zipper.rs

struct Zipper<'a, T> {
    up : Vec<&'a mut T>,
    focus : &'a mut T,
    down : Vec<&'a mut T>,
}

impl<'a, T> Zipper<'a, T> {
    fn focus_n(&'a mut self, n: i32) ->  Option<&mut Self> {
        let mut x = self.focus_top()?;
        // n focus downs from the top
        for _ in 1..n { x = x.focus_down()?; }
        return x.focus_down();
    }
    fn focus_top(&'a mut self) -> Option<&mut Self> {
        self.down.push(self.focus);
        self.up.reverse();
        self.up.append(&mut self.down);
        self.down.clear();
        std::mem::swap(&mut self.up, &mut self.down);
        self.focus = self.down.pop()?;
        Some(self)
    }

    fn focus_up(&'a mut self) -> Option<&mut Self> {
        self.down.push(self.focus);
        self.focus = self.up.pop()?;
        Some(self)
    }
    fn focus_down(&'a mut self) -> Option<&mut Self> {
        self.up.push(self.focus);
        self.focus = self.down.pop()?;
        Some(self)
    }
    fn new(data: &'a mut [T]) -> Option<Self> {
        if data.is_empty() {
            return None;
        }
        let mut dit = data.iter_mut();
        let focus = dit.next()?;
        let up = Vec::new();
        let down = dit.collect();
        Some(Self { up, focus, down })
    }
}