//! mrpgi-vault — the "front door" for the web engine: signup/login and one
//! saved world per user.
//!
//!   POST /api/signup {"user","pass"}   create account, start a session
//!   POST /api/login  {"user","pass"}   start a session
//!   POST /api/logout                   end the session
//!   GET  /api/me                       {"user":"matt"} or 401
//!   GET  /api/world                    the user's saved world JSON (404 if none)
//!   PUT  /api/world                    save (body = WorldBundle JSON, ≤ 4 MB)
//!   GET  /api/ping                     health check
//!
//! Storage is the filesystem: VAULT_DATA/users.json + VAULT_DATA/worlds/<user>.json.
//! Sessions are in-memory (a restart signs everyone out — they log back in).
//! Single-threaded on purpose: requests are tiny, ordering is trivial, and
//! there is nothing to lock.

use argon2::password_hash::{PasswordHash, PasswordHasher, PasswordVerifier, SaltString};
use argon2::Argon2;
use serde_json::{json, Value};
use std::collections::HashMap;
use std::io::Read;
use std::path::PathBuf;
use tiny_http::{Header, Method, Response, Server};

const MAX_BODY: u64 = 4 * 1024 * 1024;
const SESSION_COOKIE: &str = "mrpgi_session";

fn main() {
    let data: PathBuf = std::env::var("VAULT_DATA").unwrap_or_else(|_| "vault-data".into()).into();
    std::fs::create_dir_all(data.join("worlds")).expect("can't create data dir");
    let addr = std::env::var("VAULT_ADDR").unwrap_or_else(|_| "0.0.0.0:8200".into());
    let server = Server::http(&addr).unwrap_or_else(|e| panic!("can't bind {}: {}", addr, e));
    eprintln!("mrpgi-vault listening on {} (data: {})", addr, data.display());

    let users_path = data.join("users.json");
    let mut users: HashMap<String, String> = std::fs::read_to_string(&users_path)
        .ok()
        .and_then(|s| serde_json::from_str(&s).ok())
        .unwrap_or_default();
    let mut sessions: HashMap<String, String> = HashMap::new();
    // ponytail: naive per-minute rate limit; a real limiter if this ever gets popular
    let mut rate: HashMap<String, (u64, u32)> = HashMap::new();

    for mut req in server.incoming_requests() {
        let method = req.method().clone();
        let path = req.url().split('?').next().unwrap_or("").to_string();
        let ip = req.remote_addr().map(|a| a.ip().to_string()).unwrap_or_default();
        let session_user = cookie_token(&req).and_then(|t| sessions.get(&t).cloned());

        match (method, path.as_str()) {
            (Method::Get, "/api/ping") => respond(req, 200, json!({"ok": true}), None),

            (Method::Post, "/api/signup") | (Method::Post, "/api/login") => {
                if !allow(&mut rate, &ip) {
                    respond(req, 429, json!({"error": "slow down a little"}), None);
                    continue;
                }
                let body = read_body(&mut req);
                let v: Value = serde_json::from_str(&body).unwrap_or(Value::Null);
                let user = v["user"].as_str().unwrap_or("").trim().to_lowercase();
                let pass = v["pass"].as_str().unwrap_or("");
                if !valid_username(&user) {
                    respond(req, 400, json!({"error": "username: 2-20 chars, a-z 0-9 _ -"}), None);
                    continue;
                }
                if pass.len() < 4 || pass.len() > 100 {
                    respond(req, 400, json!({"error": "password: at least 4 characters"}), None);
                    continue;
                }
                let signup = path == "/api/signup";
                if signup {
                    if users.contains_key(&user) {
                        respond(req, 409, json!({"error": "that name is taken"}), None);
                        continue;
                    }
                    let salt = SaltString::encode_b64(&random_bytes()).expect("salt");
                    let hash = Argon2::default().hash_password(pass.as_bytes(), &salt).expect("hash").to_string();
                    users.insert(user.clone(), hash);
                    let j = serde_json::to_string_pretty(&users).expect("users json");
                    if let Err(e) = std::fs::write(&users_path, j) {
                        users.remove(&user);
                        eprintln!("users.json write failed: {}", e);
                        respond(req, 500, json!({"error": "storage hiccup, try again"}), None);
                        continue;
                    }
                } else {
                    let ok = users
                        .get(&user)
                        .and_then(|h| PasswordHash::new(h).ok())
                        .map(|h| Argon2::default().verify_password(pass.as_bytes(), &h).is_ok())
                        .unwrap_or(false);
                    if !ok {
                        respond(req, 401, json!({"error": "wrong name or password"}), None);
                        continue;
                    }
                }
                let token = hex(&random_bytes());
                sessions.insert(token.clone(), user.clone());
                respond(req, 200, json!({"user": user}), Some(set_cookie(&token, false)));
            }

