332 lines
11 KiB
C
332 lines
11 KiB
C
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 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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#define STKSIZ (8*sizeof(void*) - 2)
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/***
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*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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* maximum array size is number of elements times size of elements,
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* but is limited by the virtual address space of the processor
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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.
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* Returns neg if 1<2, 0 if 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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*******************************************************************************/
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void 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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/* Note: the number of stack entries required is no more than
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1 + log2(num), so 30 is sufficient for any array */
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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[STKSIZ], *histk[STKSIZ];
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int stkptr; /* stack for saving sub-array to be processed */
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if (num < 2)
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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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preserved, 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 partitioning element. The efficiency of the
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algorithm demands that we find one that is approximately the median
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of the values, but also that we select one fast. We choose the
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median of the first, middle, and last elements, to avoid bad
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performance in the face of already sorted data, or data that is made
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up of multiple sorted runs appended together. Testing shows that a
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median-of-three algorithm provides better performance than simply
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picking the middle element for the latter case. */
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mid = lo + (size / 2) * width; /* find middle element */
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/* Sort the first, middle, last elements into order */
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if (comp(lo, mid) > 0) {
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swap(lo, mid, width);
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}
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if (comp(lo, hi) > 0) {
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swap(lo, hi, width);
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}
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if (comp(mid, hi) > 0) {
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swap(mid, hi, width);
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}
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/* We now wish to partition the array into three pieces, one consisting
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of elements <= partition element, one of elements equal to the
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partition element, and one of elements > than it. This is done
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below; comments indicate conditions established at every step. */
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loguy = lo;
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higuy = hi;
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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,
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A[i] <= A[mid] for lo <= i <= loguy,
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A[i] > A[mid] for higuy <= i < hi,
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A[hi] >= A[mid] */
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/* The doubled loop is to avoid calling comp(mid,mid), since some
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existing comparison funcs don't work when passed the same
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value for both pointers. */
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if (mid > loguy) {
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do {
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loguy += width;
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} while (loguy < mid && comp(loguy, mid) <= 0);
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}
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if (mid <= loguy) {
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do {
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loguy += width;
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} while (loguy <= hi && comp(loguy, mid) <= 0);
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}
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/* lo < loguy <= hi+1, A[i] <= A[mid] for lo <= i < loguy,
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either loguy > hi or A[loguy] > A[mid] */
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do {
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higuy -= width;
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} while (higuy > mid && comp(higuy, mid) > 0);
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/* lo <= higuy < hi, A[i] > A[mid] for higuy < i < hi,
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either higuy == lo or A[higuy] <= A[mid] */
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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[mid], A[higuy] <= A[mid],
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loguy <= hi, higuy > lo */
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swap(loguy, higuy, width);
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/* If the partition element was moved, follow it. Only need
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to check for mid == higuy, since before the swap,
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A[loguy] > A[mid] implies loguy != mid. */
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if (mid == higuy)
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mid = loguy;
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/* A[loguy] <= A[mid], A[higuy] > A[mid]; so condition at top
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of loop is re-established */
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}
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/* A[i] <= A[mid] for lo <= i < loguy,
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A[i] > A[mid] for higuy < i < hi,
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A[hi] >= A[mid]
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higuy < loguy
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implying:
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higuy == loguy-1
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or higuy == hi - 1, loguy == hi + 1, A[hi] == A[mid] */
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/* Find adjacent elements equal to the partition element. The
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doubled loop is to avoid calling comp(mid,mid), since some
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existing comparison funcs don't work when passed the same value
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for both pointers. */
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higuy += width;
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if (mid < higuy) {
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do {
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higuy -= width;
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} while (higuy > mid && comp(higuy, mid) == 0);
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}
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if (mid >= higuy) {
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do {
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higuy -= width;
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} while (higuy > lo && comp(higuy, mid) == 0);
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}
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/* OK, now we have the following:
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higuy < loguy
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lo <= higuy <= hi
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A[i] <= A[mid] for lo <= i <= higuy
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A[i] == A[mid] for higuy < i < loguy
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A[i] > A[mid] for loguy <= i < hi
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A[hi] >= A[mid] */
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/* We've finished the partition, now we want to sort the subarrays
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[lo, higuy] 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 - lo >= hi - loguy ) {
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if (lo < higuy) {
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lostk[stkptr] = lo;
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histk[stkptr] = higuy;
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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 < higuy) {
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hi = higuy;
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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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*shortsort_s(hi, lo, width, comp, context) - 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, together with a pointer to a context
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* (if present). Returns neg if 1<2, 0 if 1=2, pos if 1>2.
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* void *context - pointer to the context in which the function is
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* called. This context is passed to the comparison function.
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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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