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298 lines
9.4 KiB
298 lines
9.4 KiB
/***
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*qsort.c - quicksort algorithm; qsort() library function for sorting arrays
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*
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* Copyright (c) 1985-1993, Microsoft Corporation. All rights reserved.
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*
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*Purpose:
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* To implement the qsort() routine for sorting arrays.
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*
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* This routine has been modified to suit the reference implementation
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*
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*******************************************************************************/
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#include "qsort.h"
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/* prototypes for local routines */
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static void shortsort(char *lo, char *hi, unsigned width,
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int (*comp)(const void *, const void *));
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static void swap(char *p, char *q, unsigned int width);
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/* this parameter defines the cutoff between using quick sort and
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insertion sort for arrays; arrays with lengths shorter or equal to the
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below value use insertion sort */
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#define CUTOFF 8 /* testing shows that this is good value */
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/***
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*ref_qsort(base, num, wid, comp) - quicksort function for sorting arrays
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*
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*Purpose:
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* quicksort the array of elements
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* side effects: sorts in place
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*
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*Entry:
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* char *base = pointer to base of array
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* unsigned num = number of elements in the array
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* unsigned width = width in bytes of each array element
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* int (*comp)() = pointer to function returning analog of strcmp for
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* strings, but supplied by user for comparing the array elements.
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* it accepts 2 pointers to elements and returns neg if 1<2, 0 if
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* 1=2, pos if 1>2.
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*
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*Exit:
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* returns void
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*
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*Exceptions:
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*
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*******************************************************************************/
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/* sort the array between lo and hi (inclusive) */
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void ref_qsort (
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void *base,
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unsigned num,
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unsigned width,
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int (*comp)(const void *, const void *)
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)
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{
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char *lo, *hi; /* ends of sub-array currently sorting */
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char *mid; /* points to middle of subarray */
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char *loguy, *higuy; /* traveling pointers for partition step */
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unsigned size; /* size of the sub-array */
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char *lostk[30], *histk[30];
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int stkptr; /* stack for saving sub-array to be processed */
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/* Note: the number of stack entries required is no more than
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1 + log2(size), so 30 is sufficient for any array */
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if (num < 2 || width == 0)
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return; /* nothing to do */
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stkptr = 0; /* initialize stack */
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lo = (char *) base;
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hi = (char *)base + width * (num-1); /* initialize limits */
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/* this entry point is for pseudo-recursion calling: setting
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lo and hi and jumping to here is like recursion, but stkptr is
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prserved, locals aren't, so we preserve stuff on the stack */
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recurse:
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size = (hi - lo) / width + 1; /* number of el's to sort */
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/* below a certain size, it is faster to use a O(n^2) sorting method */
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if (size <= CUTOFF) {
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shortsort(lo, hi, width, comp);
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}
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else {
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/* First we pick a partititioning element. The efficiency of the
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algorithm demands that we find one that is approximately the
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median of the values, but also that we select one fast. Using
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the first one produces bad performace if the array is already
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sorted, so we use the middle one, which would require a very
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wierdly arranged array for worst case performance. Testing shows
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that a median-of-three algorithm does not, in general, increase
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performance. */
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mid = lo + (size / 2) * width; /* find middle element */
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swap(mid, lo, width); /* swap it to beginning of array */
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/* We now wish to partition the array into three pieces, one
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consisiting of elements <= partition element, one of elements
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equal to the parition element, and one of element >= to it. This
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is done below; comments indicate conditions established at every
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step. */
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loguy = lo;
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higuy = hi + width;
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/* Note that higuy decreases and loguy increases on every iteration,
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so loop must terminate. */
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for (;;) {
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/* lo <= loguy < hi, lo < higuy <= hi + 1,
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A[i] <= A[lo] for lo <= i <= loguy,
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A[i] >= A[lo] for higuy <= i <= hi */
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do {
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loguy += width;
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} while (loguy <= hi && comp(loguy, lo) <= 0);
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/* lo < loguy <= hi+1, A[i] <= A[lo] for lo <= i < loguy,
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either loguy > hi or A[loguy] > A[lo] */
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do {
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higuy -= width;
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} while (higuy > lo && comp(higuy, lo) >= 0);
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/* lo-1 <= higuy <= hi, A[i] >= A[lo] for higuy < i <= hi,
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either higuy <= lo or A[higuy] < A[lo] */
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if (higuy < loguy)
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break;
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/* if loguy > hi or higuy <= lo, then we would have exited, so
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A[loguy] > A[lo], A[higuy] < A[lo],
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loguy < hi, highy > lo */
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swap(loguy, higuy, width);
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/* A[loguy] < A[lo], A[higuy] > A[lo]; so condition at top
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of loop is re-established */
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}
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/* A[i] >= A[lo] for higuy < i <= hi,
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A[i] <= A[lo] for lo <= i < loguy,
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higuy < loguy, lo <= higuy <= hi
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implying:
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A[i] >= A[lo] for loguy <= i <= hi,
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A[i] <= A[lo] for lo <= i <= higuy,
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A[i] = A[lo] for higuy < i < loguy */
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swap(lo, higuy, width); /* put partition element in place */
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/* OK, now we have the following:
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A[i] >= A[higuy] for loguy <= i <= hi,
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A[i] <= A[higuy] for lo <= i < higuy
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A[i] = A[lo] for higuy <= i < loguy */
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/* We've finished the partition, now we want to sort the subarrays
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[lo, higuy-1] and [loguy, hi].
