Most efficient way to fill a vector from back to f

Question

I am trying to populate a vector with a sequence of values. In order to calculate the first value I need to calculate the second value, which depends on the third value etc etc.

let mut bxs = Vec::with_capacity(n);

for x in info {
    let b = match bxs.last() {
        Some(bx) => union(&bx, &x.bbox),
        None => x.bbox.clone(),
    };
    bxs.push(b);
}
bxs.reverse();

Currently I just fill the vector front to back using v.push(x) and then reverse the vector using v.reverse(). Is there a way to do this in a single pass?

Answer 1

The solution using unsafe is below. The unsafe version is slightly more than 2x as fast as the safe version using reverse(). The idea is to use Vec::with_capacity(usize) to allocate the vector, then use ptr::write(dst: *mut T, src: T) to write the elements into the vector back to front. offset(self, count: isize) -> *const T is used to calculate the offset into the vector.

extern crate time;
use std::fmt::Debug;
use std::ptr;
use time::PreciseTime;

fn scanl<T, F>(u : &Vec<T>, f : F) -> Vec<T>
    where T : Clone,
          F : Fn(&T, &T) -> T {
    let mut v = Vec::with_capacity(u.len());

    for x in u.iter().rev() {
        let b = match v.last() {
            None => (*x).clone(),
            Some(y) => f(x, &y),
        };
        v.push(b);
    }
    v.reverse();
    return v;
}

fn unsafe_scanl<T, F>(u : &Vec<T> , f : F) -> Vec<T>
    where T : Clone + Debug,
          F : Fn(&T, &T) -> T {
    unsafe {
        let mut v : Vec<T> = Vec::with_capacity(u.len());

        let cap = v.capacity();
        let p = v.as_mut_ptr();

        match u.last() {
            None => return v,
            Some(x) => ptr::write(p.offset((u.len()-1) as isize), x.clone()),
        };
        for i in (0..u.len()-1).rev() {
            ptr::write(p.offset(i as isize), f(v.get_unchecked(i+1), u.get_unchecked(i)));
        }
        Vec::set_len(&mut v, cap);
        return v;
    }
}

pub fn bench_scanl() {
    let lo : u64 = 0;
    let hi : u64 = 1000000;
    let v : Vec<u64> = (lo..hi).collect();

    let start = PreciseTime::now();
    let u = scanl(&v, |x, y| x + y);
    let end= PreciseTime::now();
    println!("{:?}\n in {}", u.len(), start.to(end));

    let start2 = PreciseTime::now();
    let u = unsafe_scanl(&v, |x, y| x + y);
    let end2 = PreciseTime::now();
    println!("2){:?}\n in {}", u.len(), start2.to(end2));
}

Answer 2

Is there a way to do this in a single pass?

If you don't mind adapting the vector, it's relatively easy.

struct RevVec<T> {
    data: Vec<T>,
}

impl<T> RevVec<T> {
    fn push_front(&mut self, t: T) { self.data.push(t); }
}

impl<T> Index<usize> for RevVec<T> {
    type Output = T;
    fn index(&self, index: usize) -> &T {
        &self.data[self.len() - index - 1]
    }
}

impl<T> IndexMut<usize> for RevVec<T> {
    fn index_mut(&mut self, index: usize) -> &mut T {
        let len = self.len();
        &mut self.data[len - index - 1]
    }
}