            (Method::Post, "/api/logout") => {
                if let Some(t) = cookie_token(&req) {
                    sessions.remove(&t);
                }
                respond(req, 200, json!({"ok": true}), Some(set_cookie("", true)));
            }

            (Method::Get, "/api/me") => match session_user {
                Some(u) => respond(req, 200, json!({"user": u}), None),
                None => respond(req, 401, json!({"error": "not signed in"}), None),
            },

            (Method::Get, "/api/world") => match session_user {
                Some(u) => match std::fs::read_to_string(data.join("worlds").join(format!("{}.json", u))) {
                    Ok(w) => respond_raw(req, 200, w.into_bytes()),
                    Err(_) => respond(req, 404, json!({"error": "no saved world yet"}), None),
                },
                None => respond(req, 401, json!({"error": "not signed in"}), None),
            },

            (Method::Put, "/api/world") => match session_user {
                Some(u) => {
                    let body = read_body(&mut req);
                    // must at least parse as JSON — the vault stores worlds, not mystery bytes
                    if serde_json::from_str::<Value>(&body).is_err() {
                        respond(req, 400, json!({"error": "not valid JSON"}), None);
                        continue;
                    }
                    match std::fs::write(data.join("worlds").join(format!("{}.json", u)), &body) {
                        Ok(()) => respond(req, 200, json!({"ok": true, "bytes": body.len()}), None),
                        Err(e) => {
                            eprintln!("world write failed for {}: {}", u, e);
                            respond(req, 500, json!({"error": "storage hiccup, try again"}), None)
                        }
                    }
                }
                None => respond(req, 401, json!({"error": "not signed in"}), None),
            },

            _ => respond(req, 404, json!({"error": "no such route"}), None),
        }
    }
}

/// ponytail: 20 auth attempts per IP per minute, forgotten on restart.
fn allow(rate: &mut HashMap<String, (u64, u32)>, ip: &str) -> bool {
    let min = std::time::SystemTime::now()
        .duration_since(std::time::UNIX_EPOCH)
        .map(|d| d.as_secs() / 60)
        .unwrap_or(0);
    let count = {
        let e = rate.entry(ip.to_string()).or_insert((min, 0));
        if e.0 != min {
            *e = (min, 0);
        }
        e.1 += 1;
        e.1
    };
    if rate.len() > 10_000 {
        rate.retain(|_, v| v.0 == min);
    }
    count <= 20
}

fn valid_username(u: &str) -> bool {
    (2..=20).contains(&u.len()) && u.chars().all(|c| c.is_ascii_lowercase() || c.is_ascii_digit() || c == '_' || c == '-')
}

fn random_bytes() -> [u8; 32] {
    let mut b = [0u8; 32];
    getrandom::getrandom(&mut b).expect("os rng");
    b
}

fn hex(b: &[u8]) -> String {
    b.iter().map(|x| format!("{:02x}", x)).collect()
}

fn read_body(req: &mut tiny_http::Request) -> String {
    let mut body = String::new();
    let _ = req.as_reader().take(MAX_BODY).read_to_string(&mut body);
    body
}

fn cookie_token(req: &tiny_http::Request) -> Option<String> {
    req.headers()
        .iter()
        .find(|h| h.field.equiv("Cookie"))
        .and_then(|h| {
            h.value
                .as_str()
                .split(';')
                .map(|c| c.trim())
                .find_map(|c| c.strip_prefix(&format!("{}=", SESSION_COOKIE)))
                .map(|t| t.to_string())
        })
}

fn set_cookie(token: &str, clear: bool) -> Header {
    let v = if clear {
        format!("{}=; Path=/; Max-Age=0; HttpOnly; SameSite=Lax", SESSION_COOKIE)
    } else {
        // Secure is fine even though the origin hop is http: the browser only
        // ever sees this through Cloudflare's https.
        format!("{}={}; Path=/; Max-Age=2592000; HttpOnly; Secure; SameSite=Lax", SESSION_COOKIE, token)
    };
    Header::from_bytes("Set-Cookie", v).expect("cookie header")
}

fn respond(req: tiny_http::Request, code: u16, v: Value, cookie: Option<Header>) {
    let mut resp = Response::from_data(v.to_string().into_bytes()).with_status_code(code);
    resp.add_header(Header::from_bytes("Content-Type", "application/json").expect("header"));
    if let Some(c) = cookie {
        resp.add_header(c);
    }
    let _ = req.respond(resp);
}

fn respond_raw(req: tiny_http::Request, code: u16, bytes: Vec<u8>) {
    let mut resp = Response::from_data(bytes).with_status_code(code);
    resp.add_header(Header::from_bytes("Content-Type", "application/json").expect("header"));
    let _ = req.respond(resp);
}