//! Process HTTP connections on the server.

use std::pin::Pin;

use async_std::io;
use async_std::io::prelude::*;
use async_std::task::{Context, Poll};
use http_types::headers::{CONTENT_LENGTH, DATE, TRANSFER_ENCODING};
use http_types::{Method, Response};

use crate::chunked::ChunkedEncoder;
use crate::date::fmt_http_date;

/// A streaming HTTP encoder.
#[derive(Debug)]
pub struct Encoder {
    /// The current level of recursion the encoder is in.
    depth: u16,
    /// HTTP headers to be sent.
    res: Response,
    /// The state of the encoding process
    state: State,
    /// Track bytes read in a call to poll_read.
    bytes_written: usize,
    /// The data we're writing as part of the head section.
    head: Vec<u8>,
    /// The amount of bytes read from the head section.
    head_bytes_written: usize,
    /// The total length of the body.
    /// This is only used in the known-length body encoder.
    body_len: usize,
    /// The amount of bytes read from the body.
    /// This is only used in the known-length body encoder.
    body_bytes_written: usize,
    /// An encoder for chunked encoding.
    chunked: ChunkedEncoder,
    /// the http method that this response is in reply to
    method: Method,
}

#[derive(Debug)]
enum State {
    /// Starting state.
    Start,
    /// Write the HEAD section to an intermediate buffer.
    ComputeHead,
    /// Stream out the HEAD section.
    EncodeHead,
    /// Stream out a body whose length is known ahead of time.
    EncodeFixedBody,
    /// Stream out a body whose length is *not* know ahead of time.
    EncodeChunkedBody,
    /// Stream has ended.
    End,
}

impl Read for Encoder {
    fn poll_read(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &mut [u8],
    ) -> Poll<io::Result<usize>> {
        self.bytes_written = 0;
        let res = self.run(cx, buf);
        log::trace!("ServerEncoder {} bytes written", self.bytes_written);
        res
    }
}

impl Encoder {
    /// Create a new instance of Encoder.
    pub fn new(res: Response, method: Method) -> Self {
        Self {
            res,
            depth: 0,
            state: State::Start,
            bytes_written: 0,
            head: vec![],
            head_bytes_written: 0,
            body_len: 0,
            body_bytes_written: 0,
            chunked: ChunkedEncoder::new(),
            method,
        }
    }

    /// Switch the internal state to a new state.
    fn dispatch(
        &mut self,
        state: State,
        cx: &mut Context<'_>,
        buf: &mut [u8],
    ) -> Poll<io::Result<usize>> {
        use State::*;
        log::trace!("ServerEncoder state: {:?} -> {:?}", self.state, state);

        #[cfg(debug_assertions)]
        match self.state {
            Start => assert!(matches!(state, ComputeHead)),
            ComputeHead => assert!(matches!(state, EncodeHead)),
            EncodeHead => assert!(matches!(state, EncodeChunkedBody | EncodeFixedBody | End)),
            EncodeFixedBody => assert!(matches!(state, End)),
            EncodeChunkedBody => assert!(matches!(state, End)),
            End => panic!("No state transitions allowed after the ServerEncoder has ended"),
        }

        self.state = state;
        self.run(cx, buf)
    }

    /// Execute the right method for the current state.
    fn run(&mut self, cx: &mut Context<'_>, buf: &mut [u8]) -> Poll<io::Result<usize>> {
        match self.state {
            State::Start => self.dispatch(State::ComputeHead, cx, buf),
            State::ComputeHead => self.compute_head(cx, buf),
            State::EncodeHead => self.encode_head(cx, buf),
            State::EncodeFixedBody => self.encode_fixed_body(cx, buf),
            State::EncodeChunkedBody => self.encode_chunked_body(cx, buf),
            State::End => Poll::Ready(Ok(self.bytes_written)),
        }
    }

    /// Encode the headers to a buffer, the first time we poll.
    fn compute_head(&mut self, cx: &mut Context<'_>, buf: &mut [u8]) -> Poll<io::Result<usize>> {
        let reason = self.res.status().canonical_reason();
        let status = self.res.status();
        std::io::Write::write_fmt(
            &mut self.head,
            format_args!("HTTP/1.1 {} {}\r\n", status, reason),
        )?;

