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authorRoland McGrath <roland@gnu.org>1995-02-18 01:27:10 +0000
committerRoland McGrath <roland@gnu.org>1995-02-18 01:27:10 +0000
commit28f540f45bbacd939bfd07f213bcad2bf730b1bf (patch)
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initial import
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diff --git a/stdlib/qsort.c b/stdlib/qsort.c
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+/* Copyright (C) 1991, 1992 Free Software Foundation, Inc.
+This file is part of the GNU C Library.
+Written by Douglas C. Schmidt (schmidt@ics.uci.edu).
+
+The GNU C Library is free software; you can redistribute it and/or
+modify it under the terms of the GNU Library General Public License as
+published by the Free Software Foundation; either version 2 of the
+License, or (at your option) any later version.
+
+The GNU C Library is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+Library General Public License for more details.
+
+You should have received a copy of the GNU Library General Public
+License along with the GNU C Library; see the file COPYING.LIB. If
+not, write to the Free Software Foundation, Inc., 675 Mass Ave,
+Cambridge, MA 02139, USA. */
+
+#include <ansidecl.h>
+#include <stdlib.h>
+#include <string.h>
+
+/* Byte-wise swap two items of size SIZE. */
+#define SWAP(a, b, size) \
+ do \
+ { \
+ register size_t __size = (size); \
+ register char *__a = (a), *__b = (b); \
+ do \
+ { \
+ char __tmp = *__a; \
+ *__a++ = *__b; \
+ *__b++ = __tmp; \
+ } while (--__size > 0); \
+ } while (0)
+
+/* Discontinue quicksort algorithm when partition gets below this size.
+ This particular magic number was chosen to work best on a Sun 4/260. */
+#define MAX_THRESH 4
+
+/* Stack node declarations used to store unfulfilled partition obligations. */
+typedef struct
+ {
+ char *lo;
+ char *hi;
+ } stack_node;
+
+/* The next 4 #defines implement a very fast in-line stack abstraction. */
+#define STACK_SIZE (8 * sizeof(unsigned long int))
+#define PUSH(low, high) ((void) ((top->lo = (low)), (top->hi = (high)), ++top))
+#define POP(low, high) ((void) (--top, (low = top->lo), (high = top->hi)))
+#define STACK_NOT_EMPTY (stack < top)
+
+
+/* Order size using quicksort. This implementation incorporates
+ four optimizations discussed in Sedgewick:
+
+ 1. Non-recursive, using an explicit stack of pointer that store the
+ next array partition to sort. To save time, this maximum amount
+ of space required to store an array of MAX_INT is allocated on the
+ stack. Assuming a 32-bit integer, this needs only 32 *
+ sizeof(stack_node) == 136 bits. Pretty cheap, actually.
+
+ 2. Chose the pivot element using a median-of-three decision tree.
+ This reduces the probability of selecting a bad pivot value and
+ eliminates certain extraneous comparisons.
+
+ 3. Only quicksorts TOTAL_ELEMS / MAX_THRESH partitions, leaving
+ insertion sort to order the MAX_THRESH items within each partition.
+ This is a big win, since insertion sort is faster for small, mostly
+ sorted array segements.
+
+ 4. The larger of the two sub-partitions is always pushed onto the
+ stack first, with the algorithm then concentrating on the
+ smaller partition. This *guarantees* no more than log (n)
+ stack size is needed (actually O(1) in this case)! */
+
+void
+DEFUN(_quicksort, (pbase, total_elems, size, cmp),
+ PTR CONST pbase AND size_t total_elems AND size_t size AND
+ int EXFUN((*cmp), (CONST PTR, CONST PTR)))
+{
+ register char *base_ptr = (char *) pbase;
+
+ /* Allocating SIZE bytes for a pivot buffer facilitates a better
+ algorithm below since we can do comparisons directly on the pivot. */
+ char *pivot_buffer = (char *) __alloca (size);
+ CONST size_t max_thresh = MAX_THRESH * size;
+
+ if (total_elems == 0)
+ /* Avoid lossage with unsigned arithmetic below. */
+ return;
+
+ if (total_elems > MAX_THRESH)
+ {
+ char *lo = base_ptr;
+ char *hi = &lo[size * (total_elems - 1)];
+ /* Largest size needed for 32-bit int!!! */
+ stack_node stack[STACK_SIZE];
+ stack_node *top = stack + 1;
+
+ while (STACK_NOT_EMPTY)
+ {
+ char *left_ptr;
+ char *right_ptr;
+
+ char *pivot = pivot_buffer;
+
+ /* Select median value from among LO, MID, and HI. Rearrange
+ LO and HI so the three values are sorted. This lowers the
+ probability of picking a pathological pivot value and
+ skips a comparison for both the LEFT_PTR and RIGHT_PTR. */
+
+ char *mid = lo + size * ((hi - lo) / size >> 1);
+
+ if ((*cmp)((PTR) mid, (PTR) lo) < 0)
+ SWAP(mid, lo, size);
+ if ((*cmp)((PTR) hi, (PTR) mid) < 0)
+ SWAP(mid, hi, size);
+ else
+ goto jump_over;
+ if ((*cmp)((PTR) mid, (PTR) lo) < 0)
+ SWAP(mid, lo, size);
+ jump_over:;
+ memcpy(pivot, mid, size);
+ pivot = pivot_buffer;
+
+ left_ptr = lo + size;
+ right_ptr = hi - size;
+
+ /* Here's the famous ``collapse the walls'' section of quicksort.
