// The state machine used to represent one end of a conversation.
// This includes inducing transitions and actions taken during transitions,
// so a lot of the messenger network code is here.

use mgen::{log, MessageHeader, SerializedMessage};
use rand_distr::Distribution;
use rand_xoshiro::Xoshiro256PlusPlus;
use tokio::io::AsyncReadExt;
use tokio::net::TcpStream;
use tokio::sync::mpsc;
use tokio::time::Instant;
use std::collections::HashMap;
use std::sync::Arc;

use crate::messenger::dists::Distributions;
use crate::messenger::error::MessengerError;
use crate::messenger::message::{construct_message, construct_receipt};

/// All possible Conversation state machine states
pub enum StateMachine {
    Idle(Conversation<Idle>),
    Active(Conversation<Active>),
}

impl StateMachine {
    pub fn start(dists: Distributions, rng: &mut Xoshiro256PlusPlus) -> StateMachine {
        Self::Idle(Conversation::<Idle>::start(dists, rng))
    }
}

/// The state machine representing a conversation state and its transitions.
pub struct Conversation<S: State> {
    dists: Distributions,
    delay: Instant,
    state: S,
}

pub trait State {
    fn sent(conversation: Conversation<Self>, rng: &mut Xoshiro256PlusPlus) -> StateMachine
    where
        Self: Sized;
    fn received(conversation: Conversation<Self>, rng: &mut Xoshiro256PlusPlus) -> StateMachine
    where
        Self: Sized;
    fn to_machine(conversation: Conversation<Self>) -> StateMachine
    where
        Self: Sized;
}

pub struct Idle {}
pub struct Active {
    wait: Instant,
}

impl State for Idle {
    fn sent(conversation: Conversation<Self>, rng: &mut Xoshiro256PlusPlus) -> StateMachine {
        if conversation.dists.s.sample(rng) {
            log!("Idle: [sent] tranisition to [Active]");
            let delay = Instant::now() + conversation.dists.a_s.sample_secs(rng);
            let wait = Instant::now() + conversation.dists.w.sample_secs(rng);
            StateMachine::Active({
                Conversation::<Active> {
                    dists: conversation.dists,
                    delay,
                    state: Active { wait },
                }
            })
        } else {
            log!("Idle: [sent] tranisition to [Idle]");
            let delay = Instant::now() + conversation.dists.i.sample_secs(rng);
            StateMachine::Idle({
                Conversation::<Idle> {
                    dists: conversation.dists,
                    delay,
                    state: Idle {},
                }
            })
        }
    }

    fn received(conversation: Conversation<Self>, rng: &mut Xoshiro256PlusPlus) -> StateMachine {
        if conversation.dists.r.sample(rng) {
            log!("Idle: [recv'd] tranisition to [Active]");
            let wait = Instant::now() + conversation.dists.w.sample_secs(rng);
            let delay = Instant::now() + conversation.dists.a_r.sample_secs(rng);
            StateMachine::Active({
                Conversation::<Active> {
                    dists: conversation.dists,
                    delay,
                    state: Active { wait },
                }
            })
        } else {
            log!("Idle: [recv'd] tranisition to [Idle]");
            StateMachine::Idle(conversation)
        }
    }

    fn to_machine(conversation: Conversation<Self>) -> StateMachine {
        StateMachine::Idle(conversation)
    }
}

impl State for Active {
    fn sent(conversation: Conversation<Self>, rng: &mut Xoshiro256PlusPlus) -> StateMachine {
        log!("Active: [sent] transition to [Active]");
        let delay = Instant::now() + conversation.dists.a_s.sample_secs(rng);
        StateMachine::Active(Conversation::<Active> {
            dists: conversation.dists,
            delay,
            state: conversation.state,
        })
    }

    fn received(conversation: Conversation<Self>, rng: &mut Xoshiro256PlusPlus) -> StateMachine {
        log!("Active: [recv'd] transition to [Active]");
        let delay = Instant::now() + conversation.dists.a_r.sample_secs(rng);
        StateMachine::Active(Conversation::<Active> {
            dists: conversation.dists,
            delay,
            state: conversation.state,
        })
    }

    fn to_machine(conversation: Conversation<Self>) -> StateMachine {
        StateMachine::Active(conversation)
    }
}

impl Conversation<Idle> {
    fn start(dists: Distributions, rng: &mut Xoshiro256PlusPlus) -> Self {
        let delay = Instant::now() + dists.i.sample_secs(rng);
        log!("[start]");
        Self {
            dists,
            delay,
            state: Idle {},
        }
    }
}

