//! This reader composes frames of bytes started with a 4 byte frame header indicating the size of
//! the buffer. An exact size buffer will be allocated once the 4 byte frame header is received.

use std::io::{self, Read, Error, ErrorKind};
use std::collections::VecDeque;
use std::mem;

#[derive(Debug)]
pub struct FrameReader {
    frames: Frames
}

impl FrameReader {
    pub fn new(max_frame_size: u32) -> FrameReader {
        FrameReader {
            frames: Frames::new(max_frame_size)
        }
    }

    pub fn read<T: Read>(&mut self, reader: &mut T) -> io::Result<usize> {
        self.frames.read(reader)
    }

    pub fn iter_mut(&mut self) -> Iter {
        Iter {
            frames: &mut self.frames
        }
    }
}

pub struct Iter<'a> {
    frames: &'a mut Frames
}

impl<'a> Iterator for Iter<'a> {
    type Item = Vec<u8>;

    fn next(&mut self) -> Option<Vec<u8>> {
        self.frames.completed_frames.pop_front()
    }
}

#[derive(Debug)]
struct Frames {
    max_frame_size: u32,
    bytes_read: usize,
    header: [u8; 4],
    reading_header: bool,
    current: Vec<u8>,
    completed_frames: VecDeque<Vec<u8>>
}

impl Frames {
    pub fn new(max_frame_size: u32) -> Frames {
        Frames {
            max_frame_size: max_frame_size,
            bytes_read: 0,
            header: [0; 4],
            reading_header: true,
            current: Vec::with_capacity(0),
            completed_frames: VecDeque::new()
        }
    }

    /// Will read as much data as possible and build up frames to be retrieved from the iterator.
    ///
    /// Will stop reading when 0 bytes are retrieved from the latest call to `do_read` or the error
    /// kind is io::ErrorKind::WouldBlock.
    ///
    /// Returns an error or the total amount of bytes read.
    fn read<T: Read>(&mut self, reader: &mut T) -> io::Result<usize> {
        let mut total_bytes_read = 0;
        loop {
            match self.do_read(reader) {
                Ok(0) => {
                    if total_bytes_read == 0 {
                        return Err(Error::new(ErrorKind::UnexpectedEof, "Read 0 bytes"));
                    }
                    return Ok(total_bytes_read);
                },
                Ok(bytes_read) => {
                    total_bytes_read += bytes_read;
                },
                Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => return Ok(total_bytes_read),
                Err(e) => return Err(e)
            }
        }
    }

    fn do_read<T: Read>(&mut self, reader: &mut T) -> io::Result<usize> {
        if self.reading_header {
            self.read_header(reader)
        } else {
            self.read_value(reader)
        }
    }

    // TODO: Return an error if size is greater than max_frame_size
    fn read_header<T: Read>(&mut self, reader: &mut T) -> io::Result<usize> {
        let bytes_read = try!(reader.read(&mut self.header[self.bytes_read..]));
        self.bytes_read += bytes_read;
        if self.bytes_read == 4 {
           let len = unsafe { u32::from_be(mem::transmute(self.header)) };
           self.bytes_read = 0;
           self.reading_header = false;
           self.current = Vec::with_capacity(len as usize);
           unsafe { self.current.set_len(len as usize); }
        }
        Ok(bytes_read)
    }

    fn read_value<T: Read>(&mut self, reader: &mut T) -> io::Result<usize> {
        let bytes_read = try!(reader.read(&mut self.current[self.bytes_read..]));
        self.bytes_read += bytes_read;
        if self.bytes_read == self.current.len() {
           self.completed_frames.push_back(mem::replace(&mut self.current, Vec::new()));
           self.bytes_read = 0;
           self.reading_header = true;
        }
        Ok(bytes_read)
    }
}

#[cfg(test)]
mod tests {
    use std::{mem, thread};
    use std::io::Cursor;
    use std::io::Write;
    use std::net::{TcpListener, TcpStream};
    use super::FrameReader;

    #[test]
    fn partial_and_complete_reads() {
        let buf1 = String::from("Hello World").into_bytes();
        let buf2 = String::from("Hi.").into_bytes();
        let header1: [u8; 4] = unsafe { mem::transmute((buf1.len() as u32).to_be()) };
        let header2: [u8; 4] = unsafe { mem::transmute((buf2.len() as u32).to_be()) };

        let mut reader = FrameReader::new(1024);

        // Write a partial header
        let mut header = Cursor::new(&header1[0..2]);
        let bytes_read = reader.read(&mut header).unwrap();
        assert_eq!(2, bytes_read);
        assert_eq!(None, reader.iter_mut().next());

        // Complete writing just the header
        let mut header = Cursor::new(&header1[2..]);
        let bytes_read = reader.read(&mut header).unwrap();
        assert_eq!(2, bytes_read);
        assert_eq!(None, reader.iter_mut().next());

        // Write a partial value
        let mut data = Cursor::new(&buf1[0..5]);
        let bytes_read = reader.read(&mut data).unwrap();
        assert_eq!(5, bytes_read);
        assert_eq!(None, reader.iter_mut().next());

        // Complete writing the first value
        let mut data = Cursor::new(&buf1[5..]);
        let bytes_read = reader.read(&mut data).unwrap();
        assert_eq!(6, bytes_read);
        let val = reader.iter_mut().next().unwrap();
        assert_eq!(buf1, val);

        // Write an entire header and value
        let mut data = Cursor::new(Vec::with_capacity(7));
        assert_eq!(4, data.write(&header2).unwrap());
        assert_eq!(3, data.write(&buf2).unwrap());
        data.set_position(0);
        let bytes_read = reader.read(&mut data).unwrap();
        assert_eq!(7, bytes_read);
        assert_eq!(buf2, reader.iter_mut().next().unwrap());
    }

    const IP: &'static str = "127.0.0.1:5003";
    /// Test that we never get an io error, but instead get Ok(0) when the call to read would block
    #[test]
    fn would_block() {
        let listener = TcpListener::bind(IP).unwrap();
        let h = thread::spawn(move || {
            for stream in listener.incoming() {
                if let Ok(mut conn) = stream {
                    conn.set_nonblocking(true).unwrap();
                    let mut reader = FrameReader::new(1024);
                    let result = reader.read(&mut conn);
                    assert_matches!(result, Ok(0));
                    return;
                }
            }
        });
        // Assign to a variable so the sock isn't dropped early
        // Name it with a preceding underscore so we don't get an unused variable warning
        let _sock = TcpStream::connect(IP).unwrap();
        h.join().unwrap();

    }
}