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Improve performance of partition

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This commit is contained in:
Andrei Alexandrescu 2016-01-11 12:25:29 -05:00
parent 9c7f0dabc3
commit 06cb165a47
1 changed files with 53 additions and 25 deletions
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78
std/algorithm/sorting.d
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@ -362,38 +362,66 @@ Range partition(alias predicate,
{ {
// Inspired from www.stepanovpapers.com/PAM3-partition_notes.pdf, // Inspired from www.stepanovpapers.com/PAM3-partition_notes.pdf,
// section "Bidirectional Partition Algorithm (Hoare)" // section "Bidirectional Partition Algorithm (Hoare)"
auto result = r; static if (isDynamicArray!Range)
for (;;)
{ {
// For dynamic arrays prefer index-based manipulation
if (!r.length) return r;
size_t lo = 0, hi = r.length - 1;
for (;;) for (;;)
{ {
if (r.empty) return result; for (;;)
if (!pred(r.front)) break; {
if (lo > hi) return r[lo .. $];
if (!pred(r[lo])) break;
++lo;
}
// found the left bound
assert(lo <= hi);
for (;;)
{
if (lo == hi) return r[lo .. $];
if (pred(r[hi])) break;
--hi;
}
// found the right bound, swap & make progress
swap(r[lo++], r[hi--]);
}
}
else
{
auto result = r;
for (;;)
{
for (;;)
{
if (r.empty) return result;
if (!pred(r.front)) break;
r.popFront();
result.popFront();
}
// found the left bound
assert(!r.empty);
for (;;)
{
if (pred(r.back)) break;
r.popBack();
if (r.empty) return result;
}
// found the right bound, swap & make progress
static if (is(typeof(swap(r.front, r.back))))
{
swap(r.front, r.back);
}
else
{
auto t1 = moveFront(r), t2 = moveBack(r);
r.front = t2;
r.back = t1;
}
r.popFront(); r.popFront();
result.popFront(); result.popFront();
}
// found the left bound
assert(!r.empty);
for (;;)
{
if (pred(r.back)) break;
r.popBack(); r.popBack();
if (r.empty) return result;
} }
// found the right bound, swap & make progress
static if (is(typeof(swap(r.front, r.back))))
{
swap(r.front, r.back);
}
else
{
auto t1 = moveFront(r), t2 = moveBack(r);
r.front = t2;
r.back = t1;
}
r.popFront();
result.popFront();
r.popBack();
} }
} }
} }