Improve shootout-reverse-complement.rs using unsafe code

[TeXitoi]

This is some improvement as asked and discused here: http://www.reddit.com/r/rust/comments/2j2ij3/benchmark_improvement_reverse_compliment/

Before:

real    0m0.396s
user    0m0.280s
sys     0m0.112s

after:

real    0m0.293s
user    0m0.216s
sys     0m0.076s

best C version:

real    0m0.135s
user    0m0.132s
sys     0m0.060s

Another possibility will be to add a DoubleEndedIterator::next_two_side() with a deffault implementation, and specialising it for slices, and use it here (MutableSlice::reverse() can then become safe). This benchmark will then be safe.

What do you think?

[rust-highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

[huonw]

This is probably faster storing raw pointers (and is somewhat simpler):

struct TwoSidedIterator<'a, T: 'a> {
    start: *mut T,
    end: *mut T,
    _marker: marker::ContravariantLifetime<'a>
}

fn new(x: &'a mut [T]) -> TwoSideIterator<'a, T> {
    let start = x.as_mut_ptr();
    let end = if x.len() == 0 {
        start
    } else {
        unsafe {start.offset(x.len() - 1)}
    };

    TwoSideIterator {
        start: start,
        end: end,
        _marker: marker::ContravariantLifetime
    }
}

fn next(&mut self) -> Option<(&'a mut T, &'a mut T)> {
    if self.start < self.end {
        let ret = unsafe { (&mut *self.start, &mut *self.end) };
        self.start = self.start.offset(1);
        self.end = self.end.offset(-1);
        ret
    } else {
        None
    }
}

If not that, decrementing self.last instead of doing this subtraction would be better (would avoid the need to store nb too).

[TeXitoi]

After using pointers in TwoSideIter as suggested by @huon, I have

real    0m0.264s
user    0m0.184s
sys     0m0.076s

so better :)

On the other hand, the code is trickier:

• as we don't take the slice, markers are needed, which is error prone (@huon forgot NoCopy for example)
• if std::mem::size_of::<T>() == 0 I don't know what append.

Finally, such a modification may be interesting to do in MutableSlice::reverse().