<html><head><meta http-equiv="Content-Type" content="text/html charset=windows-1252"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><br><div><div>On Jul 22, 2014, at 8:48 PM, David Majnemer <<a href="mailto:david.majnemer@gmail.com">david.majnemer@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div dir="ltr">Should this be merged for 3.5?</div></blockquote><div><br></div>Yes.</div><div><br></div><div>— Marshall</div><div><br><blockquote type="cite"><div class="gmail_extra"><br><br><div class="gmail_quote">On Mon, Jul 21, 2014 at 11:07 PM, David Majnemer <span dir="ltr"><<a href="mailto:david.majnemer@gmail.com" target="_blank">david.majnemer@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Author: majnemer<br>
Date: Tue Jul 22 01:07:09 2014<br>
New Revision: 213615<br>
<br>
URL: <a href="http://llvm.org/viewvc/llvm-project?rev=213615&view=rev" target="_blank">http://llvm.org/viewvc/llvm-project?rev=213615&view=rev</a><br>
Log:<br>
Fix std::make_heap's worst case time complexity<br>
<br>
std::make_heap is currently implemented by iteratively applying a<br>
siftup-type algorithm. Since sift-up is O(ln n), this gives<br>
std::make_heap a worst case time complexity of O(n ln n).<br>
<br>
The C++ standard mandates that std::make_heap make no more than O(3n)<br>
comparisons, this makes our std::make_heap out of spec.<br>
<br>
Fix this by introducing an implementation of __sift_down and switch<br>
std::make_heap to create the heap using it.<br>
This gives std::make_heap linear time complexity in the worst case.<br>
<br>
This fixes PR20161.<br>
<br>
Modified:<br>
libcxx/trunk/include/algorithm<br>
libcxx/trunk/test/algorithms/alg.sorting/alg.heap.operations/make.heap/make_heap_comp.pass.cpp<br>
<br>
Modified: libcxx/trunk/include/algorithm<br>
URL: <a href="http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/algorithm?rev=213615&r1=213614&r2=213615&view=diff" target="_blank">http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/algorithm?rev=213615&r1=213614&r2=213615&view=diff</a><br>
==============================================================================<br>
--- libcxx/trunk/include/algorithm (original)<br>
+++ libcxx/trunk/include/algorithm Tue Jul 22 01:07:09 2014<br>
@@ -4794,49 +4794,8 @@ is_heap(_RandomAccessIterator __first, _<br>
<br>
template <class _Compare, class _RandomAccessIterator><br>
void<br>
-__push_heap_front(_RandomAccessIterator __first, _RandomAccessIterator, _Compare __comp,<br>
- typename iterator_traits<_RandomAccessIterator>::difference_type __len)<br>
-{<br>
- typedef typename iterator_traits<_RandomAccessIterator>::difference_type difference_type;<br>
- typedef typename iterator_traits<_RandomAccessIterator>::value_type value_type;<br>
- if (__len > 1)<br>
- {<br>
- difference_type __p = 0;<br>
- _RandomAccessIterator __pp = __first;<br>
- difference_type __c = 2;<br>
- _RandomAccessIterator __cp = __first + __c;<br>
- if (__c == __len || __comp(*__cp, *(__cp - 1)))<br>
- {<br>
- --__c;<br>
- --__cp;<br>
- }<br>
- if (__comp(*__pp, *__cp))<br>
- {<br>
- value_type __t(_VSTD::move(*__pp));<br>
- do<br>
- {<br>
- *__pp = _VSTD::move(*__cp);<br>
- __pp = __cp;<br>
- __p = __c;<br>
- __c = (__p + 1) * 2;<br>
- if (__c > __len)<br>
- break;<br>
- __cp = __first + __c;<br>
- if (__c == __len || __comp(*__cp, *(__cp - 1)))<br>
- {<br>
- --__c;<br>
- --__cp;<br>
- }<br>
- } while (__comp(__t, *__cp));<br>
- *__pp = _VSTD::move(__t);<br>
- }<br>
- }<br>
-}<br>
-<br>
-template <class _Compare, class _RandomAccessIterator><br>
-void<br>
-__push_heap_back(_RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp,<br>
