use std::convert::TryInto;

#[inline(never)] #[no_mangle]
fn grab_bit_slice(buffer: &[u8], offset: usize) -> u64 {
    let offset_bytes = offset / 8;
    let offset_bits = offset - offset_bytes * 8;
    let offset_tail = offset_bytes + 8;
    let len = buffer.len();

    // special case if the buffer is too small.
    let bytes_available = len - offset_bytes;
    if bytes_available < 9 {
        let mut mini_buffer = [0u8; 9];
        (&mut mini_buffer[0..bytes_available]).copy_from_slice(&buffer[offset_bytes..len]);
        return grab_bit_slice(&mini_buffer, offset_bits)
    }

    let read_body =
        u64::from_be_bytes((&buffer[offset_bytes..offset_tail]).try_into().unwrap());

    let read_tail = buffer[offset_tail] as u64;

    (read_body << offset_bits) | (read_tail >> (8 - offset_bits))
}

fn main() {
    let bytes = b"my hands, they are typing words";
    println!("{:p}", bytes);
    for i in 0..100 {
        println!("{:064b}", grab_bit_slice(bytes, i))
    }
}