        // If the body isn't streaming, we can set the content-length ahead of time. Else we need to
        // send all items in chunks.
        if let Some(len) = self.res.len() {
            std::io::Write::write_fmt(&mut self.head, format_args!("content-length: {}\r\n", len))?;
        } else {
            std::io::Write::write_fmt(
                &mut self.head,
                format_args!("transfer-encoding: chunked\r\n"),
            )?;
        }

        if self.res.header(DATE).is_none() {
            let date = fmt_http_date(std::time::SystemTime::now());
            std::io::Write::write_fmt(&mut self.head, format_args!("date: {}\r\n", date))?;
        }

        let iter = self
            .res
            .iter()
            .filter(|(h, _)| h != &&CONTENT_LENGTH)
            .filter(|(h, _)| h != &&TRANSFER_ENCODING);
        for (header, values) in iter {
            for value in values.iter() {
                std::io::Write::write_fmt(
                    &mut self.head,
                    format_args!("{}: {}\r\n", header, value),
                )?
            }
        }

        std::io::Write::write_fmt(&mut self.head, format_args!("\r\n"))?;

        self.dispatch(State::EncodeHead, cx, buf)
    }

    /// Encode the status code + headers.
    fn encode_head(&mut self, cx: &mut Context<'_>, buf: &mut [u8]) -> Poll<io::Result<usize>> {
        // Read from the serialized headers, url and methods.
        let head_len = self.head.len();
        let len = std::cmp::min(head_len - self.head_bytes_written, buf.len());
        let range = self.head_bytes_written..self.head_bytes_written + len;
        buf[0..len].copy_from_slice(&self.head[range]);
        self.bytes_written += len;
        self.head_bytes_written += len;

        // If we've read the total length of the head we're done
        // reading the head and can transition to reading the body
        if self.head_bytes_written == head_len {
            if self.method == Method::Head {
                // If we are responding to a HEAD request, we MUST NOT send
                // body content
                self.dispatch(State::End, cx, buf)
            } else {
                // The response length lets us know if we are encoding
                // our body in chunks or not
                match self.res.len() {
                    Some(body_len) => {
                        self.body_len = body_len;
                        self.dispatch(State::EncodeFixedBody, cx, buf)
                    }
                    None => self.dispatch(State::EncodeChunkedBody, cx, buf),
                }
            }
        } else {
            // If we haven't read the entire header it means `buf` isn't
            // big enough. Break out of loop and return from `poll_read`
            Poll::Ready(Ok(self.bytes_written))
        }
    }

    /// Encode the body with a known length.
    fn encode_fixed_body(
        &mut self,
        cx: &mut Context<'_>,
        buf: &mut [u8],
    ) -> Poll<io::Result<usize>> {
        // Double check that we didn't somehow read more bytes than
        // can fit in our buffer
        debug_assert!(self.bytes_written <= buf.len());

        // ensure we have at least room for 1 more byte in our buffer
        if self.bytes_written == buf.len() {
            return Poll::Ready(Ok(self.bytes_written));
        }

        // Figure out how many bytes we can read.
        let upper_bound =
            (self.bytes_written + self.body_len - self.body_bytes_written).min(buf.len());
        // Read bytes from body
        let range = self.bytes_written..upper_bound;
        let inner_poll_result = Pin::new(&mut self.res).poll_read(cx, &mut buf[range]);
        let new_body_bytes_written = match inner_poll_result {
            Poll::Ready(Ok(n)) => n,
            Poll::Ready(Err(e)) => return Poll::Ready(Err(e)),
            Poll::Pending => match self.bytes_written {
                0 => return Poll::Pending,
                n => return Poll::Ready(Ok(n)),
            },
        };
        self.body_bytes_written += new_body_bytes_written;
        self.bytes_written += new_body_bytes_written;

        // Double check we did not read more body bytes than the total
        // length of the body
        debug_assert!(
            self.body_bytes_written <= self.body_len,
            "Too many bytes read. Expected: {}, read: {}",
            self.body_len,
            self.body_bytes_written
        );

        if self.body_len == self.body_bytes_written {
            // If we've read the `len` number of bytes, end
            self.dispatch(State::End, cx, buf)
        } else if new_body_bytes_written == 0 {
            // If we've reached unexpected EOF, end anyway
            // TODO: do something?
            self.dispatch(State::End, cx, buf)
        } else {
            // Else continue encoding
            self.encode_fixed_body(cx, buf)
        }
    }

    /// Encode an AsyncBufRead using "chunked" framing. This is used for streams
    /// whose length is not known up front.
    fn encode_chunked_body(
        &mut self,
        cx: &mut Context<'_>,
        buf: &mut [u8],
    ) -> Poll<io::Result<usize>> {
        let buf = &mut buf[self.bytes_written..];
        match self.chunked.encode(&mut self.res, cx, buf) {
            Poll::Ready(Ok(read)) => {
                self.bytes_written += read;
                match self.bytes_written {
                    0 => self.dispatch(State::End, cx, buf),
                    _ => Poll::Ready(Ok(self.bytes_written)),
                }
            }
            Poll::Ready(Err(err)) => Poll::Ready(Err(err)),
            Poll::Pending => match self.bytes_written {
                0 => Poll::Pending,
                _ => Poll::Ready(Ok(self.bytes_written)),
            },
        }
    }
}