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