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binarysort() cleanup: Documented the key invariants, explained why they

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imply this is a stable sort, and added some asserts.
This commit is contained in:
Tim Peters 2002-07-19 06:12:32 +00:00
parent 326b44871e
commit 0fe977c4a9
1 changed files with 13 additions and 2 deletions
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15
Objects/listobject.c
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@ -871,14 +871,25 @@ binarysort(PyObject **lo, PyObject **hi, PyObject **start, PyObject *compare)
l = lo;
r = start;
pivot = *r;
/* Invariants:
* pivot >= all in [lo, l).
* pivot < all in [r, start).
* The second is vacuously true at the start.
*/
assert(l < r);
do {
p = l + ((r - l) >> 1);
IFLT(pivot, *p)
r = p;
else
l = p + 1;
l = p+1;
} while (l < r);
/* Pivot should go at l -- slide over to make room.
assert(l == r);
/* The invariants still hold, so pivot >= all in [lo, l) and
pivot < all in [l, start), so pivot belongs at l. Note
that if there are elements equal to pivot, l points to the
first slot after them -- that's why this sort is stable.
Slide over to make room.
Caution: using memmove is much slower under MSVC 5;
we're not usually moving many slots. */
for (p = start; p > l; --p)