#[derive(Copy, Clone)]

enum Value {

    Short(u16),

    Long(u32),

    Rational(u32, u32),

    DataOffset,

}

pub fn wrap(data: &[u8], width: u32, height: u32) -> Vec<u8> {

    use Value::*;

    let header_data = [

        (256, Long(width)), // ImageWidth

        (257, Long(height)), // ImageLength

        (259, Short(4)), // Compression

        (262, Short(0)), // PhotometricInterpretation

        (273, DataOffset), // StripOffsets

        (274, Short(1)), // Orientation

        (278, Long(height)), // RowsPerStrip

        (279, Long(data.len() as u32)), // StripByteCounts

        (282, Rational(200, 1)), // XResolution

        (283, Rational(200, 1)), // YResolution

        (296, Short(2)), // ResolutionUnit

    ];

    let rat_data_len = 2 * 8; // number of rationals * 8

    let ifd_end = 

        4 + // magic

        4 + // IFD offset

        2 + // IFD entry count

        12 * header_data.len() + // IFD enties

        4; // null pointer at end of IFD

    let header_size = ifd_end + rat_data_len;

    let mut out = Vec::with_capacity(header_size + data.len());

    out.extend_from_slice(&[73, 73, 42, 0]);

    let ifd_offset: u32 = 8;

    out.extend_from_slice(&ifd_offset.to_le_bytes());

    out.extend_from_slice(&u16::to_le_bytes(header_data.len() as u16));

    let mut num_rat = 0;

    for &(tag, val) in header_data.iter() {

        let (typ_num, val) = match val {

            Short(n) => (3, n as u32),

            Long(n) => (4, n),

            Rational(_, _) => {

                let o = ifd_end + 8 * num_rat;

                num_rat += 1;

                (5, o as u32)

            }

            DataOffset => (4, header_size as u32)

        };

        let count = 1;

        out.extend_from_slice(&u16::to_le_bytes(tag));

        out.extend_from_slice(&u16::to_le_bytes(typ_num));

        out.extend_from_slice(&u32::to_le_bytes(count));

        out.extend_from_slice(&u32::to_le_bytes(val));

    }

    // NULL at IFD end

    out.extend_from_slice(&[0; 4]);

    // write additional data

    for &(_, val) in header_data.iter() {

        if let Value::Rational(nom, denom) = val {

            out.extend_from_slice(&nom.to_le_bytes());

            out.extend_from_slice(&denom.to_le_bytes());

        }

    }

    assert_eq!(out.len(), header_size);

    out.extend_from_slice(data);

    out

}