data-encoding 1.2.0

This crate provides generic data encoding functions. It is meant to guarantee mathematical properties, to conform to RFC 4648, to be efficient, and to give choice between allocating and in-place functions. It also provides an exhaustive example with similar functionality to the base64 GNU program. It supports common bases (base2, base4, base8, base16, hex, base32, base32hex, base64, and base64url) and custom bases (defined on the command-line by their padding and symbols in value order).
Documentation 
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macro_rules! check {
    ($e: expr, $c: expr) => {
        if !$c {
            return Err($e);
        }
    };
}

pub fn div_ceil(x: usize, m: usize) -> usize {
    (x + m - 1) / m
}

pub unsafe fn chunk_unchecked(x: &[u8], n: usize, i: usize) -> &[u8] {
    let ptr = x.as_ptr().offset((n * i) as isize);
    ::std::slice::from_raw_parts(ptr, n)
}

pub unsafe fn chunk_mut_unchecked
    (x: &mut [u8], n: usize, i: usize) -> &mut [u8]
{
    let ptr = x.as_mut_ptr().offset((n * i) as isize);
    ::std::slice::from_raw_parts_mut(ptr, n)
}

pub fn chunk(x: &[u8], n: usize, i: usize) -> &[u8] {
    assert!(n * (i + 1) <= x.len());
    unsafe { chunk_unchecked(x, n, i) }
}

pub fn chunk_mut(x: &mut [u8], n: usize, i: usize) -> &mut [u8] {
    assert!(n * (i + 1) <= x.len());
    unsafe { chunk_mut_unchecked(x, n, i) }
}