Someone is already working on std.combinatorics
That's me. I'm very bust atm with work, so I haven't been able to
do much with it lately, but it is progressing.
1) To avoid excessive allocations, it would have to be a
transient
range. Which means it may have unexpected results if you use it
with an
algorithm that doesn't handle transient ranges correctly.
This has been discussed previously. bearophile suggested a policy
to control whether the buffer is permuted in-place, with it
defaulting to creating duplicates. The slow down from duplicates
on DMD was ~3x.
2) If the starting range is not sorted, then the permutation
range needs
to make a copy of the original range so that it knows when all
permutations have been enumerated.
I sort on range creation (if not already sorted).
If transience is acceptable, and the starting range is always
sorted,
then it's almost trivial to write a wrapper range:
auto permutations(R)(R forwardRange)
if (isForwardRange!R)
{
struct Permutations
{
R front;
bool empty = false;
this(R _src) { front = _src; }
void popFront() { empty = !nextPermutation(front); }
}
return Permutations(forwardRange);
}
This is missing some features:
- Bidirectionality (this complicates things).
- Length (this complicates things, because it easily overflows).
- Random-access (non-trivial, but useful)
A library solution should address all these.
