use desert::{ToBytes,FromBytes,Error};

#[derive(Debug)]
enum Item { A((f32,f32)), B(u32) }

#[derive(Debug)]
struct Custom { foo: u64, items: Vec<Item> }

impl ToBytes for Custom {
  fn to_bytes(&self) -> Result<Vec<u8>,Error> {
    let mut bytes = vec![];
    // Store foo (in big endian).
    bytes.extend(&self.foo.to_bytes()?);

    // Store the number of items (in big endian).
    bytes.extend(&(self.items.len() as u16).to_bytes()?);

    // Use a bitfield to more compactly represent
    // whether an Item is an A or B.
    let mut bitfield = vec![0u8;(self.items.len()+7)/8];
    for (i,item) in self.items.iter().enumerate() {
      bitfield[i/8] |= match item {
        Item::A(_) => 0,
        Item::B(_) => 1,
      } << (i%8);
    }
    bytes.extend(bitfield);

    // Write out each item serially.
    for item in self.items.iter() {
      bytes.extend(match item {
        Item::A(x) => x.to_bytes()?,
        Item::B(x) => x.to_bytes()?
      });
    }
    Ok(bytes)
  }
}

impl FromBytes for Custom {
  fn from_bytes(src: &[u8]) -> Result<(usize,Self),Error> {
    let mut offset = 0;

    // Read foo (in big endian).
    let (size,foo) = u64::from_bytes(&src[offset..])?;
    offset += size;

    // Read the number of items (in big endian).
    let (size,item_len) = u16::from_bytes(&src[offset..])?;
    offset += size;

    // Read the bitfield data but keep it as a u8 slice.
    let bitfield_len = ((item_len+7)/8) as usize;
    let bitfield = &src[offset..offset+bitfield_len];
    offset += bitfield_len;

    // Read the items, checking the bitfield to know whether an item
    // is an A or a B.
    let mut items = vec![];
    for i in 0..item_len as usize {
      if (bitfield[i/8]>>(i%8))&1 == 0 {
        let (size,x) = <(f32,f32)>::from_bytes(&src[offset..])?;
        items.push(Item::A(x));
        offset += size;
      } else {
        let (size,x) = u32::from_bytes(&src[offset..])?;
        items.push(Item::B(x));
        offset += size;
      }
    }
    Ok((offset, Custom { foo, items }))
  }
}

fn main() -> Result<(),Error> {
  let bytes = Custom {
    foo: 1234567890123456789,
    items: vec![
      Item::A((3.0,4.2)),
      Item::B(1337),
      Item::A((5.5,6.6))
    ]
  }.to_bytes()?;
  println!["serialized: {:?}", bytes];

  let (size,custom) = Custom::from_bytes(&bytes)?;
  println!["deserialized {} bytes: {:?}", size, custom];
  Ok(())
}