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Annotation of 43BSDReno/contrib/emacs-18.55/etc/qsort.c, revision 1.1

1.1     ! root        1: /*
        !             2:  * qsort.c:
        !             3:  * Our own version of the system qsort routine which is faster by an average
        !             4:  * of 25%, with lows and highs of 10% and 50%.
        !             5:  * The THRESHold below is the insertion sort threshold, and has been adjusted
        !             6:  * for records of size 48 bytes.
        !             7:  * The MTHREShold is where we stop finding a better median.
        !             8:  */
        !             9: 
        !            10: #define                THRESH          4               /* threshold for insertion */
        !            11: 
        !            12: #define                MTHRESH         6               /* threshold for median */
        !            13: 
        !            14: 
        !            15: 
        !            16: static  int            qsz;                    /* size of each record */
        !            17: static  int            (*qcmp)();              /* the comparison routine */
        !            18: 
        !            19: static  int            thresh;                 /* THRESHold in chars */
        !            20: static  int            mthresh;                /* MTHRESHold in chars */
        !            21: 
        !            22: /*
        !            23:  * qsort:
        !            24:  * First, set up some global parameters for qst to share.  Then, quicksort
        !            25:  * with qst(), and then a cleanup insertion sort ourselves.  Sound simple?
        !            26:  * It's not...
        !            27:  */
        !            28: qsort (base, n, size, compar)
        !            29:      char *base;
        !            30:      int n;
        !            31:      int size;
        !            32:      int (*compar)();
        !            33: {
        !            34:   register char *i, *j, *lo, *hi, *min;
        !            35:   register int c;
        !            36:   char *max;
        !            37: 
        !            38:   if (n <= 1)  return;
        !            39:   qsz = size;
        !            40:   qcmp = compar;
        !            41:   thresh = qsz*THRESH;
        !            42:   mthresh = qsz*MTHRESH;
        !            43:   max = base + n*qsz;
        !            44:   if (n >= THRESH)
        !            45:     {
        !            46:       qst (base, max);
        !            47:       hi = base + thresh;
        !            48:     }
        !            49:   else
        !            50:     {
        !            51:       hi = max;
        !            52:     }
        !            53:   /*
        !            54:    * First put smallest element, which must be in the first THRESH, in
        !            55:    * the first position as a sentinel.  This is done just by searching
        !            56:    * the first THRESH elements (or the first n if n < THRESH), finding
        !            57:    * the min, and swapping it into the first position.
        !            58:    */
        !            59:   for (j = lo = base; (lo += qsz) < hi; )
        !            60:     {
        !            61:       if ((*qcmp) (j, lo) > 0)
        !            62:        j = lo;
        !            63:     }
        !            64:   if (j != base)
        !            65:     {                  /* swap j into place */
        !            66:       for (i = base, hi = base + qsz; i < hi;)
        !            67:        {
        !            68:          c = *j;
        !            69:          *j++ = *i;
        !            70:          *i++ = c;
        !            71:        }
        !            72:     }
        !            73:   /*
        !            74:    * With our sentinel in place, we now run the following hyper-fast
        !            75:    * insertion sort.  For each remaining element, min, from [1] to [n-1],
        !            76:    * set hi to the index of the element AFTER which this one goes.
        !            77:    * Then, do the standard insertion sort shift on a character at a time
        !            78:    * basis for each element in the frob.
        !            79:    */
        !            80:   for (min = base; (hi = min += qsz) < max;)
        !            81:     {
        !            82:       while ( (*qcmp) (hi -= qsz, min) > 0);
        !            83:       if ((hi += qsz) != min)
        !            84:        {
        !            85:          for (lo = min + qsz; --lo >= min;)
        !            86:            {
        !            87:              c = *lo;
        !            88:              for (i = j = lo; (j -= qsz) >= hi; i = j)
        !            89:                *i = *j;
        !            90:              *i = c;
        !            91:            }
        !            92:        }
        !            93:     }
        !            94: }
        !            95: 
        !            96: /*
        !            97:  * qst:
        !            98:  * Do a quicksort
        !            99:  * First, find the median element, and put that one in the first place as the
        !           100:  * discriminator.  (This "median" is just the median of the first, last and
        !           101:  * middle elements).  (Using this median instead of the first element is a big
        !           102:  * win).  Then, the usual partitioning/swapping, followed by moving the
        !           103:  * discriminator into the right place.  Then, figure out the sizes of the two
        !           104:  * partions, do the smaller one recursively and the larger one via a repeat of
        !           105:  * this code.  Stopping when there are less than THRESH elements in a partition
        !           106:  * and cleaning up with an insertion sort (in our caller) is a huge win.
