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Annotation of 42BSD/lib/libc/gen/qsort.c, revision 1.1

1.1     ! root        1: /* @(#)qsort.c 4.2 (Berkeley) 3/9/83 */
        !             2: 
        !             3: /*
        !             4:  * qsort.c:
        !             5:  * Our own version of the system qsort routine which is faster by an average
        !             6:  * of 25%, with lows and highs of 10% and 50%.
        !             7:  * The THRESHold below is the insertion sort threshold, and has been adjusted
        !             8:  * for records of size 48 bytes.
        !             9:  * The MTHREShold is where we stop finding a better median.
        !            10:  */
        !            11: 
        !            12: #define                THRESH          4               /* threshold for insertion */
        !            13: #define                MTHRESH         6               /* threshold for median */
        !            14: 
        !            15: static  int            (*qcmp)();              /* the comparison routine */
        !            16: static  int            qsz;                    /* size of each record */
        !            17: static  int            thresh;                 /* THRESHold in chars */
        !            18: static  int            mthresh;                /* MTHRESHold in chars */
        !            19: 
        !            20: /*
        !            21:  * qsort:
        !            22:  * First, set up some global parameters for qst to share.  Then, quicksort
        !            23:  * with qst(), and then a cleanup insertion sort ourselves.  Sound simple?
        !            24:  * It's not...
        !            25:  */
        !            26: 
        !            27: qsort(base, n, size, compar)
        !            28:        char    *base;
        !            29:        int     n;
        !            30:        int     size;
        !            31:        int     (*compar)();
        !            32: {
        !            33:        register char c, *i, *j, *lo, *hi;
        !            34:        char *min, *max;
        !            35: 
        !            36:        if (n <= 1)
        !            37:                return;
        !            38:        qsz = size;
        !            39:        qcmp = compar;
        !            40:        thresh = qsz * THRESH;
        !            41:        mthresh = qsz * MTHRESH;
        !            42:        max = base + n * qsz;
        !            43:        if (n >= THRESH) {
        !            44:                qst(base, max);
        !            45:                hi = base + thresh;
        !            46:        } else {
        !            47:                hi = max;
        !            48:        }
        !            49:        /*
        !            50:         * First put smallest element, which must be in the first THRESH, in
        !            51:         * the first position as a sentinel.  This is done just by searching
        !            52:         * the first THRESH elements (or the first n if n < THRESH), finding
        !            53:         * the min, and swapping it into the first position.
        !            54:         */
        !            55:        for (j = lo = base; (lo += qsz) < hi; )
        !            56:                if (qcmp(j, lo) > 0)
        !            57:                        j = lo;
        !            58:        if (j != base) {
        !            59:                /* swap j into place */
        !            60:                for (i = base, hi = base + qsz; i < hi; ) {
        !            61:                        c = *j;
        !            62:                        *j++ = *i;
        !            63:                        *i++ = c;
        !            64:                }
        !            65:        }
        !            66:        /*
        !            67:         * With our sentinel in place, we now run the following hyper-fast
        !            68:         * insertion sort.  For each remaining element, min, from [1] to [n-1],
        !            69:         * set hi to the index of the element AFTER which this one goes.
        !            70:         * Then, do the standard insertion sort shift on a character at a time
        !            71:         * basis for each element in the frob.
        !            72:         */
        !            73:        for (min = base; (hi = min += qsz) < max; ) {
        !            74:                while (qcmp(hi -= qsz, min) > 0)
        !            75:                        /* void */;
        !            76:                if ((hi += qsz) != min) {
        !            77:                        for (lo = min + qsz; --lo >= min; ) {
        !            78:                                c = *lo;
        !            79:                                for (i = j = lo; (j -= qsz) >= hi; i = j)
        !            80:                                        *i = *j;
        !            81:                                *i = c;
        !            82:                        }
        !            83:                }
        !            84:        }
        !            85: }
        !            86: 
        !            87: /*
        !            88:  * qst:
        !            89:  * Do a quicksort
        !            90:  * First, find the median element, and put that one in the first place as the
        !            91:  * discriminator.  (This "median" is just the median of the first, last and
        !            92:  * middle elements).  (Using this median instead of the first element is a big
        !            93:  * win).  Then, the usual partitioning/swapping, followed by moving the
        !            94:  * discriminator into the right place.  Then, figure out the sizes of the two
        !            95:  * partions, do the smaller one recursively and the larger one via a repeat of
        !            96:  * this code.  Stopping when there are less than THRESH elements in a partition
        !            97:  * and cleaning up with an insertion sort (in our caller) is a huge win.
