use std::io::{Read, Seek, SeekFrom, Result};

pub struct Subfile<T: Read + Seek> {
    stream: T,
    offset: u64
}

impl <T: Read + Seek> Subfile<T> {
    pub fn new(mut stream: T, offset: u64) -> Subfile<T> {
        stream.seek(SeekFrom::Start(offset)).unwrap();
        Subfile {
            stream: stream,
            offset: offset
        }
    }
}

impl <T: Read + Seek> Read for Subfile<T> {
    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
        self.stream.read(buf)
    }
}

impl <T: Read + Seek> Seek for Subfile<T> {
    fn seek(&mut self, mut pos: SeekFrom) -> Result<u64> {
        pos = match pos {
            SeekFrom::Start(offset) => SeekFrom::Start(offset + self.offset),
            x => x
        };

        let newpos = try!(self.stream.seek(pos));

        if newpos > self.offset {
            return Ok(newpos - self.offset)
        } else {
            return Ok(0)
        }
    }
}