        !           107:  * All data swaps are done in-line, which is space-losing but time-saving.
        !           108:  * (And there are only three places where this is done).
        !           109:  */
        !           110: 
        !           111: qst (base, max)
        !           112:      char *base, *max;
        !           113: {
        !           114:   register char *i, *j, *jj, *mid;
        !           115:   register int ii, c;
        !           116:   char *tmp;
        !           117:   int lo, hi;
        !           118: 
        !           119:   lo = max - base;             /* number of elements as chars */
        !           120:   do
        !           121:     {
        !           122:       /*
        !           123:        * At the top here, lo is the number of characters of elements in the
        !           124:        * current partition.  (Which should be max - base).
        !           125:        * Find the median of the first, last, and middle element and make that the
        !           126:        * middle element.  Set j to largest of first and middle.  If max is larger
        !           127:        * than that guy, then it's that guy, else compare max with loser of first
        !           128:        * and take larger.  Things are set up to prefer the middle, then the first
        !           129:        * in case of ties.
        !           130:        */
        !           131:       mid = i = base + qsz * ((lo/qsz) >> 1);
        !           132:       if (lo >= mthresh)
        !           133:        {
        !           134:          j = ((*qcmp) ((jj = base), i) > 0 ? jj : i);
        !           135:          if ((*qcmp) (j, (tmp = max - qsz)) > 0)
        !           136:            {
        !           137:              j = (j == jj ? i : jj);   /* switch to first loser */
        !           138:              if ((*qcmp) (j, tmp) < 0)
        !           139:                j = tmp;
        !           140:            }
        !           141:          if (j != i)
        !           142:            {
        !           143:              ii = qsz;
        !           144:              do
        !           145:                {
        !           146:                  c = *i;
        !           147:                  *i++ = *j;
        !           148:                  *j++ = c;
        !           149:                }
        !           150:              while(  --ii  );
        !           151:            }
        !           152:        }
        !           153:       /*
        !           154:        * Semi-standard quicksort partitioning/swapping
        !           155:        */
        !           156:       for (i = base, j = max - qsz; ;)
        !           157:        {
        !           158:          while (i < mid && (*qcmp) (i, mid) <= 0)
        !           159:            i += qsz;
        !           160:          while (j > mid)
        !           161:            {
        !           162:              if ((*qcmp) (mid, j) <= 0)
        !           163:                {
        !           164:                  j -= qsz;
        !           165:                  continue;
        !           166:                }
        !           167:              tmp = i + qsz;            /* value of i after swap */
        !           168:              if (i == mid)
        !           169:                {       /* j <-> mid, new mid is j */
        !           170:                  mid = jj = j;
        !           171:                }
        !           172:              else
        !           173:                {                       /* i <-> j */
        !           174:                  jj = j;
        !           175:                  j -= qsz;
        !           176:                }
        !           177:              goto  swap;
        !           178:            }
        !           179:          if (i == mid)
        !           180:            {
        !           181:              break;
        !           182:            }
        !           183:          else
        !           184:            {                           /* i <-> mid, new mid is i */
        !           185:              jj = mid;
        !           186:              tmp = mid = i;            /* value of i after swap */
        !           187:              j -= qsz;
        !           188:            }
        !           189:        swap:
        !           190:          ii = qsz;
        !           191:          do
        !           192:            {
        !           193:              c = *i;
        !           194:              *i++ = *jj;
        !           195:              *jj++ = c;
        !           196:            }
        !           197:          while (--ii);
        !           198:          i = tmp;
        !           199:        }
        !           200:       /*
        !           201:        * Look at sizes of the two partitions, do the smaller one first by
        !           202:        * recursion, then do the larger one by making sure lo is its size,
        !           203:        * base and max are update correctly, and branching back.
        !           204:        * But only repeat (recursively or by branching) if the partition is
        !           205:        * of at least size THRESH.
        !           206:        */
        !           207:       i = (j = mid) + qsz;
        !           208:       if ((lo = j - base) <= (hi = max - i))
        !           209:        {
        !           210:          if (lo >= thresh)
        !           211:            qst (base, j);
        !           212:          base = i;
        !           213:          lo = hi;
        !           214:        }
        !           215:       else
        !           216:        {
        !           217:          if (hi >= thresh)
        !           218:            qst (i, max);
        !           219:          max = j;
        !           220:        }
        !           221:     }
        !           222:   while (lo >= thresh);
        !           223: }
        !           224: 
        !           225: 
        !           226: 

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