSorter->aTree[1] ];
          assert( pIter->pFile );
          nByte = pIter->nKey + sqlite3VarintLen(pIter->nKey);
          rc = sqlite3OsWrite(pTemp2, pIter->aAlloc, nByte, iWrite2);
          iWrite2 += nByte;
          if( rc==SQLITE_OK ){
            rc = sqlite3VdbeSorterNext(db, pCsr, &bEof);
          }
        }
        iRoot++;
      }
    }while( rc==SQLITE_OK && iNext<pSorter->nOffset );

    if( iRoot==0 ){
      break;
    }else{
      sqlite3_file *pTmp = pSorter->pTemp1;
      pSorter->nOffset = iRoot;
      pSorter->pTemp1 = pTemp2;
      pTemp2 = pTmp;
      pSorter->iWriteOff = iWrite2;
      iWrite2 = 0;
    }
  }