146 lines
4.2 KiB
C
146 lines
4.2 KiB
C
/*
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* Copyright (c) 2014
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* The President and Fellows of Harvard College.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* 1. Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution.
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* 3. Neither the name of the University nor the names of its contributors
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* may be used to endorse or promote products derived from this software
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* without specific prior written permission.
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*
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* THIS SOFTWARE IS PROVIDED BY THE UNIVERSITY AND CONTRIBUTORS ``AS IS'' AND
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* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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* ARE DISCLAIMED. IN NO EVENT SHALL THE UNIVERSITY OR CONTRIBUTORS BE LIABLE
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* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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* SUCH DAMAGE.
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*/
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#include <stdlib.h>
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#include <string.h>
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#include <assert.h>
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/*
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* qsort() for OS/161, where it isn't in libc.
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*/
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void
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qsort(void *vdata, unsigned num, size_t size,
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int (*f)(const void *, const void *))
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{
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unsigned pivot, head, tail;
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char *data = vdata;
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char tmp[size];
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#define COMPARE(aa, bb) \
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((aa) == (bb) ? 0 : f(data + (aa) * size, data + (bb) * size))
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#define EXCHANGE(aa, bb) \
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memcpy(tmp, data + (aa) * size, size); \
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memcpy(data + (aa) * size, data + (bb) * size, size); \
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memcpy(data + (bb) * size, tmp, size)
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if (num <= 1) {
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return;
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}
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if (num == 2) {
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if (COMPARE(0, 1) > 0) {
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EXCHANGE(0, 1);
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return;
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}
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}
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/*
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* 1. Pick a pivot value. For simplicity, always use the
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* middle of the array.
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*/
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pivot = num / 2;
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/*
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* 2. Shift all values less than or equal to the pivot value
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* to the front of the array.
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*/
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head = 0;
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tail = num - 1;
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while (head < tail) {
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if (COMPARE(head, pivot) <= 0) {
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head++;
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}
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else if (COMPARE(tail, pivot) > 0) {
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tail--;
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}
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else {
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EXCHANGE(head, tail);
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if (pivot == head) {
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pivot = tail;
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}
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else if (pivot == tail) {
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pivot = head;
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}
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head++;
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tail--;
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}
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}
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/*
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* 3. If there's an even number of elements and we swapped the
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* last two, the head and tail indexes will cross. In this
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* case the first entry on the tail side is tail+1. If there's
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* an odd number of elements, we stop with head == tail, and
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* the first entry on the tail side is this value (hence,
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* tail) if it's is greater than the pivot value, and the next
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* element (hence, tail+1) if it's less than or equal to the
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* pivot value.
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*
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* Henceforth use "tail" to hold the index of the first entry
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* of the back portion of the array.
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*/
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if (head > tail || COMPARE(head, pivot) <= 0) {
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tail++;
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}
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/*
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* 4. If we got a bad pivot that gave us only one partition,
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* because of the order of the advances in the loop above it
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* will always put everything in the front portion of the
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* array (so tail == num). This happens if we picked the
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* largest value. Move the pivot to the end, if necessary, lop
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* off all values equal to it, and recurse on the rest. (If
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* there is no rest, the array is already sorted and we're
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* done.)
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*/
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if (tail == num) {
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if (pivot < num - 1) {
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if (COMPARE(pivot, num - 1) > 0) {
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EXCHANGE(pivot, num - 1);
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}
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}
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tail = num - 1;
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while (tail > 0 && COMPARE(tail - 1, tail) == 0) {
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tail--;
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}
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if (tail > 0) {
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qsort(vdata, tail, size, f);
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}
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return;
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}
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assert(tail > 0 && tail < num);
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/*
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* 5. Recurse on each subpart of the array.
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*/
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qsort(vdata, tail, size, f);
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qsort((char *)vdata + tail * size, num - tail, size, f);
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}
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