//! # Rdsys Backend Distributor API
//!
//! `rdsys_backend` is an implementation of the rdsys backend API
//! https://gitlab.torproject.org/tpo/anti-censorship/rdsys/-/blob/main/doc/backend-api.md

use bytes::{self, Buf, Bytes};
use futures_util::StreamExt;
use reqwest::Client;
use std::io::{self, BufRead};
use std::sync::mpsc;

pub mod proto;

#[derive(Debug)]
pub enum Error {
    Reqwest(reqwest::Error),
    Io(io::Error),
    JSON(serde_json::Error),
}

impl From<serde_json::Error> for Error {
    fn from(value: serde_json::Error) -> Self {
        Self::JSON(value)
    }
}

impl From<reqwest::Error> for Error {
    fn from(value: reqwest::Error) -> Self {
        Self::Reqwest(value)
    }
}

impl From<io::Error> for Error {
    fn from(value: io::Error) -> Self {
        Self::Io(value)
    }
}

/// An iterable wrapper of ResourceDiff items for the streamed chunks of Bytes
/// received from the connection to the rdsys backend
pub struct ResourceStream {
    rx: mpsc::Receiver<Bytes>,
    buf: Vec<u8>,
    partial: Option<bytes::buf::Reader<Bytes>>,
}

impl Iterator for ResourceStream {
    type Item = proto::ResourceDiff;
    fn next(&mut self) -> Option<Self::Item> {
        let mut parse =
            |buffer: &mut bytes::buf::Reader<Bytes>| -> Result<Option<Self::Item>, Error> {
                match buffer.read_until(b'\r', &mut self.buf) {
                    Ok(_) => match self.buf.pop() {
                        Some(b'\r') => match serde_json::from_slice(&self.buf) {
                            Ok(diff) => {
                                self.buf.clear();
                                Ok(Some(diff))
                            }
                            Err(e) => Err(Error::JSON(e)),
                        },
                        Some(n) => {
                            self.buf.push(n);
                            Ok(None)
                        }
                        None => Ok(None),
                    },
                    Err(e) => Err(Error::Io(e)),
                }
            };

        if let Some(p) = &mut self.partial {
            match parse(p) {
                Ok(Some(diff)) => return Some(diff),
                Ok(None) => self.partial = None,
                Err(_) => return None,
            }
        }
        for chunk in &self.rx {
            let mut buffer = chunk.reader();
            match parse(&mut buffer) {
                Ok(Some(diff)) => {
                    self.partial = Some(buffer);
                    return Some(diff);
                }
                Ok(None) => continue,
                Err(_) => return None,
            };
        }
        None
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn parse_resource() {
        let chunk = Bytes::from_static(
            b"{\"new\": null,\"changed\": null,\"gone\": null,\"full_update\": true}\r",
        );
        let (tx, rx) = mpsc::channel();
        tx.send(chunk).unwrap();
        let mut diffs = ResourceStream {
            rx: rx,
            partial: None,
            buf: vec![],
        };
        let res = diffs.next();
        assert_ne!(res, None);
        if let Some(diff) = res {
            assert_eq!(diff.new, None);
            assert_eq!(diff.full_update, true);
        }
    }

    #[test]
    fn parse_across_chunks() {
        let chunk1 = Bytes::from_static(b"{\"new\": null,\"changed\": null,");
        let chunk2 = Bytes::from_static(b"\"gone\": null,\"full_update\": true}\r");
        let (tx, rx) = mpsc::channel();
        tx.send(chunk1).unwrap();
        tx.send(chunk2).unwrap();
        let mut diffs = ResourceStream {
            rx: rx,
            partial: None,
            buf: vec![],
        };
        let res = diffs.next();
        assert_ne!(res, None);
        if let Some(diff) = res {
            assert_eq!(diff.new, None);
            assert_eq!(diff.full_update, true);
        }
    }

    #[test]
    fn parse_multi_diff_partial_chunks() {
        let chunk1 = Bytes::from_static(b"{\"new\": null,\"changed\": null,");
        let chunk2 =
            Bytes::from_static(b"\"gone\": null,\"full_update\": true}\r{\"new\": null,\"changed");
        let chunk3 = Bytes::from_static(b"\": null,\"gone\": null,\"full_update\": true}");
        let chunk4 = Bytes::from_static(b"\r");
        let (tx, rx) = mpsc::channel();
        tx.send(chunk1).unwrap();
        tx.send(chunk2).unwrap();
        tx.send(chunk3).unwrap();
        tx.send(chunk4).unwrap();
        let mut diffs = ResourceStream {
            rx: rx,
            partial: None,
            buf: vec![],
        };
        let mut res = diffs.next();
        assert_ne!(res, None);
        if let Some(diff) = res {
            assert_eq!(diff.new, None);
            assert_eq!(diff.full_update, true);
        }
        res = diffs.next();
        assert_ne!(res, None);
        if let Some(diff) = res {
            assert_eq!(diff.new, None);
            assert_eq!(diff.full_update, true);
        }
    }
}

/// Makes an http connection to the rdsys backend api endpoint and returns a ResourceStream
/// if successful
///
/// # Examples
///
/// ```ignore
/// use rdsys_backend::start_stream;
///
/// let endpoint = String::from("http://127.0.0.1:7100/resource-stream");
/// let name = String::from("https");
/// let token = String::from("HttpsApiTokenPlaceholder");
/// let types = vec![String::from("obfs2"), String::from("scramblesuit")];
/// let rx = start_stream(endpoint, name, token, types).await.unwrap();
/// for diff in rx {
///     println!("Received diff: {:?}", diff);
/// }
/// ```
///
pub async fn start_stream(
    api_endpoint: String,
    name: String,
    token: String,
    resource_types: Vec<String>,
) -> Result<ResourceStream, Error> {
    let (tx, rx) = mpsc::channel();

    let req = proto::ResourceRequest {
        request_origin: name,
        resource_types,
    };
    let json = serde_json::to_string(&req)?;

    let auth_value = format!("Bearer {}", token);

    let client = Client::new();

    let mut stream = client
        .get(api_endpoint)
        .header("Authorization", &auth_value)
        .body(json)
        .send()
        .await?
        .bytes_stream();

    tokio::spawn(async move {
        while let Some(chunk) = stream.next().await {
            let bytes = match chunk {
                Ok(b) => b,
                Err(_e) => {
                    return;
                }
            };
            tx.send(bytes).unwrap();
        }
    });
    Ok(ResourceStream {
        rx,
        buf: vec![],
        partial: None,
    })
}