+ Gotta like those tight inner loops! They are the main reason
+ that this algorithm runs much faster than others. */
+ do
+ {
+ while ((*cmp)((PTR) left_ptr, (PTR) pivot) < 0)
+ left_ptr += size;
+
+ while ((*cmp)((PTR) pivot, (PTR) right_ptr) < 0)
+ right_ptr -= size;
+
+ if (left_ptr < right_ptr)
+ {
+ SWAP(left_ptr, right_ptr, size);
+ left_ptr += size;
+ right_ptr -= size;
+ }
+ else if (left_ptr == right_ptr)
+ {
+ left_ptr += size;
+ right_ptr -= size;
+ break;
+ }
+ }
+ while (left_ptr <= right_ptr);
+
+ /* Set up pointers for next iteration. First determine whether
+ left and right partitions are below the threshold size. If so,
+ ignore one or both. Otherwise, push the larger partition's
+ bounds on the stack and continue sorting the smaller one. */
+
+ if ((size_t) (right_ptr - lo) <= max_thresh)
+ {
+ if ((size_t) (hi - left_ptr) <= max_thresh)
+ /* Ignore both small partitions. */
+ POP(lo, hi);
+ else
+ /* Ignore small left partition. */
+ lo = left_ptr;
+ }
+ else if ((size_t) (hi - left_ptr) <= max_thresh)
+ /* Ignore small right partition. */
+ hi = right_ptr;
+ else if ((right_ptr - lo) > (hi - left_ptr))
+ {
+ /* Push larger left partition indices. */
+ PUSH(lo, right_ptr);
+ lo = left_ptr;
+ }
+ else
+ {
+ /* Push larger right partition indices. */
+ PUSH(left_ptr, hi);
+ hi = right_ptr;
+ }
+ }
+ }
+
+ /* Once the BASE_PTR array is partially sorted by quicksort the rest
+ is completely sorted using insertion sort, since this is efficient
+ for partitions below MAX_THRESH size. BASE_PTR points to the beginning
+ of the array to sort, and END_PTR points at the very last element in
+ the array (*not* one beyond it!). */
+
+#define min(x, y) ((x) < (y) ? (x) : (y))
+
+ {
+ char *CONST end_ptr = &base_ptr[size * (total_elems - 1)];
+ char *tmp_ptr = base_ptr;
+ char *thresh = min(end_ptr, base_ptr + max_thresh);
+ register char *run_ptr;
+
+ /* Find smallest element in first threshold and place it at the
+ array's beginning. This is the smallest array element,
+ and the operation speeds up insertion sort's inner loop. */
+
+ for (run_ptr = tmp_ptr + size; run_ptr <= thresh; run_ptr += size)
+ if ((*cmp)((PTR) run_ptr, (PTR) tmp_ptr) < 0)
+ tmp_ptr = run_ptr;
+
+ if (tmp_ptr != base_ptr)
+ SWAP(tmp_ptr, base_ptr, size);
+
+ /* Insertion sort, running from left-hand-side up to right-hand-side. */
+
+ run_ptr = base_ptr + size;
+ while ((run_ptr += size) <= end_ptr)
+ {
+ tmp_ptr = run_ptr - size;
+ while ((*cmp)((PTR) run_ptr, (PTR) tmp_ptr) < 0)
+ tmp_ptr -= size;
+
+ tmp_ptr += size;
+ if (tmp_ptr != run_ptr)
+ {
+ char *trav;
+
+ trav = run_ptr + size;
+ while (--trav >= run_ptr)
+ {
+ char c = *trav;
+ char *hi, *lo;
+
+ for (hi = lo = trav; (lo -= size) >= tmp_ptr; hi = lo)
+ *hi = *lo;
+ *hi = c;
+ }
+ }
+ }
+ }
+}
+