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bc8e453867
qsort in Windows 2000 (and possibly other older Windows' C libraries)
is a Quicksort with the usual O(n^2) worst case. Unfortunately, sorting
Git trees seems to get very close to that worst case quite often:
$ /git/gitbad runstatus
# On branch master
qsort, nmemb = 30842
done, 237838087 comparisons.
This patch adds a simplified version of the merge sort that is glibc's
qsort(3). As a merge sort, this needs a temporary array equal in size
to the array that is to be sorted.
The complexity that was removed is:
* Doing direct stores for word-size and -aligned data.
* Falling back to quicksort if the allocation required to perform the
merge sort would likely push the machine into swap.
Even with these simplifications, this seems to outperform the Windows
qsort(3) implementation, even in Windows XP (where it is "fixed" and
doesn't trigger O(n^2) complexity on trees).
[jes: moved into compat/qsort.c, as per Johannes Sixt's suggestion]
Signed-off-by: Brian Downing <bdowning@lavos.net>
Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
61 lines
1.1 KiB
C
61 lines
1.1 KiB
C
#include "../git-compat-util.h"
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/* This merge sort implementation is simplified from glibc's. */
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static void msort_with_tmp(void *b, size_t n, size_t s,
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int (*cmp)(const void *, const void *),
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char *t)
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{
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char *tmp;
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char *b1, *b2;
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size_t n1, n2;
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if (n <= 1)
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return;
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n1 = n / 2;
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n2 = n - n1;
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b1 = b;
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b2 = (char *)b + (n1 * s);
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msort_with_tmp(b1, n1, s, cmp, t);
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msort_with_tmp(b2, n2, s, cmp, t);
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tmp = t;
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while (n1 > 0 && n2 > 0) {
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if (cmp(b1, b2) <= 0) {
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memcpy(tmp, b1, s);
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tmp += s;
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b1 += s;
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--n1;
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} else {
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memcpy(tmp, b2, s);
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tmp += s;
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b2 += s;
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--n2;
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}
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}
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if (n1 > 0)
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memcpy(tmp, b1, n1 * s);
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memcpy(b, t, (n - n2) * s);
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}
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void git_qsort(void *b, size_t n, size_t s,
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int (*cmp)(const void *, const void *))
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{
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const size_t size = n * s;
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if (size < 1024) {
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char buf[size]; /* gcc-ism */
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/* The temporary array is small, so put it on
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the stack. */
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msort_with_tmp(b, n, s, cmp, buf);
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} else {
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/* It's somewhat large, so malloc it. */
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char *tmp = malloc(size);
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msort_with_tmp(b, n, s, cmp, tmp);
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free(tmp);
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}
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}
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