- typename iterator_traits<_RandomAccessIterator>::difference_type __len)<br>
+__sift_up(_RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp,<br>
+ typename iterator_traits<_RandomAccessIterator>::difference_type __len)<br>
{<br>
typedef typename iterator_traits<_RandomAccessIterator>::difference_type difference_type;<br>
typedef typename iterator_traits<_RandomAccessIterator>::value_type value_type;<br>
@@ -4869,10 +4828,10 @@ push_heap(_RandomAccessIterator __first,<br>
#ifdef _LIBCPP_DEBUG<br>
typedef typename add_lvalue_reference<__debug_less<_Compare> >::type _Comp_ref;<br>
__debug_less<_Compare> __c(__comp);<br>
- __push_heap_back<_Comp_ref>(__first, __last, __c, __last - __first);<br>
+ __sift_up<_Comp_ref>(__first, __last, __c, __last - __first);<br>
#else // _LIBCPP_DEBUG<br>
typedef typename add_lvalue_reference<_Compare>::type _Comp_ref;<br>
- __push_heap_back<_Comp_ref>(__first, __last, __comp, __last - __first);<br>
+ __sift_up<_Comp_ref>(__first, __last, __comp, __last - __first);<br>
#endif // _LIBCPP_DEBUG<br>
}<br>
<br>
@@ -4887,6 +4846,60 @@ push_heap(_RandomAccessIterator __first,<br>
// pop_heap<br>
<br>
template <class _Compare, class _RandomAccessIterator><br>
+void<br>
+__sift_down(_RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp,<br>
+ typename iterator_traits<_RandomAccessIterator>::difference_type __len,<br>
+ _RandomAccessIterator __start)<br>
+{<br>
+ typedef typename iterator_traits<_RandomAccessIterator>::difference_type difference_type;<br>
+ typedef typename iterator_traits<_RandomAccessIterator>::value_type value_type;<br>
+ // left-child of __start is at 2 * __start + 1<br>
+ // right-child of __start is at 2 * __start + 2<br>
+ difference_type __child = __start - __first;<br>
+<br>
+ if (__len < 2 || (__len - 2) / 2 < __child)<br>
+ return;<br>
+<br>
+ __child = 2 * __child + 1;<br>
+ _RandomAccessIterator __child_i = __first + __child;<br>
+<br>
+ if ((__child + 1) < __len && __comp(*__child_i, *(__child_i + 1))) {<br>
+ // right-child exists and is greater than left-child<br>
+ ++__child_i;<br>
+ ++__child;<br>
+ }<br>
+<br>
+ // check if we are in heap-order<br>
+ if (__comp(*__child_i, *__start))<br>
+ // we are, __start is larger than it's largest child<br>
+ return;<br>
+<br>
+ value_type __top(_VSTD::move(*__start));<br>
+ do<br>
+ {<br>
+ // we are not in heap-order, swap the parent with it's largest child<br>
+ *__start = _VSTD::move(*__child_i);<br>
+ __start = __child_i;<br>
+<br>
+ if ((__len - 2) / 2 < __child)<br>
+ break;<br>
+<br>
+ // recompute the child based off of the updated parent<br>
+ __child = 2 * __child + 1;<br>
+ __child_i = __first + __child;<br>
+<br>
+ if ((__child + 1) < __len && __comp(*__child_i, *(__child_i + 1))) {<br>
+ // right-child exists and is greater than left-child<br>
+ ++__child_i;<br>
+ ++__child;<br>
+ }<br>
+<br>
+ // check if we are in heap-order<br>
+ } while (!__comp(*__child_i, __top));<br>
+ *__start = _VSTD::move(__top);<br>
+}<br>
+<br>
+template <class _Compare, class _RandomAccessIterator><br>
inline _LIBCPP_INLINE_VISIBILITY<br>
void<br>
__pop_heap(_RandomAccessIterator __first, _RandomAccessIterator __last, _Compare __comp,<br>
@@ -4895,7 +4908,7 @@ __pop_heap(_RandomAccessIterator __first<br>
if (__len > 1)<br>
{<br>
swap(*__first, *--__last);<br>
- __push_heap_front<_Compare>(__first, __last, __comp, __len-1);<br>
+ __sift_down<_Compare>(__first, __last, __comp, __len - 1, __first);<br>
}<br>
}<br>
<br>
@@ -4932,10 +4945,11 @@ __make_heap(_RandomAccessIterator __firs<br>
difference_type __n = __last - __first;<br>
if (__n > 1)<br>
{<br>
- __last = __first;<br>
