use crate::{
    extension::{Link, Signature},
    util::split_at_pos,
};

pub const SIGNATURE: Signature = *b"link";

pub struct Bitmaps {
    pub delete: git_bitmap::ewah::Vec,
    pub replace: git_bitmap::ewah::Vec,
}

pub mod decode {
    use quick_error::quick_error;

    quick_error! {
        #[derive(Debug)]
        pub enum Error {
            Corrupt(message: &'static str) {
                display("{}", message)
            }
            BitmapDecode{err: git_bitmap::ewah::decode::Error, kind: &'static str} {
                display("{} bitmap corrupt", kind)
                source(err)
            }
        }
    }
}

pub(crate) fn decode(data: &[u8], object_hash: git_hash::Kind) -> Result<Link, decode::Error> {
    let (id, data) = split_at_pos(data, object_hash.len_in_bytes())
        .ok_or(decode::Error::Corrupt(
            "link extension too short to read share index checksum",
        ))
        .map(|(id, d)| (git_hash::ObjectId::from(id), d))?;

    if data.is_empty() {
        return Ok(Link {
            shared_index_checksum: id,
            bitmaps: None,
        });
    }

    let (delete, data) =
        git_bitmap::ewah::decode(data).map_err(|err| decode::Error::BitmapDecode { kind: "delete", err })?;
    let (replace, data) =
        git_bitmap::ewah::decode(data).map_err(|err| decode::Error::BitmapDecode { kind: "replace", err })?;

    if !data.is_empty() {
        return Err(decode::Error::Corrupt("garbage trailing link extension"));
    }

    Ok(Link {
        shared_index_checksum: id,
        bitmaps: Some(Bitmaps { delete, replace }),
    })
}