It is not currently possible to do it without threading the vector through.
However, if we implement the so-called INHTPAMA changes I proposed here:
http://www.smallcultfollowing.com/babysteps/blog/2012/11/18/imagine-never-hearing-the-phrase-aliasable/
then it would work fine with &mut.
Niko
Ben Alpert wrote:
Is there any way to avoid passing v around all the time to make the
code cleaner?
After I sent the message last night, I attempted to make the helper
take v:& ~mut [T] instead (or&mut ~[T] based on your last change):
https://gist.github.com/0362eef759791558d13d
but this gives the error "illegal borrow unless pure: creating
immutable alias to mutable vec content" on the put(*v). It seems to me
like this is safe so I filed
https://github.com/mozilla/rust/issues/4331
but please correct me if I'm wrong here.
I suppose it's probably not possible to get rid of the unsafe block
with that approach.
Ben
On Wed, Jan 2, 2013 at 5:27 PM, Niko Matsakis<[email protected]> wrote:
Here is a revised version:
https://gist.github.com/4439672
The only real change is the workaround for the lack of mutable function
arguments. Basically instead of a parameter `v: ~[mut T]` I changed it to
`v: ~[T]` and added `let mut v = v`. This takes advantage of inherited
mutability. It also means there is no need for unsafe code because the
compiler can see that the vector is owned by the permutation function and
thus allows it to be temporarily declared as immutable during the callback.
Niko
Ben Alpert wrote:
Hi all,
I was perusing the source code of vec::each_permutation and noticed the
comment:
This does not seem like the most efficient implementation. You
could make far fewer copies if you put your mind to it.
so I decided to try my hand at a faster version, which I have posted here:
https://gist.github.com/48b14f37051e7b91c22c
Code review would be appreciated. In particular, I did some awkward
shenanigans to pass v around my recursive function called 'helper' in
order to satisfy the ownership checker. Originally I had helper(v:
&mut [T], ...) -> bool, but that causes an error because put takes a
&[T]. It would be great if there was a simpler way to do this.
For comparison, it seems to be around 100x faster on this trivial test:
let mut i = 0;
for each_permutation(~[1,2,3,4,5,6,7,8,9,10,11]) |_| { i += 1; }
assert i == 39916800;
Let me know whether it makes sense to open a pull request with this
new implementation.
Thanks,
Ben
P.S. I am puzzled by the fact that each_permutation is listed in the
docs but it's not marked as pub so it appears to be inaccessible from
outside the vec.rs
file.
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