use anyhow::Context;
use async_trait::async_trait;
use parking_lot::Mutex;
use pq_proto::StartupMessageParams;
use std::fmt;
use std::{collections::HashMap, sync::Arc};
use tokio::time;

use crate::{auth, console};
use crate::{compute, config};

use super::sql_over_http::MAX_RESPONSE_SIZE;

use crate::proxy::ConnectMechanism;

use tracing::error;
use tracing::info;

pub const APP_NAME: &str = "sql_over_http";
const MAX_CONNS_PER_ENDPOINT: usize = 20;

#[derive(Debug)]
pub struct ConnInfo {
    pub username: String,
    pub dbname: String,
    pub hostname: String,
    pub password: String,
}

impl ConnInfo {
    // hm, change to hasher to avoid cloning?
    pub fn db_and_user(&self) -> (String, String) {
        (self.dbname.clone(), self.username.clone())
    }
}

impl fmt::Display for ConnInfo {
    // use custom display to avoid logging password
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{}@{}/{}", self.username, self.hostname, self.dbname)
    }
}

struct ConnPoolEntry {
    conn: tokio_postgres::Client,
    _last_access: std::time::Instant,
}

// Per-endpoint connection pool, (dbname, username) -> Vec<ConnPoolEntry>
// Number of open connections is limited by the `max_conns_per_endpoint`.
pub struct EndpointConnPool {
    pools: HashMap<(String, String), Vec<ConnPoolEntry>>,
    total_conns: usize,
}

pub struct GlobalConnPool {
    // endpoint -> per-endpoint connection pool
    //
    // That should be a fairly conteded map, so return reference to the per-endpoint
    // pool as early as possible and release the lock.
    global_pool: Mutex<HashMap<String, Arc<Mutex<EndpointConnPool>>>>,

    // Maximum number of connections per one endpoint.
    // Can mix different (dbname, username) connections.
    // When running out of free slots for a particular endpoint,
    // falls back to opening a new connection for each request.
    max_conns_per_endpoint: usize,

    proxy_config: &'static crate::config::ProxyConfig,
}

impl GlobalConnPool {
    pub fn new(config: &'static crate::config::ProxyConfig) -> Arc<Self> {
        Arc::new(Self {
            global_pool: Mutex::new(HashMap::new()),
            max_conns_per_endpoint: MAX_CONNS_PER_ENDPOINT,
            proxy_config: config,
        })
    }

    pub async fn get(
        &self,
        conn_info: &ConnInfo,
        force_new: bool,
    ) -> anyhow::Result<tokio_postgres::Client> {
        let mut client: Option<tokio_postgres::Client> = None;

        if !force_new {
            let pool = self.get_endpoint_pool(&conn_info.hostname).await;

            // find a pool entry by (dbname, username) if exists
            let mut pool = pool.lock();
            let pool_entries = pool.pools.get_mut(&conn_info.db_and_user());
            if let Some(pool_entries) = pool_entries {
                if let Some(entry) = pool_entries.pop() {
                    client = Some(entry.conn);
                    pool.total_conns -= 1;
                }
            }
        }

        // ok return cached connection if found and establish a new one otherwise
        if let Some(client) = client {
            if client.is_closed() {
                info!("pool: cached connection '{conn_info}' is closed, opening a new one");
                connect_to_compute(self.proxy_config, conn_info).await
            } else {
                info!("pool: reusing connection '{conn_info}'");
                Ok(client)
            }
        } else {
            info!("pool: opening a new connection '{conn_info}'");
            connect_to_compute(self.proxy_config, conn_info).await
        }
    }

    pub async fn put(
        &self,
        conn_info: &ConnInfo,
        client: tokio_postgres::Client,
    ) -> anyhow::Result<()> {
        let pool = self.get_endpoint_pool(&conn_info.hostname).await;

