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Improved siftDown and percolate, also documented both

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This commit is contained in:
Andrei Alexandrescu 2016-01-15 17:24:49 -05:00
parent 96ece512a2
commit 01bdccf310
1 changed files with 44 additions and 24 deletions
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68
std/algorithm/sorting.d
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@ -1277,15 +1277,18 @@ package(std) template HeapOps(alias less, Range)
for (size_t i = r.length - 1; i > 0; --i)
{
r.swapAt(0, i);
siftDown(r, 0, i);
percolate(r, 0, i);
}
}
//template because of @@@12410@@@
void buildHeap()(Range r)
{
size_t i = r.length / 2;
while (i-- > 0) siftDown(r, i, r.length);
immutable n = r.length;
for (size_t i = n / 2; i-- > 0; )
{
siftDown(r, i, n);
}
assert(isHeap(r));
}
@ -1301,53 +1304,70 @@ package(std) template HeapOps(alias less, Range)
return true;
}
//template because of @@@12410@@@
// Sifts down r[parent] (which is initially assumed to be messed up) so the
// heap property is restored for r[parent .. end].
// template because of @@@12410@@@
void siftDown()(Range r, size_t parent, immutable size_t end)
{
for (;;)
{
auto child = parent * 2 + 1;
if (child >= end) break;
auto child1 = child + 1;
if (child1 < end && lessFun(r[child], r[child1])) child = child1;
auto child = (parent + 1) * 2;
if (child >= end)
{
// Leftover left child?
if (child == end && lessFun(r[parent], r[--child]))
r.swapAt(parent, child);
break;
}
auto leftChild = child - 1;
if (lessFun(r[child], r[leftChild])) child = leftChild;
if (!lessFun(r[parent], r[child])) break;
r.swapAt(parent, child);
parent = child;
}
}
// Alternate version of siftDown that performs fewer comparisons, see
// https://en.wikipedia.org/wiki/Heapsort#Bottom-up_heapsort. The percolate
// process first sifts the parent all the way down (without comparing it
// against the leaves), and then a bit up until the heap property is
// restored. So there are more swaps but fewer comparisons. Gains are made
// when the final position is likely to end toward the bottom of the heap,
// so not a lot of sifts back are performed.
//template because of @@@12410@@@
void percolate()(Range r, size_t parent, immutable size_t end)
{
immutable root = parent;
size_t child = void;
// Sift down
while(true)
for (;;)
{
child = parent * 2 + 1;
auto child = (parent + 1) * 2;
if(child >= end) break;
auto child1 = child + 1;
if (child1 < end && lessFun(r[child], r[child1])) child = child1;
if (child >= end)
{
if (child == end)
{
// Leftover left node.
--child;
r.swapAt(parent, child);
parent = child;
}
break;
}
auto leftChild = child - 1;
if (lessFun(r[child], r[leftChild])) child = leftChild;
r.swapAt(parent, child);
parent = child;
}
child = parent;
// Sift up
while(child > root)
for (auto child = parent; child > root; child = parent)
{
parent = (child - 1) / 2;
if(lessFun(r[parent], r[child]))
{
r.swapAt(parent, child);
child = parent;
}
else break;
if (!lessFun(r[parent], r[child])) break;
r.swapAt(parent, child);
}
}
}