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250 lines
8.2 KiB
C
250 lines
8.2 KiB
C
/* Copyright (C) 1991-2017 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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Written by Douglas C. Schmidt (schmidt@ics.uci.edu).
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<http://www.gnu.org/licenses/>. */
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/* If you consider tuning this algorithm, you should consult first:
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Engineering a sort function; Jon Bentley and M. Douglas McIlroy;
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Software - Practice and Experience; Vol. 23 (11), 1249-1265, 1993. */
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#include <alloca.h>
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#include <limits.h>
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#include <stdlib.h>
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#include <string.h>
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/* Byte-wise swap two items of size SIZE. */
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#define SWAP(a, b, size) \
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do \
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{ \
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size_t __size = (size); \
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char *__a = (a), *__b = (b); \
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do \
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{ \
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char __tmp = *__a; \
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*__a++ = *__b; \
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*__b++ = __tmp; \
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} while (--__size > 0); \
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} while (0)
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/* Discontinue quicksort algorithm when partition gets below this size.
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This particular magic number was chosen to work best on a Sun 4/260. */
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#define MAX_THRESH 4
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/* Stack node declarations used to store unfulfilled partition obligations. */
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typedef struct
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{
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char *lo;
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char *hi;
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} stack_node;
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/* The next 4 #defines implement a very fast in-line stack abstraction. */
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/* The stack needs log (total_elements) entries (we could even subtract
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log(MAX_THRESH)). Since total_elements has type size_t, we get as
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upper bound for log (total_elements):
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bits per byte (CHAR_BIT) * sizeof(size_t). */
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#define STACK_SIZE (CHAR_BIT * sizeof(size_t))
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#define PUSH(low, high) ((void) ((top->lo = (low)), (top->hi = (high)), ++top))
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#define POP(low, high) ((void) (--top, (low = top->lo), (high = top->hi)))
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#define STACK_NOT_EMPTY (stack < top)
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/* Order size using quicksort. This implementation incorporates
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four optimizations discussed in Sedgewick:
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1. Non-recursive, using an explicit stack of pointer that store the
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next array partition to sort. To save time, this maximum amount
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of space required to store an array of SIZE_MAX is allocated on the
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stack. Assuming a 32-bit (64 bit) integer for size_t, this needs
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only 32 * sizeof(stack_node) == 256 bytes (for 64 bit: 1024 bytes).
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Pretty cheap, actually.
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2. Chose the pivot element using a median-of-three decision tree.
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This reduces the probability of selecting a bad pivot value and
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eliminates certain extraneous comparisons.
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3. Only quicksorts TOTAL_ELEMS / MAX_THRESH partitions, leaving
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insertion sort to order the MAX_THRESH items within each partition.
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This is a big win, since insertion sort is faster for small, mostly
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sorted array segments.
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4. The larger of the two sub-partitions is always pushed onto the
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stack first, with the algorithm then concentrating on the
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smaller partition. This *guarantees* no more than log (total_elems)
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stack size is needed (actually O(1) in this case)! */
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void
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_quicksort (void *const pbase, size_t total_elems, size_t size,
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__compar_d_fn_t cmp, void *arg)
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{
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char *base_ptr = (char *) pbase;
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const size_t max_thresh = MAX_THRESH * size;
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if (total_elems == 0)
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/* Avoid lossage with unsigned arithmetic below. */
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return;
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if (total_elems > MAX_THRESH)
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{
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char *lo = base_ptr;
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char *hi = &lo[size * (total_elems - 1)];
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stack_node stack[STACK_SIZE];
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stack_node *top = stack;
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PUSH (NULL, NULL);
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while (STACK_NOT_EMPTY)
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{
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char *left_ptr;
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char *right_ptr;
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/* Select median value from among LO, MID, and HI. Rearrange
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LO and HI so the three values are sorted. This lowers the
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probability of picking a pathological pivot value and
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skips a comparison for both the LEFT_PTR and RIGHT_PTR in
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the while loops. */
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char *mid = lo + size * ((hi - lo) / size >> 1);
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if ((*cmp) ((void *) mid, (void *) lo, arg) < 0)
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SWAP (mid, lo, size);
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if ((*cmp) ((void *) hi, (void *) mid, arg) < 0)
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SWAP (mid, hi, size);
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else
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goto jump_over;
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if ((*cmp) ((void *) mid, (void *) lo, arg) < 0)
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SWAP (mid, lo, size);
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jump_over:;
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left_ptr = lo + size;
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right_ptr = hi - size;
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/* Here's the famous ``collapse the walls'' section of quicksort.
