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authorBrian Downing <bdowning@lavos.net>2008-02-05 21:10:44 (GMT)
committerJunio C Hamano <gitster@pobox.com>2008-02-07 06:35:28 (GMT)
commit43fe901b71f81cdff142048dfc27e492c445d225 (patch)
tree1e3e8ad7f8f760a950643f4970f2daee2364b1b7 /compat/qsort.c
parent7a2078b4b00fb1c5d7b0bf8155778f79377b8f2f (diff)
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compat: Add simplified merge sort implementation from glibc
qsort in Windows 2000 (and various other 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, but has a worst-case performance of O(n log n). 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] [bcd: removed gcc-ism, thanks to Edgar Toernig. renamed make variable per Junio's comment.] 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> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Diffstat (limited to 'compat/qsort.c')
-rw-r--r--compat/qsort.c62
1 files changed, 62 insertions, 0 deletions
diff --git a/compat/qsort.c b/compat/qsort.c
new file mode 100644
index 0000000..d93dce2
--- /dev/null
+++ b/compat/qsort.c
@@ -0,0 +1,62 @@
+#include "../git-compat-util.h"
+
+/*
+ * A merge sort implementation, simplified from the qsort implementation
+ * by Mike Haertel, which is a part of the GNU C Library.
+ */
+
+static void msort_with_tmp(void *b, size_t n, size_t s,
+ int (*cmp)(const void *, const void *),
+ char *t)
+{
+ char *tmp;
+ char *b1, *b2;
+ size_t n1, n2;
+
+ if (n <= 1)
+ return;
+
+ n1 = n / 2;
+ n2 = n - n1;
+ b1 = b;
+ b2 = (char *)b + (n1 * s);
+
+ msort_with_tmp(b1, n1, s, cmp, t);
+ msort_with_tmp(b2, n2, s, cmp, t);
+
+ tmp = t;
+
+ while (n1 > 0 && n2 > 0) {
+ if (cmp(b1, b2) <= 0) {
+ memcpy(tmp, b1, s);
+ tmp += s;
+ b1 += s;
+ --n1;
+ } else {
+ memcpy(tmp, b2, s);
+ tmp += s;
+ b2 += s;
+ --n2;
+ }
+ }
+ if (n1 > 0)
+ memcpy(tmp, b1, n1 * s);
+ memcpy(b, t, (n - n2) * s);
+}
+
+void git_qsort(void *b, size_t n, size_t s,
+ int (*cmp)(const void *, const void *))
+{
+ const size_t size = n * s;
+ char buf[1024];
+
+ if (size < sizeof(buf)) {
+ /* The temporary array fits on the small on-stack buffer. */
+ msort_with_tmp(b, n, s, cmp, buf);
+ } else {
+ /* It's somewhat large, so malloc it. */
+ char *tmp = malloc(size);
+ msort_with_tmp(b, n, s, cmp, tmp);
+ free(tmp);
+ }
+}