gistfile1.rs

use std::ops::{Index, IndexMut};

macro_rules! fastidx_arr(
    ($contents: expr) => (
        unsafe {
            FastIdxArr::new($contents, || ())
        }
    )
);

pub struct FastIdxArr<T, Phantom: Fn() -> ()> {
    contents: Box<[T]>
}
impl<T, Phantom: Fn() -> ()> FastIdxArr<T, Phantom> {
    pub unsafe fn new(contents: Box<[T]>, _: Phantom) -> FastIdxArr<T, Phantom> {
        FastIdxArr {
            contents: contents
        }
    }

    pub fn fast_index(&self, idx: uint) -> Result<FastArrIdx<T, Phantom>, ()> {
        if idx < self.contents.len() {
            Ok(FastArrIdx { idx: idx })
        } else {
            Err(())
        }
    }
}

pub struct FastArrIdx<T, Phantom> {
    idx: uint
}
impl<T, Phantom> Index<FastArrIdx<T, Phantom>> for FastIdxArr<T, Phantom> {
    type Output = T;
    fn index<'a>(&'a self, index: &FastArrIdx<T, Phantom>) -> &'a T {
        unsafe {
            self.contents.get_unchecked(index.idx)
        }
    }
}
impl<T, Phantom> IndexMut<FastArrIdx<T, Phantom>> for FastIdxArr<T, Phantom> {
    type Output = T;
    fn index_mut<'a>(&'a mut self, index: &FastArrIdx<T, Phantom>) -> &'a mut T {
        unsafe {
            self.contents.get_unchecked_mut(index.idx)
        }
    }
}

#[test]
fn simple() {
    let arr = fastidx_arr!(vec![1u32, 2, 3].into_boxed_slice());
    let idx = arr.fast_index(1).unwrap();
    assert_eq!(arr[idx], 2);
}

#[test]
#[should_fail]
fn out_of_bounds() {
    let arr = fastidx_arr!(vec![1u32, 2, 3].into_boxed_slice());
    arr.fast_index(3).unwrap(); 
}