//! Which posts `gatewayd` acts on, which session each belongs to, commands, and the queue of
//! messages per session (M4a spec, section 7). Pure: the state file and the network are elsewhere.

use std::collections::{BTreeSet, HashMap};

use proto::SessionId;

use crate::mm::Post;

pub const M4B_COMMAND: &str = "approvals over Mattermost arrive in M4b; use `bxctl approvals`";
pub const UNKNOWN_COMMAND: &str = "unknown command; the commands are !approve and !deny";
pub const BUSY: &str = "busy: too many messages are waiting in this conversation";

/// Names that name nobody: every agent in the channel would answer them.
const EVERYONE: [&str; 3] = ["channel", "here", "all"];

/// Why a post was not acted on. Only `NotAllowed` is logged, by user id and post id.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Ignored {
    Own,
    System,
    NotAllowed,
    NotForUs,
}

/// Where an answer goes: a channel, and the root of the thread in it.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Thread {
    pub channel: String,
    pub root: String,
}

/// A message for a session.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Message {
    pub session: SessionId,
    pub thread: Thread,
    /// A reply in a thread: the session should exist already.
    pub resume: bool,
    pub text: String,
    /// A thread in a channel or group message that this Boxmaker now takes part in.
    pub joins_thread: bool,
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Route {
    Ignore(Ignored),
    /// Answer in the thread without a turn (a command).
    Reply {
        thread: Thread,
        text: String,
    },
    Queue(Message),
}

pub struct Router {
    me_id: String,
    me_name: String,
    users: BTreeSet<String>,
    channels: BTreeSet<String>,
}

/// Every `@name` in a message, lower-cased: `a-z`, `0-9`, `.`, `-` and `_` after an `@`, without
/// trailing dots.
pub fn named(message: &str) -> Vec<String> {
    // Find each "@". The name after it is the longest run of ASCII letters, digits, ".", "-" and
    // "_", with trailing "." removed and lower-cased. Skip empty names. Continue after the name.
    todo!()
}

impl Router {
    pub fn new(me_id: &str, me_name: &str, users: &[String], channels: &[String]) -> Router {
        // Store the ids, the username lower-cased, and the two lists as sets.
        todo!()
    }

    /// Is a post in a channel or group message for this Boxmaker? `known` says whether it has a
    /// session for a thread root.
    fn for_us(&self, post: &Post, known: &dyn Fn(&str) -> bool) -> bool {
        // Named this bot -> true. Otherwise true only for a reply (root_id not empty) in a known
        // thread (`known(root_id)`) that names nobody but channel, here or all.
        todo!()
    }

    /// What to do with a new post (a post seen before is dropped by the caller first).
    pub fn route(&self, post: &Post, channel_type: &str, known: &dyn Fn(&str) -> bool) -> Route {
        // Section 7 of the spec, in this order: own post -> Ignore(Own); kind not empty ->
        // Ignore(System); user not allowed -> Ignore(NotAllowed); then the channel: "D" is always
        // ours; "O", "P" or "G" is ours when its id is allowed and `for_us`; anything else ->
        // Ignore(NotForUs). The thread root is root_id, or the post id when root_id is empty. Then
        // commands: "!!..." drops one "!" and goes on as a message; "!" then a first word approve
        // or deny -> Reply M4B_COMMAND; any other "!" -> Reply UNKNOWN_COMMAND. Then Queue: session
        // "mm-<root>" (if SessionId::new fails, Ignore(NotForUs)), resume = root_id not empty,
        // joins_thread = not "D".
        todo!()
    }
}

/// A turn to send: every message that was waiting, joined with a blank line.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Batch {
    pub session: SessionId,
    pub thread: Thread,
    pub resume: bool,
    pub text: String,
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Pushed {
    /// No turn was running: send this one now.
    Start(Batch),
    /// A turn is running; the message waits for the next.
    Waiting,
    /// Too many are waiting: the message is dropped, answer `BUSY`.
    Full(Thread),
}

struct Pending {
    thread: Thread,
    resume: bool,
    waiting: Vec<String>,
}

/// The sessions with a turn running, and the messages waiting for each.
pub struct Queues {
    limit: usize,
    running: HashMap<SessionId, Pending>,
}

impl Queues {
    /// `limit` is the most messages that may wait per session.
    pub fn new(limit: usize) -> Queues {
        // An empty map.
        todo!()
    }

    pub fn push(&mut self, message: Message) -> Pushed {
        // If the session is running: with `limit` messages already waiting -> Full(thread); else
        // add the text to waiting -> Waiting. Otherwise insert it as running (its later batches
        // resume: true) and return Start with this message alone.
        todo!()
    }

    /// A session's turn ended: the next batch, or `None`, and the session is no longer running.
    pub fn finish(&mut self, session: &SessionId) -> Option<Batch> {
        // Not running -> None. Nothing waiting -> remove it, None. Otherwise take every waiting
        // text, joined with "\n\n", as the next Batch (it stays running).
        todo!()
    }

    /// How many sessions have a turn running.
    pub fn running(&self) -> usize {
        // How many sessions are running.
        todo!()
    }

    /// The threads with a turn running, for showing this bot as typing in them.
    pub fn threads(&self) -> Vec<Thread> {
        // The thread of every running session.
        todo!()
    }
}