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(quicksort): Copy pivot out of the array while constructing the

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partitions; it could get overwritten otherwise.  Because of the
ultimate insertion sort, this bug did not cause quicksort to fail, it
just put all the burdon on the insertion sort and was thus very slow.
Thanks to Rolan Orre for reporting the slowness!
This commit is contained in:
Marius Vollmer 2004-10-19 22:49:51 +00:00
parent 192cd79259
commit 14e9281bb3
1 changed files with 4 additions and 2 deletions
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6
libguile/sort.c
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@ -127,6 +127,7 @@ quicksort (SCM *const base_ptr, size_t nr_elems, scm_t_trampoline_2 cmp, SCM les
{ {
size_t left; size_t left;
size_t right; size_t right;
SCM pivot;
/* Select median value from among LO, MID, and HI. Rearrange /* Select median value from among LO, MID, and HI. Rearrange
LO and HI so the three values are sorted. This lowers the LO and HI so the three values are sorted. This lowers the
@ -145,6 +146,7 @@ quicksort (SCM *const base_ptr, size_t nr_elems, scm_t_trampoline_2 cmp, SCM les
SWAP (base_ptr[mid], base_ptr[lo]); SWAP (base_ptr[mid], base_ptr[lo]);
jump_over:; jump_over:;
pivot = base_ptr[mid];
left = lo + 1; left = lo + 1;
right = hi - 1; right = hi - 1;
@ -153,7 +155,7 @@ quicksort (SCM *const base_ptr, size_t nr_elems, scm_t_trampoline_2 cmp, SCM les
that this algorithm runs much faster than others. */ that this algorithm runs much faster than others. */
do do
{ {
while (scm_is_true ((*cmp) (less, base_ptr[left], base_ptr[mid]))) while (scm_is_true ((*cmp) (less, base_ptr[left], pivot)))
{ {
left++; left++;
/* The comparison predicate may be buggy */ /* The comparison predicate may be buggy */
@ -161,7 +163,7 @@ quicksort (SCM *const base_ptr, size_t nr_elems, scm_t_trampoline_2 cmp, SCM les
scm_misc_error (NULL, s_buggy_less, SCM_EOL); scm_misc_error (NULL, s_buggy_less, SCM_EOL);
} }
while (scm_is_true ((*cmp) (less, base_ptr[mid], base_ptr[right]))) while (scm_is_true ((*cmp) (less, pivot, base_ptr[right])))
{ {
right--; right--;
/* The comparison predicate may be buggy */ /* The comparison predicate may be buggy */