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/* Copyright (C) 1991, 1992 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 Library General Public License as
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published by the Free Software Foundation; either version 2 of the
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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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Library General Public License for more details.
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You should have received a copy of the GNU Library General Public
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License along with the GNU C Library; see the file COPYING.LIB. If
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not, write to the Free Software Foundation, Inc., 675 Mass Ave,
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Cambridge, MA 02139, USA. */
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/* #include <ansidecl.h> */
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#define __alloca __builtin_alloca
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#define _quicksort qsort
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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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register size_t __size = (size); \
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register char *__a = (a), *__b = (b); \
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} while (--__size > 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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/* Stack node declarations used to store unfulfilled partition obligations. */
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/* The next 4 #defines implement a very fast in-line stack abstraction. */
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#define STACK_SIZE (8 * sizeof(unsigned long int))
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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 MAX_INT is allocated on the
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stack. Assuming a 32-bit integer, this needs only 32 *
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sizeof(stack_node) == 136 bits. 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 segements.
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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 (n)
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stack size is needed (actually O(1) in this case)! */
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_quicksort (void *pbase, size_t total_elems, size_t size, __compar_fn_t cmp)
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register char *base_ptr = (char *) pbase;
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/* Allocating SIZE bytes for a pivot buffer facilitates a better
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algorithm below since we can do comparisons directly on the pivot. */
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char *pivot_buffer = (char *) __alloca (size);
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const size_t max_thresh = MAX_THRESH * size;
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/* Avoid lossage with unsigned arithmetic below. */
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if (total_elems > MAX_THRESH)
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char *hi = &lo[size * (total_elems - 1)];
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/* Largest size needed for 32-bit int!!! */
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stack_node stack[STACK_SIZE];
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stack_node *top = stack + 1;
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while (STACK_NOT_EMPTY)
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char *pivot = pivot_buffer;
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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. */
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char *mid = lo + size * ((hi - lo) / size >> 1);
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if ((*cmp)((void *) mid, (void *) lo) < 0)
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if ((*cmp)((void *) hi, (void *) mid) < 0)
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if ((*cmp)((void *) mid, (void *) lo) < 0)
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memcpy(pivot, mid, size);
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pivot = pivot_buffer;
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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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while ((*cmp)((void *) left_ptr, (void *) pivot) < 0)
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while ((*cmp)((void *) pivot, (void *) right_ptr) < 0)
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if (left_ptr < right_ptr)
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SWAP(left_ptr, right_ptr, size);
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else if (left_ptr == right_ptr)
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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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if ((size_t) (hi - left_ptr) <= max_thresh)
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/* Ignore both small partitions. */
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/* Ignore small left partition. */
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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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else if ((right_ptr - lo) > (hi - left_ptr))
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/* Push larger left partition indices. */
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/* Push larger right partition indices. */
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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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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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register 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) < 0)
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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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tmp_ptr = run_ptr - size;
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while ((*cmp)((void *) run_ptr, (void *) tmp_ptr) < 0)
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if (tmp_ptr != run_ptr)
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trav = run_ptr + size;
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while (--trav >= run_ptr)
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for (hi = lo = trav; (lo -= size) >= tmp_ptr; hi = lo)
added
removed
libc/gnu/qsort.c