        !            98:  * All data swaps are done in-line, which is space-losing but time-saving.
        !            99:  * (And there are only three places where this is done).
        !           100:  */
        !           101: 
        !           102: static
        !           103: qst(base, max)
        !           104:        char *base, *max;
        !           105: {
        !           106:        register char c, *i, *j, *jj;
        !           107:        register int ii;
        !           108:        char *mid, *tmp;
        !           109:        int lo, hi;
        !           110: 
        !           111:        /*
        !           112:         * At the top here, lo is the number of characters of elements in the
        !           113:         * current partition.  (Which should be max - base).
        !           114:         * Find the median of the first, last, and middle element and make
        !           115:         * that the middle element.  Set j to largest of first and middle.
        !           116:         * If max is larger than that guy, then it's that guy, else compare
        !           117:         * max with loser of first and take larger.  Things are set up to
        !           118:         * prefer the middle, then the first in case of ties.
        !           119:         */
        !           120:        lo = max - base;                /* number of elements as chars */
        !           121:        do      {
        !           122:                mid = i = base + qsz * ((lo / qsz) >> 1);
        !           123:                if (lo >= mthresh) {
        !           124:                        j = (qcmp((jj = base), i) > 0 ? jj : i);
        !           125:                        if (qcmp(j, (tmp = max - qsz)) > 0) {
        !           126:                                /* switch to first loser */
        !           127:                                j = (j == jj ? i : jj);
        !           128:                                if (qcmp(j, tmp) < 0)
        !           129:                                        j = tmp;
        !           130:                        }
        !           131:                        if (j != i) {
        !           132:                                ii = qsz;
        !           133:                                do      {
        !           134:                                        c = *i;
        !           135:                                        *i++ = *j;
        !           136:                                        *j++ = c;
        !           137:                                } while (--ii);
        !           138:                        }
        !           139:                }
        !           140:                /*
        !           141:                 * Semi-standard quicksort partitioning/swapping
        !           142:                 */
        !           143:                for (i = base, j = max - qsz; ; ) {
        !           144:                        while (i < mid && qcmp(i, mid) <= 0)
        !           145:                                i += qsz;
        !           146:                        while (j > mid) {
        !           147:                                if (qcmp(mid, j) <= 0) {
        !           148:                                        j -= qsz;
        !           149:                                        continue;
        !           150:                                }
        !           151:                                tmp = i + qsz;  /* value of i after swap */
        !           152:                                if (i == mid) {
        !           153:                                        /* j <-> mid, new mid is j */
        !           154:                                        mid = jj = j;
        !           155:                                } else {
        !           156:                                        /* i <-> j */
        !           157:                                        jj = j;
        !           158:                                        j -= qsz;
        !           159:                                }
        !           160:                                goto swap;
        !           161:                        }
        !           162:                        if (i == mid) {
        !           163:                                break;
        !           164:                        } else {
        !           165:                                /* i <-> mid, new mid is i */
        !           166:                                jj = mid;
        !           167:                                tmp = mid = i;  /* value of i after swap */
        !           168:                                j -= qsz;
        !           169:                        }
        !           170:                swap:
        !           171:                        ii = qsz;
        !           172:                        do      {
        !           173:                                c = *i;
        !           174:                                *i++ = *jj;
        !           175:                                *jj++ = c;
        !           176:                        } while (--ii);
        !           177:                        i = tmp;
        !           178:                }
        !           179:                /*
        !           180:                 * Look at sizes of the two partitions, do the smaller
        !           181:                 * one first by recursion, then do the larger one by
        !           182:                 * making sure lo is its size, base and max are update
        !           183:                 * correctly, and branching back.  But only repeat
        !           184:                 * (recursively or by branching) if the partition is
        !           185:                 * of at least size THRESH.
        !           186:                 */
        !           187:                i = (j = mid) + qsz;
        !           188:                if ((lo = j - base) <= (hi = max - i)) {
        !           189:                        if (lo >= thresh)
        !           190:                                qst(base, j);
        !           191:                        base = i;
        !           192:                        lo = hi;
        !           193:                } else {
        !           194:                        if (hi >= thresh)
        !           195:                                qst(i, max);
        !           196:                        max = j;
        !           197:                }
        !           198:        } while (lo >= thresh);
        !           199: }

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