const crypto = require('crypto');

const COOKIE = 'sdp_session';
const STATE_COOKIE = 'sdp_oauth_state';
// renew this long before expiry so a slow SDP query never runs on a dead token
const RENEW_EARLY_MS = 5 * 60 * 1000;

function parseCookies(header) {
    const cookies = {};
    for (const part of (header || '').split(';')) {
        const eq = part.indexOf('=');
        if (eq < 1) continue;
        try {
            cookies[part.slice(0, eq).trim()] = decodeURIComponent(part.slice(eq + 1).trim());
        } catch (err) {
            // a malformed cookie from somewhere else on the domain; ignore it
        }
    }
    return cookies;
}

// The whole session lives in the browser: the user's Zoho tokens, encrypted and authenticated
// with SESSION_SECRET, in an httpOnly cookie page scripts can't read. The server keeps nothing,
// so restarts and new image versions don't sign anyone out.
function createSessions({ secret, secure, sessionDays, zoho }) {
    const key = crypto.createHash('sha256').update(secret).digest();
    const maxAgeMs = sessionDays * 24 * 60 * 60 * 1000;
    const cookieBase = { httpOnly: true, secure, sameSite: 'lax', path: '/' };

    function seal(payload) {
        const iv = crypto.randomBytes(12);
        const cipher = crypto.createCipheriv('aes-256-gcm', key, iv);
        const body = Buffer.concat([cipher.update(JSON.stringify(payload), 'utf8'), cipher.final()]);
        return Buffer.concat([iv, cipher.getAuthTag(), body]).toString('base64url');
    }

    function open(value) {
        try {
            const raw = Buffer.from(value, 'base64url');
            const decipher = crypto.createDecipheriv('aes-256-gcm', key, raw.subarray(0, 12));
            decipher.setAuthTag(raw.subarray(12, 28));
            const body = Buffer.concat([decipher.update(raw.subarray(28)), decipher.final()]);
            return JSON.parse(body.toString('utf8'));
        } catch (err) {
            // tampered, truncated, or sealed with an old SESSION_SECRET
            return null;
        }
    }

    function write(res, session) {
        // the cookie dies with the session; renewing the access token doesn't extend it
        const maxAge = Math.max(0, session.startedAt + maxAgeMs - Date.now());
        res.cookie(COOKIE, seal(session), { ...cookieBase, maxAge });
    }

    function clear(res) {
        res.clearCookie(COOKIE, cookieBase);
    }

    function read(req) {
        const session = open(parseCookies(req.headers.cookie)[COOKIE] || '');
        if (!session || !session.refreshToken) return null;
        if (Date.now() > session.startedAt + maxAgeMs) return null;
        return session;
    }

    function start(res, tokens, user) {
        const session = { ...tokens, user, startedAt: Date.now() };
        write(res, session);
        return session;
    }

    // A page load fires several calls at once, all carrying the same expired cookie. Renew once
    // per refresh token and hand the result to all of them, rather than minting a token each.
    const renewals = new Map();

    function renew(refreshToken) {
        const id = crypto.createHash('sha256').update(refreshToken).digest('hex');
        const cached = renewals.get(id);
        if (cached) return cached;

        const pending = zoho.refreshAccessToken(refreshToken);
        renewals.set(id, pending);
        pending.then(
            // keep the answer until shortly before it too needs renewing, then forget it
            tokens => setTimeout(() => renewals.delete(id), Math.max(0, tokens.expiresAt - RENEW_EARLY_MS - Date.now())).unref(),
            () => renewals.delete(id)
        );
        return pending;
    }

    // middleware: puts the signed-in session on req.session, renewing the access token if due
    async function requireSession(req, res, next) {
        const session = read(req);
        if (!session) {
            clear(res);
            return res.status(401).json({ error: 'Not signed in' });
        }

        if (Date.now() >= session.expiresAt - RENEW_EARLY_MS) {
            try {
                Object.assign(session, await renew(session.refreshToken));
                write(res, session);
            } catch (err) {
                if (err.zoho || err.response?.status === 400) {
                    // the refresh token was revoked or has been dropped by zoho; sign in again
                    clear(res);
                    return res.status(401).json({ error: 'Your Zoho sign-in has expired' });
                }
                return res.status(502).json({ error: 'Could not reach Zoho: ' + err.message });
            }
        }

        req.session = session;
        next();
    }

    // one-time value that ties Zoho's callback to the browser that started the sign-in
    function startState(res) {
        const state = crypto.randomBytes(16).toString('hex');
        res.cookie(STATE_COOKIE, state, { ...cookieBase, maxAge: 10 * 60 * 1000 });
        return state;
    }

    function checkState(req, res, given) {
        const expected = parseCookies(req.headers.cookie)[STATE_COOKIE];
        res.clearCookie(STATE_COOKIE, cookieBase);
        if (!expected || typeof given !== 'string') return false;
        // hash first so the comparison is fixed-length whatever the query string holds
        const hash = value => crypto.createHash('sha256').update(value).digest();
        return crypto.timingSafeEqual(hash(expected), hash(given));
    }

    return {
        read,
        start,
        clear,
        requireSession,
        startState,
        checkState
    };
}

module.exports = {
    createSessions
};