use anyhow::Result;
use log::trace;
use std::{
    io::{Read, Seek, SeekFrom},
    sync::{Arc, Mutex},
};

/// Splits an reader into many by seeking in between reads.
pub struct MultiReader<T> {
    position: u64,
    inner: Arc<Mutex<(u64, T)>>,
}
impl<T: Seek> MultiReader<T> {
    pub fn new(mut inner: T) -> Result<Self> {
        let position = inner.stream_position()?;
        Ok(Self {
            position,
            inner: Arc::new(Mutex::new((position, inner))),
        })
    }
}
impl<T> Clone for MultiReader<T> {
    fn clone(&self) -> Self {
        Self {
            position: self.position,
            inner: self.inner.clone(),
        }
    }
}
impl<T: Read + Seek> Read for MultiReader<T> {
    fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
        let mut g = self.inner.lock().unwrap();
        if g.0 != self.position {
            trace!(
                "seeking to match stream position ({} actual vs {} required)",
                g.0, self.position
            );
            g.1.seek(SeekFrom::Start(self.position))?;
            g.0 = self.position;
        }
        let size = g.1.read(buf)?;
        g.0 += size as u64;
        self.position += size as u64;
        Ok(size)
    }
}
impl<T: Seek> Seek for MultiReader<T> {
    fn seek(&mut self, pos: SeekFrom) -> std::io::Result<u64> {
        self.position = match pos {
            SeekFrom::Start(x) => x,
            SeekFrom::Current(x) => self.position.saturating_add_signed(x),
            SeekFrom::End(_) => unimplemented!(),
        };
        Ok(self.position)
    }
}