- ++__last;<br>
- for (difference_type __i = 1; __i < __n;)<br>
- __push_heap_back<_Compare>(__first, ++__last, __comp, ++__i);<br>
+ // start from the first parent, there is no need to consider children<br>
+ for (difference_type __start = (__n - 2) / 2; __start >= 0; --__start)<br>
+ {<br>
+ __sift_down<_Compare>(__first, __last, __comp, __n, __first + __start);<br>
+ }<br>
}<br>
}<br>
<br>
@@ -5010,7 +5024,7 @@ __partial_sort(_RandomAccessIterator __f<br>
if (__comp(*__i, *__first))<br>
{<br>
swap(*__i, *__first);<br>
- __push_heap_front<_Compare>(__first, __middle, __comp, __len);<br>
+ __sift_down<_Compare>(__first, __middle, __comp, __len, __first);<br>
}<br>
}<br>
__sort_heap<_Compare>(__first, __middle, __comp);<br>
@@ -5051,15 +5065,15 @@ __partial_sort_copy(_InputIterator __fir<br>
_RandomAccessIterator __r = __result_first;<br>
if (__r != __result_last)<br>
{<br>
- typename iterator_traits<_RandomAccessIterator>::difference_type __len = 0;<br>
- for (; __first != __last && __r != __result_last; ++__first, ++__r, ++__len)<br>
+ for (; __first != __last && __r != __result_last; ++__first, ++__r)<br>
*__r = *__first;<br>
__make_heap<_Compare>(__result_first, __r, __comp);<br>
+ typename iterator_traits<_RandomAccessIterator>::difference_type __len = __r - __result_first;<br>
for (; __first != __last; ++__first)<br>
if (__comp(*__first, *__result_first))<br>
{<br>
*__result_first = *__first;<br>
- __push_heap_front<_Compare>(__result_first, __r, __comp, __len);<br>
+ __sift_down<_Compare>(__result_first, __r, __comp, __len, __result_first);<br>
}<br>
__sort_heap<_Compare>(__result_first, __r, __comp);<br>
}<br>
<br>
Modified: libcxx/trunk/test/algorithms/alg.sorting/alg.heap.operations/make.heap/make_heap_comp.pass.cpp<br>
URL: <a href="http://llvm.org/viewvc/llvm-project/libcxx/trunk/test/algorithms/alg.sorting/alg.heap.operations/make.heap/make_heap_comp.pass.cpp?rev=213615&r1=213614&r2=213615&view=diff" target="_blank">http://llvm.org/viewvc/llvm-project/libcxx/trunk/test/algorithms/alg.sorting/alg.heap.operations/make.heap/make_heap_comp.pass.cpp?rev=213615&r1=213614&r2=213615&view=diff</a><br>
==============================================================================<br>
--- libcxx/trunk/test/algorithms/alg.sorting/alg.heap.operations/make.heap/make_heap_comp.pass.cpp (original)<br>
+++ libcxx/trunk/test/algorithms/alg.sorting/alg.heap.operations/make.heap/make_heap_comp.pass.cpp Tue Jul 22 01:07:09 2014<br>
@@ -35,14 +35,35 @@ struct indirect_less<br>
void test(unsigned N)<br>
{<br>
int* ia = new int [N];<br>
+ {<br>
for (int i = 0; i < N; ++i)<br>
ia[i] = i;<br>
- {<br>
std::random_shuffle(ia, ia+N);<br>
std::make_heap(ia, ia+N, std::greater<int>());<br>
assert(std::is_heap(ia, ia+N, std::greater<int>()));<br>
}<br>
<br>
+// Ascending<br>
+ {<br>
+ binary_counting_predicate<std::greater<int>, int, int> pred ((std::greater<int>()));<br>
+ for (int i = 0; i < N; ++i)<br>
+ ia[i] = i;<br>
+ std::make_heap(ia, ia+N, std::ref(pred));<br>
+ assert(pred.count() <= 3*N);<br>
+ assert(std::is_heap(ia, ia+N, pred));<br>
+ }<br>
+<br>
+// Descending<br>
+ {<br>
+ binary_counting_predicate<std::greater<int>, int, int> pred ((std::greater<int>()));<br>
+ for (int i = 0; i < N; ++i)<br>
+ ia[N-1-i] = i;<br>
+ std::make_heap(ia, ia+N, std::ref(pred));<br>
+ assert(pred.count() <= 3*N);<br>
+ assert(std::is_heap(ia, ia+N, pred));<br>
+ }<br>
+<br>
+// Random<br>
{<br>
binary_counting_predicate<std::greater<int>, int, int> pred ((std::greater<int>()));<br>
std::random_shuffle(ia, ia+N);<br>
<br>
<br>
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