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We do the smaller one first to minimize stack usage.
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We only sort arrays of length 2 or more.*/
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if ( higuy - 1 - lo >= hi - loguy ) {
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if (lo + width < higuy) {
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lostk[stkptr] = lo;
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histk[stkptr] = higuy - width;
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++stkptr;
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} /* save big recursion for later */
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if (loguy < hi) {
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lo = loguy;
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goto recurse; /* do small recursion */
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}
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}
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else {
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if (loguy < hi) {
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lostk[stkptr] = loguy;
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histk[stkptr] = hi;
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++stkptr; /* save big recursion for later */
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}
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if (lo + width < higuy) {
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hi = higuy - width;
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goto recurse; /* do small recursion */
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}
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}
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}
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/* We have sorted the array, except for any pending sorts on the stack.
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Check if there are any, and do them. */
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--stkptr;
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if (stkptr >= 0) {
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lo = lostk[stkptr];
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hi = histk[stkptr];
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goto recurse; /* pop subarray from stack */
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}
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else
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return; /* all subarrays done */
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}
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/***
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*shortsort(hi, lo, width, comp) - insertion sort for sorting short arrays
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*
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*Purpose:
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* sorts the sub-array of elements between lo and hi (inclusive)
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* side effects: sorts in place
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* assumes that lo < hi
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*
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*Entry:
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* char *lo = pointer to low element to sort
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* char *hi = pointer to high element to sort
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* unsigned width = width in bytes of each array element
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* int (*comp)() = pointer to function returning analog of strcmp for
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* strings, but supplied by user for comparing the array elements.
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* it accepts 2 pointers to elements and returns neg if 1<2, 0 if
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* 1=2, pos if 1>2.
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*
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*Exit:
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* returns void
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*
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*Exceptions:
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*
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*******************************************************************************/
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static void shortsort (
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char *lo,
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char *hi,
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unsigned width,
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int (*comp)(const void *, const void *)
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)
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{
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char *p, *max;
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/* Note: in assertions below, i and j are alway inside original bound of
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array to sort. */
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while (hi > lo) {
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/* A[i] <= A[j] for i <= j, j > hi */
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max = lo;
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for (p = lo+width; p <= hi; p += width) {
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/* A[i] <= A[max] for lo <= i < p */
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if (comp(p, max) > 0) {
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max = p;
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}
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/* A[i] <= A[max] for lo <= i <= p */
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}
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/* A[i] <= A[max] for lo <= i <= hi */
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swap(max, hi, width);
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/* A[i] <= A[hi] for i <= hi, so A[i] <= A[j] for i <= j, j >= hi */
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hi -= width;
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/* A[i] <= A[j] for i <= j, j > hi, loop top condition established */
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}
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/* A[i] <= A[j] for i <= j, j > lo, which implies A[i] <= A[j] for i < j,
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so array is sorted */
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}
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/***
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*swap(a, b, width) - swap two elements
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*
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*Purpose:
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* swaps the two array elements of size width
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*
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*Entry:
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* char *a, *b = pointer to two elements to swap
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* unsigned width = width in bytes of each array element
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*
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*Exit:
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* returns void
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*
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*Exceptions:
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*
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*******************************************************************************/
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static void swap (
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char *a,
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char *b,
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unsigned width
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)
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{
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char tmp;
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if ( a != b )
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/* Do the swap one character at a time to avoid potential alignment
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problems. */
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while ( width-- ) {
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tmp = *a;
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*a++ = *b;
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*b++ = tmp;
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}
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}
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