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Gotta like those tight inner loops! They are the main reason
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that this algorithm runs much faster than others. */
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do
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{
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while ((*cmp) ((void *) left_ptr, (void *) mid, arg) < 0)
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left_ptr += size;
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while ((*cmp) ((void *) mid, (void *) right_ptr, arg) < 0)
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right_ptr -= size;
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if (left_ptr < right_ptr)
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{
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SWAP (left_ptr, right_ptr, size);
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if (mid == left_ptr)
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mid = right_ptr;
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else if (mid == right_ptr)
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mid = left_ptr;
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left_ptr += size;
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right_ptr -= size;
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}
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else if (left_ptr == right_ptr)
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{
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left_ptr += size;
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right_ptr -= size;
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break;
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}
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}
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while (left_ptr <= right_ptr);
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/* Set up pointers for next iteration. First determine whether
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left and right partitions are below the threshold size. If so,
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ignore one or both. Otherwise, push the larger partition's
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bounds on the stack and continue sorting the smaller one. */
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if ((size_t) (right_ptr - lo) <= max_thresh)
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{
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if ((size_t) (hi - left_ptr) <= max_thresh)
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/* Ignore both small partitions. */
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POP (lo, hi);
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else
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/* Ignore small left partition. */
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lo = left_ptr;
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}
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else if ((size_t) (hi - left_ptr) <= max_thresh)
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/* Ignore small right partition. */
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hi = right_ptr;
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else if ((right_ptr - lo) > (hi - left_ptr))
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{
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/* Push larger left partition indices. */
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PUSH (lo, right_ptr);
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lo = left_ptr;
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}
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else
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{
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/* Push larger right partition indices. */
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PUSH (left_ptr, hi);
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hi = right_ptr;
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}
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}
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}
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/* Once the BASE_PTR array is partially sorted by quicksort the rest
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is completely sorted using insertion sort, since this is efficient
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for partitions below MAX_THRESH size. BASE_PTR points to the beginning
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of the array to sort, and END_PTR points at the very last element in
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the array (*not* one beyond it!). */
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#define min(x, y) ((x) < (y) ? (x) : (y))
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{
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char *const end_ptr = &base_ptr[size * (total_elems - 1)];
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char *tmp_ptr = base_ptr;
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char *thresh = min(end_ptr, base_ptr + max_thresh);
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char *run_ptr;
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/* Find smallest element in first threshold and place it at the
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array's beginning. This is the smallest array element,
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and the operation speeds up insertion sort's inner loop. */
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for (run_ptr = tmp_ptr + size; run_ptr <= thresh; run_ptr += size)
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if ((*cmp) ((void *) run_ptr, (void *) tmp_ptr, arg) < 0)
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tmp_ptr = run_ptr;
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if (tmp_ptr != base_ptr)
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SWAP (tmp_ptr, base_ptr, size);
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/* Insertion sort, running from left-hand-side up to right-hand-side. */
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run_ptr = base_ptr + size;
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while ((run_ptr += size) <= end_ptr)
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{
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tmp_ptr = run_ptr - size;
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while ((*cmp) ((void *) run_ptr, (void *) tmp_ptr, arg) < 0)
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tmp_ptr -= size;
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tmp_ptr += size;
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if (tmp_ptr != run_ptr)
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{
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char *trav;
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trav = run_ptr + size;
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while (--trav >= run_ptr)
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{
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char c = *trav;
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char *hi, *lo;
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for (hi = lo = trav; (lo -= size) >= tmp_ptr; hi = lo)
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*hi = *lo;
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*hi = c;
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}
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}
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}
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}
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}
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