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Fix std::make_heap's worst case time complexity

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std::make_heap is currently implemented by iteratively applying a
siftup-type algorithm.  Since sift-up is O(ln n), this gives
std::make_heap a worst case time complexity of O(n ln n).

The C++ standard mandates that std::make_heap make no more than O(3n)
comparisons, this makes our std::make_heap out of spec.

Fix this by introducing an implementation of __sift_down and switch
std::make_heap to create the heap using it.
This gives std::make_heap linear time complexity in the worst case.

This fixes PR20161.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@213615 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
David Majnemer
2014-07-22 06:07:09 +00:00
parent db5e54d2a3
commit cb8757aca7
2 changed files with 90 additions and 55 deletions
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23
test/algorithms/alg.sorting/alg.heap.operations/make.heap/make_heap_comp.pass.cpp
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@@ -35,14 +35,35 @@ struct indirect_less
void test(unsigned N)
{
int* ia = new int [N];
{
for (int i = 0; i < N; ++i)
ia[i] = i;
{
std::random_shuffle(ia, ia+N);
std::make_heap(ia, ia+N, std::greater<int>());
assert(std::is_heap(ia, ia+N, std::greater<int>()));
}
// Ascending
{
binary_counting_predicate<std::greater<int>, int, int> pred ((std::greater<int>()));
for (int i = 0; i < N; ++i)
ia[i] = i;
std::make_heap(ia, ia+N, std::ref(pred));
assert(pred.count() <= 3*N);
assert(std::is_heap(ia, ia+N, pred));
}
// Descending
{
binary_counting_predicate<std::greater<int>, int, int> pred ((std::greater<int>()));
for (int i = 0; i < N; ++i)
ia[N-1-i] = i;
std::make_heap(ia, ia+N, std::ref(pred));
assert(pred.count() <= 3*N);
assert(std::is_heap(ia, ia+N, pred));
}
// Random
{
binary_counting_predicate<std::greater<int>, int, int> pred ((std::greater<int>()));
std::random_shuffle(ia, ia+N);