impl Conversation<Active> {
    fn waited(self, rng: &mut Xoshiro256PlusPlus) -> Conversation<Idle> {
        log!("Active: [waited] tranision to [Idle]");
        let delay = Instant::now() + self.dists.i.sample_secs(rng);
        Conversation::<Idle> {
            dists: self.dists,
            delay,
            state: Idle {},
        }
    }

    async fn sleep(delay: Instant, wait: Instant) -> ActiveGroupActions {
        if delay < wait {
            log!("delaying for {:?}", delay - Instant::now());
            tokio::time::sleep_until(delay).await;
            ActiveGroupActions::Send
        } else {
            log!("waiting for {:?}", wait - Instant::now());
            tokio::time::sleep_until(wait).await;
            ActiveGroupActions::Idle
        }
    }
}

/// Attempt to read some portion of the header size from the stream.
/// The number of bytes written is returned in the Ok case.
/// The caller must read any remaining bytes less than 4.
// N.B.: This must be written cancellation safe!
// https://docs.rs/tokio/1.26.0/tokio/macro.select.html#cancellation-safety
async fn read_header_size(
    stream: &mut TcpStream,
    header_size: &mut [u8; 4],
) -> Result<usize, MessengerError> {
    let read = stream.read(header_size).await?;

    if read == 0 {
        Err(tokio::io::Error::new(
            tokio::io::ErrorKind::WriteZero,
            "failed to read any bytes from message with bytes remaining",
        )
        .into())
    } else {
        Ok(read)
    }
}

async fn read_message(
    stream: &mut TcpStream,
    n: usize,
    mut header_size: [u8; 4],
) -> Result<MessageHeader, MessengerError> {
    if n < 4 {
        // we didn't get the whole size, but we can use read_exact now
        stream.read_exact(&mut header_size[n..]).await?;
    }

    let (header, _) = mgen::get_message_with_header_size(stream, header_size).await?;
    Ok(header)
}

async fn send_action<'a, T: State, I: Iterator<Item = &'a mut mpsc::UnboundedSender<Arc<SerializedMessage>>>> (
    conversation: Conversation<T>,
    streams: I,
    our_id: &str,
    recipients: Vec<&str>,
    rng: &mut Xoshiro256PlusPlus,
) -> StateMachine {
    let size = conversation.dists.m.sample(rng);
    log!(
        "sending message from {} to {:?} of size {}",
        our_id,
        recipients,
        size
    );
    let m = Arc::new(construct_message(
        our_id.to_string(),
        recipients.iter().map(|s| s.to_string()).collect(),
        size,
    ));

    for stream in streams {
        stream.send(m.clone()).expect("Internal stream closed with messages still being sent");
    }

    T::sent(conversation, rng)
}

async fn receive_action<T: State>(
    msg: MessageHeader,
    conversation: Conversation<T>,
    stream_map: &HashMap<String, mpsc::UnboundedSender<Arc<SerializedMessage>>>,
    our_id: &str,
    rng: &mut Xoshiro256PlusPlus,
) -> StateMachine {

    match msg.body {
        mgen::MessageBody::Size(size) => {
            log!(
                "{:?} got message from {} of size {}",
                msg.recipients,
                msg.sender,
                size
            );
            let stream = stream_map.get(&msg.sender).expect(&format!("No such contact: {}", msg.sender));
            let m = construct_receipt(our_id.to_string(), msg.sender);
            stream.send(Arc::new(m));
            T::received(conversation, rng)
        }
        mgen::MessageBody::Receipt => T::to_machine(conversation),
    }
}

/*
enum IdleActions {
    Send,
    Receive(usize),
}

/// Handle a state transition from Idle, including I/O, for a single-connection conversation.
/// Used for Idle client-server and single-contact p2p conversations.
pub async fn manage_single_idle_conversation(
    conversation: Conversation<Idle>,
    stream: &mut TcpStream,
    our_id: &str,
    recipients: Vec<&str>,
    rng: &mut Xoshiro256PlusPlus,
) -> Result<StateMachine, (StateMachine, MessengerError)> {
    log!("delaying for {:?}", conversation.delay - Instant::now());
    let mut header_size = [0; 4];
    let action = tokio::select! {
        () = tokio::time::sleep_until(conversation.delay) => {
            Ok(IdleActions::Send)
        }

