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use crate::{Result, Tape, Walue};

/// An offset.
#[derive(Clone, Copy, Debug, Eq, Ord, PartialEq, PartialOrd)]
pub struct Offset(pub u32);

/// An offset size.
pub type OffsetSize = u8;

impl Walue<'static> for Offset {
    type Parameter = OffsetSize;

    fn read<T: Tape>(tape: &mut T, size: OffsetSize) -> Result<Self> {
        #[cfg(target_endian = "big")]
        macro_rules! assemble(($hi:expr, $me:expr, $lo:expr) => ([0, $hi, $me, $lo]));
        #[cfg(target_endian = "little")]
        macro_rules! assemble(($hi:expr, $me:expr, $lo:expr) => ([$lo, $me, $hi, 0]));
        Ok(Offset(match size {
            1 => tape.take::<u8>()? as u32,
            2 => tape.take::<u16>()? as u32,
            3 => {
                let trio: [u8; 3] = tape.take()?;
                unsafe { std::mem::transmute::<_, u32>(assemble!(trio[0], trio[1], trio[2])) }
            }
            4 => tape.take::<u32>()?,
            _ => raise!("found a malformed offset"),
        }))
    }
}

#[cfg(test)]
mod tests {
    use std::io::Cursor;

    use crate::compact1::Offset;
    use crate::Walue;

    #[test]
    fn read() {
        let mut tape = Cursor::new(vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);

        assert_eq!(Offset::read(&mut tape, 1).unwrap().0, 0x01);
        assert_eq!(Offset::read(&mut tape, 2).unwrap().0, 0x0203);
        assert_eq!(Offset::read(&mut tape, 3).unwrap().0, 0x040506);
        assert_eq!(Offset::read(&mut tape, 4).unwrap().0, 0x0708090a);
    }
}