Best way to get the 3 bytes of a small u32?

help
6

Thanks for all the comments. I ended up reconfiguring a bunch of my function parameters from taking a Write to taking a Cursor<Vec<u8>> instead. That allowed me to take advantage of the byteorder crate without worrying too much about the extra vector allocation.

fn write_u24_be(cursor: &mut Cursor<Vec<u8>>, value: u32) -> Result<()> {
    debug_assert!(value <= 16777215, "Value is greater than what can fit in 3 bytes");

    try!(cursor.write_u32::<BigEndian>(value));
    
    {
        let mut inner = cursor.get_mut();
        let index_to_remove = inner.len() - 1 - 3;
        inner.remove(index_to_remove);
    }

    try!(cursor.seek(SeekFrom::End(0)));
    Ok(())
}

#[cfg(test)]
mod test {
    use super::write_u24_be;
    use std::io::Cursor;
    use byteorder::WriteBytesExt;

    #[test]
    fn can_write_u24() {
        let mut cursor = Cursor::new(Vec::new());
        write_u24_be(&mut cursor, 16777215).unwrap();

        // Make sure next writes are at the 4th byte
        cursor.write_u8(8).unwrap();
        
        assert_eq!(cursor.into_inner(), vec![255, 255, 255, 8]);
    }
}

If profiling shows this being a critical hot path then I'll swap that function out for bitwise stuff.