use std::io::SeekFrom;
use async_std::fs::File;
use futures::{AsyncSeekExt, AsyncReadExt};


async fn read_at( f   :&mut File
                    , pos :SeekFrom
                    , buf :&mut [u8]) -> std::io::Result<()> {
    f.seek(pos).await?;
    f.read_exact(buf).await
}

pub async fn get_sample( f   :&mut File
                       , buf :&mut [u8]) -> std::io::Result<()> {
    let file_len = f.metadata().await?.len();
    let chunk_size = buf.len() / 3;

    read_at(f, SeekFrom::Start(0), &mut buf[0..chunk_size]).await?;
    if file_len >= 2 * chunk_size as u64 {
        read_at( f
               , SeekFrom::End(-(chunk_size as i64))
               , &mut buf[2*chunk_size..]).await?;
    }
    if file_len >= 3 * chunk_size as u64 {
        read_at( f
               , SeekFrom::Start((file_len-chunk_size as u64) / 2)
               , &mut buf[chunk_size..2*chunk_size]).await?;
    }

    Ok(())
}