//! `<home>/gateway/state.json`: what `gatewayd` has handled, the threads it takes part in, and the
//! turns in flight (M4a spec, section 9). Written atomically after every change. A file that
//! cannot be read, parsed or written stops `gatewayd`: guessing would answer posts twice.

use std::collections::BTreeMap;
use std::fs;
use std::io::{self, Write};
use std::os::unix::fs::{DirBuilderExt, OpenOptionsExt};
use std::path::{Path, PathBuf};

use serde::{Deserialize, Serialize};

use crate::config::valid_id;

pub const RECENT_KEPT: usize = 500;
pub const THREADS_KEPT: usize = 5000;
pub const RUNBOOK: &str = "see docs/runbook.md#gateway-state-damaged";

#[derive(Debug)]
pub enum StateError {
    Read(PathBuf, String),
    Write(PathBuf, io::Error),
}

impl std::fmt::Display for StateError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            StateError::Read(path, why) => write!(f, "{}: {why}\n{RUNBOOK}", path.display()),
            StateError::Write(path, err) => {
                write!(f, "{}: cannot write: {err}\n{RUNBOOK}", path.display())
            }
        }
    }
}

impl std::error::Error for StateError {}

/// A turn sent to `loopd` and not yet answered.
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
#[serde(deny_unknown_fields)]
pub struct InFlight {
    pub session: String,
    pub channel: String,
    pub root: String,
}

#[derive(Debug, Clone, Default, PartialEq, Eq, Serialize, Deserialize)]
#[serde(deny_unknown_fields)]
struct StateFile {
    channels: BTreeMap<String, i64>,
    recent: Vec<String>,
    threads: Vec<String>,
    in_flight: Vec<InFlight>,
    /// Optional, so an M4a state file loads unchanged.
    #[serde(default)]
    approvals: Vec<ApprovalPosts>,
}

/// The posts of an approval shown in Mattermost and not yet ended (M4b spec, section 8).
#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
#[serde(deny_unknown_fields)]
pub struct ApprovalPosts {
    pub approval: u64,
    pub channel: String,
    /// First part first; the last is the one that is edited when it ends.
    pub posts: Vec<String>,
}

impl StateFile {
    /// Every id is a Mattermost id; a session is `mm-<id>`.
    fn problem(&self) -> Option<String> {
        // Every key of channels, every entry of recent and threads must be `valid_id` ("not a
        // Mattermost id: <id with {:?}>"). Each in_flight entry: session is "mm-" + a valid id,
        // channel and root valid ids ("a turn in flight is not valid: <session with {:?}>"). None
        // when all are good.
        for channel in self.channels.keys() {
            if !valid_id(channel) {
                return Some(format!("not a Mattermost id: {channel:?}"));
            }
        }
        for post in &self.recent {
            if !valid_id(post) {
                return Some(format!("not a Mattermost id: {post:?}"));
            }
        }
        for root in &self.threads {
            if !valid_id(root) {
                return Some(format!("not a Mattermost id: {root:?}"));
            }
        }
        for turn in &self.in_flight {
            let ok = match turn.session.strip_prefix("mm-") {
                Some(rest) => valid_id(rest) && valid_id(&turn.channel) && valid_id(&turn.root),
                None => false,
            };
            if !ok {
                return Some(format!("a turn in flight is not valid: {:?}", turn.session));
            }
        }
        for shown in &self.approvals {
            if !valid_id(&shown.channel)
                || shown.posts.is_empty()
                || !shown.posts.iter().all(|post| valid_id(post))
            {
                return Some(format!("approval {}'s posts are not valid", shown.approval));
            }
        }
        None
    }
}

pub struct State {
    path: PathBuf,
    file: StateFile,
}

impl State {
    /// Read the state; a missing file is a first start.
    pub fn load(path: &Path) -> Result<State, StateError> {
        // Read the file. Missing (NotFound only) -> an empty StateFile. Any other read error, a
        // parse error (serde_json), or `problem()` -> Read(path, why).
        let text = match fs::read_to_string(path) {
            Ok(text) => text,
            Err(err) if err.kind() == io::ErrorKind::NotFound => {
                return Ok(State {
                    path: path.to_path_buf(),
                    file: StateFile::default(),
                });
            }
            Err(err) => return Err(StateError::Read(path.to_path_buf(), err.to_string())),
        };
        let file = match serde_json::from_str::<StateFile>(&text) {
            Ok(file) => file,
            Err(err) => return Err(StateError::Read(path.to_path_buf(), err.to_string())),
        };
        if let Some(why) = file.problem() {
            return Err(StateError::Read(path.to_path_buf(), why));
        }
        Ok(State {
            path: path.to_path_buf(),
            file,
        })
    }

    /// Write the state to its path atomically, in six steps; an error leaves the old file.
    fn save(&self) -> Result<(), StateError> {
        // `persist`, its error as Write(path, error).
        self.persist()
            .map_err(|err| StateError::Write(self.path.clone(), err))
    }

    fn persist(&self) -> io::Result<()> {
        // The six steps of brokerd/src/state.rs, persist: 1. the parent directory, recursive, 0700;
        // 2. serde_json::to_string plus "\n"; 3. "<path>.tmp" (with_extension("json.tmp")), create
        //    + truncate, mode 0600; 4. write_all and sync_all; 5. rename over the path; 6. open the
        //    directory and sync_all.
        let dir = self
            .path
            .parent()
            .ok_or_else(|| io::Error::other("state file has no parent"))?;
        fs::DirBuilder::new()
            .recursive(true)
            .mode(0o700)
            .create(dir)?;
        let json = serde_json::to_string(&self.file).map_err(io::Error::other)?;
        let bytes = format!("{json}\n");
        let tmp = self.path.with_extension("json.tmp");
        let mut file = fs::OpenOptions::new()
            .write(true)
            .create(true)
            .truncate(true)
            .mode(0o600)
            .open(&tmp)?;
        file.write_all(bytes.as_bytes())?;
        file.sync_all()?;
        fs::rename(&tmp, &self.path)?;
        fs::File::open(dir)?.sync_all()
    }