        // return connection to the pool
        let mut total_conns;
        let mut returned = false;
        let mut per_db_size = 0;
        {
            let mut pool = pool.lock();
            total_conns = pool.total_conns;

            let pool_entries: &mut Vec<ConnPoolEntry> = pool
                .pools
                .entry(conn_info.db_and_user())
                .or_insert_with(|| Vec::with_capacity(1));
            if total_conns < self.max_conns_per_endpoint {
                pool_entries.push(ConnPoolEntry {
                    conn: client,
                    _last_access: std::time::Instant::now(),
                });

                total_conns += 1;
                returned = true;
                per_db_size = pool_entries.len();

                pool.total_conns += 1;
            }
        }

        // do logging outside of the mutex
        if returned {
            info!("pool: returning connection '{conn_info}' back to the pool, total_conns={total_conns}, for this (db, user)={per_db_size}");
        } else {
            info!("pool: throwing away connection '{conn_info}' because pool is full, total_conns={total_conns}");
        }

        Ok(())
    }

    async fn get_endpoint_pool(&self, endpoint: &String) -> Arc<Mutex<EndpointConnPool>> {
        // find or create a pool for this endpoint
        let mut created = false;
        let mut global_pool = self.global_pool.lock();
        let pool = global_pool
            .entry(endpoint.clone())
            .or_insert_with(|| {
                created = true;
                Arc::new(Mutex::new(EndpointConnPool {
                    pools: HashMap::new(),
                    total_conns: 0,
                }))
            })
            .clone();
        let global_pool_size = global_pool.len();
        drop(global_pool);

        // log new global pool size
        if created {
            info!(
                "pool: created new pool for '{endpoint}', global pool size now {global_pool_size}"
            );
        }

        pool
    }
}

struct TokioMechanism<'a> {
    conn_info: &'a ConnInfo,
}

#[async_trait]
impl ConnectMechanism for TokioMechanism<'_> {
    type Connection = tokio_postgres::Client;
    type ConnectError = tokio_postgres::Error;
    type Error = anyhow::Error;

    async fn connect_once(
        &self,
        node_info: &console::CachedNodeInfo,
        timeout: time::Duration,
    ) -> Result<Self::Connection, Self::ConnectError> {
        connect_to_compute_once(node_info, self.conn_info, timeout).await
    }

    fn update_connect_config(&self, _config: &mut compute::ConnCfg) {}
}

// Wake up the destination if needed. Code here is a bit involved because
// we reuse the code from the usual proxy and we need to prepare few structures
// that this code expects.
#[tracing::instrument(skip_all)]
async fn connect_to_compute(
    config: &config::ProxyConfig,
    conn_info: &ConnInfo,
) -> anyhow::Result<tokio_postgres::Client> {
    let tls = config.tls_config.as_ref();
    let common_names = tls.and_then(|tls| tls.common_names.clone());

    let credential_params = StartupMessageParams::new([
        ("user", &conn_info.username),
        ("database", &conn_info.dbname),
        ("application_name", APP_NAME),
    ]);

    let creds = config
        .auth_backend
        .as_ref()
        .map(|_| {
            auth::ClientCredentials::parse(
                &credential_params,
                Some(&conn_info.hostname),
                common_names,
            )
        })
        .transpose()?;
    let extra = console::ConsoleReqExtra {
        session_id: uuid::Uuid::new_v4(),
        application_name: Some(APP_NAME),
    };

    let node_info = creds
        .wake_compute(&extra)
        .await?
        .context("missing cache entry from wake_compute")?;

    crate::proxy::connect_to_compute(&TokioMechanism { conn_info }, node_info, &extra, &creds).await
}

async fn connect_to_compute_once(
    node_info: &console::CachedNodeInfo,
    conn_info: &ConnInfo,
    timeout: time::Duration,
) -> Result<tokio_postgres::Client, tokio_postgres::Error> {
    let mut config = (*node_info.config).clone();

    let (client, connection) = config
        .user(&conn_info.username)
        .password(&conn_info.password)
        .dbname(&conn_info.dbname)
        .max_backend_message_size(MAX_RESPONSE_SIZE)
        .connect_timeout(timeout)
        .connect(tokio_postgres::NoTls)
        .await?;

    tokio::spawn(async move {
        if let Err(e) = connection.await {
            error!("connection error: {}", e);
        }
    });

    Ok(client)
}