        res = read_header_size(stream, &mut header_size) => {
            match res {
                Ok(n) => Ok(IdleActions::Receive(n)),
                Err(e) => Err(e),
            }
        }
    };
    let action = match action {
        Ok(action) => action,
        Err(e) => return Err((StateMachine::Idle(conversation), e)),
    };

    match action {
        IdleActions::Send => send_action(conversation, stream, our_id, recipients, rng).await,
        IdleActions::Receive(n) => {
            let msg = match read_message(stream, n, header_size).await {
                Ok(msg) => msg,
                Err(e) => return Err((StateMachine::Idle(conversation), e)),
            };
            receive_action(msg, conversation, stream, our_id, rng).await
        }
    }
}

enum ActiveActions {
    Send,
    Receive(usize),
    Idle,
}

/// Handle a state transition from Active, including I/O, for a single-connection conversation.
/// Used for Active client-server and single-contact p2p conversations.
pub async fn manage_single_active_conversation(
    conversation: Conversation<Active>,
    stream: &mut TcpStream,
    our_id: &str,
    recipients: Vec<&str>,
    rng: &mut Xoshiro256PlusPlus,
) -> Result<StateMachine, (StateMachine, MessengerError)> {
    let mut header_size = [0; 4];
    let action = tokio::select! {
        action = Conversation::<Active>::sleep(conversation.delay, conversation.state.wait) => {
            Ok(action)
        }

        res = read_header_size(stream, &mut header_size) => {
            match res {
                Ok(n) => Ok(ActiveActions::Receive(n)),
                Err(e) => Err(e),
            }
        }
    };
    let action = match action {
        Ok(action) => action,
        Err(e) => return Err((StateMachine::Active(conversation), e)),
    };

    match action {
        ActiveActions::Send => send_action(conversation, stream, our_id, recipients, rng).await,
        ActiveActions::Receive(n) => {
            let msg = match read_message(stream, n, header_size).await {
                Ok(msg) => msg,
                Err(e) => return Err((StateMachine::Active(conversation), e)),
            };
            receive_action(msg, conversation, stream, our_id, rng).await
        }
        ActiveActions::Idle => Ok(StateMachine::Idle(conversation.waited(rng))),
    }
}
*/

enum IdleGroupActions {
    Send,
    Receive(MessageHeader),
}

/// Handle a state transition from Idle, including I/O, for a multi-connection conversation.
/// Used for Idle group p2p conversations.
pub async fn manage_group_idle_conversation(
    conversation: Conversation<Idle>,
    inbound: &mut mpsc::UnboundedReceiver<MessageHeader>,
    stream_map: &mut HashMap<String, mpsc::UnboundedSender<Arc<SerializedMessage>>>,
    our_id: &str,
    recipients: Vec<&str>,
    rng: &mut Xoshiro256PlusPlus,
) -> StateMachine {
    log!("delaying for {:?}", conversation.delay - Instant::now());
    let action = tokio::select! {
        () = tokio::time::sleep_until(conversation.delay) => IdleGroupActions::Send,

        res = inbound.recv() =>
            IdleGroupActions::Receive(res.expect("inbound channel closed")),
    };

    match action {
        IdleGroupActions::Send => send_action(conversation, stream_map.values_mut(), our_id, recipients, rng).await,
        IdleGroupActions::Receive(msg) => receive_action(msg, conversation, stream_map, our_id, rng).await,
    }
}

enum ActiveGroupActions {
    Send,
    Receive(MessageHeader),
    Idle,
}

/// Handle a state transition from Active, including I/O, for a multi-connection conversation.
/// Used for Active group p2p conversations.
pub async fn manage_group_active_conversation(
    conversation: Conversation<Active>,
    inbound: &mut mpsc::UnboundedReceiver<MessageHeader>,
    stream_map: &mut HashMap<String, mpsc::UnboundedSender<Arc<SerializedMessage>>>,
    our_id: &str,
    recipients: Vec<&str>,
    rng: &mut Xoshiro256PlusPlus,
) -> StateMachine {
    let action = tokio::select! {
        action = Conversation::<Active>::sleep(conversation.delay, conversation.state.wait) => action,

        res = inbound.recv() =>
            ActiveGroupActions::Receive(res.expect("inbound channel closed")),
    };

    match action {
        ActiveGroupActions::Send => send_action(conversation, stream_map.values_mut(), our_id, recipients, rng).await,
        ActiveGroupActions::Receive(msg) => receive_action(msg, conversation, stream_map, our_id, rng).await,
        ActiveGroupActions::Idle => StateMachine::Idle(conversation.waited(rng)),
    }
}