    /// Was this post handled already?
    pub fn seen(&self, post_id: &str) -> bool {
        // Is the id in recent?
        self.file.recent.contains(&post_id.to_string())
    }

    /// A post was handled (acted on or ignored): remember its id and move its channel's mark.
    pub fn handled(
        &mut self,
        post_id: &str,
        channel: &str,
        create_at: i64,
    ) -> Result<(), StateError> {
        // If not seen: push the id to recent, then drop the oldest beyond RECENT_KEPT. The channel
        // mark becomes the larger of its old value and create_at (a new channel starts at
        // create_at). Save.
        if !self.file.recent.contains(&post_id.to_string()) {
            self.file.recent.push(post_id.to_string());
            while self.file.recent.len() > RECENT_KEPT {
                self.file.recent.remove(0);
            }
        }
        let entry = self
            .file
            .channels
            .entry(channel.to_string())
            .or_insert(create_at);
        *entry = (*entry).max(create_at);
        self.save()
    }

    /// The `create_at` of the last post handled in a channel, if any.
    pub fn since(&self, channel: &str) -> Option<i64> {
        // The mark of the channel.
        self.file.channels.get(channel).copied()
    }

    /// The channels with a mark, for catching up.
    pub fn channels(&self) -> Vec<String> {
        // The channel ids that have a mark.
        self.file.channels.keys().cloned().collect()
    }

    /// Start catching up a channel from `at` (milliseconds), if it has no mark yet.
    pub fn mark(&mut self, channel: &str, at: i64) -> Result<(), StateError> {
        // A channel that has a mark is left alone (nothing saved). Otherwise set it to `at` and
        // save.
        if self.file.channels.contains_key(channel) {
            return Ok(());
        }
        self.file.channels.insert(channel.to_string(), at);
        self.save()
    }

    pub fn knows_thread(&self, root: &str) -> bool {
        // Is the root in threads?
        self.file.threads.contains(&root.to_string())
    }

    /// This Boxmaker takes part in a thread; the newest `THREADS_KEPT` are kept.
    pub fn join_thread(&mut self, root: &str) -> Result<(), StateError> {
        // Known -> nothing. Else push it, drop the oldest beyond THREADS_KEPT, save.
        if self.file.threads.contains(&root.to_string()) {
            return Ok(());
        }
        self.file.threads.push(root.to_string());
        while self.file.threads.len() > THREADS_KEPT {
            self.file.threads.remove(0);
        }
        self.save()
    }

    pub fn start_turn(&mut self, turn: InFlight) -> Result<(), StateError> {
        // Remove any entry of the same session, push this one, save.
        self.file.in_flight.retain(|t| t.session != turn.session);
        self.file.in_flight.push(turn);
        self.save()
    }

    pub fn end_turn(&mut self, session: &str) -> Result<(), StateError> {
        // Remove the entries of the session, save.
        self.file.in_flight.retain(|t| t.session != session);
        self.save()
    }

    /// The turns left in flight by the last run, removed from the state.
    pub fn take_in_flight(&mut self) -> Result<Vec<InFlight>, StateError> {
        // Take the whole list out (std::mem::take); save only when it was not empty.
        let taken = std::mem::take(&mut self.file.in_flight);
        if taken.is_empty() {
            return Ok(taken);
        }
        self.save().map(|()| taken)
    }

    /// The turn of a session, if one is in flight.
    pub fn turn_of(&self, session: &str) -> Option<InFlight> {
        // The entry of in_flight with that session, cloned.
        self.file
            .in_flight
            .iter()
            .find(|turn| turn.session == session)
            .cloned()
    }

    /// A new posting of an approval shown in Mattermost; it replaces any earlier one.
    pub fn add_approval(&mut self, shown: ApprovalPosts) -> Result<(), StateError> {
        // Remove any entry with the same approval, push this one, save.
        self.file.approvals.retain(|a| a.approval != shown.approval);
        self.file.approvals.push(shown);
        self.save()
    }

    /// Stop tracking an approval; nothing saved when it was not tracked.
    pub fn remove_approval(&mut self, approval: u64) -> Result<(), StateError> {
        // Remove the entries with that approval, save only if one was removed.
        let before = self.file.approvals.len();
        self.file.approvals.retain(|a| a.approval != approval);
        if self.file.approvals.len() == before {
            return Ok(());
        }
        self.save()
    }

    /// The approval posts left behind by the last run, removed from the state.
    pub fn take_approvals(&mut self) -> Result<Vec<ApprovalPosts>, StateError> {
        // Take them all out (std::mem::take), save only when there were some, return them.
        let taken = std::mem::take(&mut self.file.approvals);
        if taken.is_empty() {
            return Ok(taken);
        }
        self.save().map(|()| taken)
    }
}