/**
 * @fileOverview Helper utilities for initialising the morgan logging format
 */
'use strict';
const morgan = require('morgan');
const debug = require('debug')('logging:activity');
const Writeable = require('stream').Writable;
const mainDBP = require('../ComServe/mainDB-promises');

let initialised = false;
const MAX_BUFFER = 1000;    // Max entries to buffer if the db is down

module.exports = {
    init,
    writeableStream
};

/**
 * Initialises the morgan formats if it has not already been initialised
 */
function init() {
    if (initialised) {
        return;
    }

    //
    // Define a morgan token to get the userId from the session
    //
    morgan.token('user-id', (req) => {
        if (req.session && req.session.data) {
            return req.session.data.user;
        } else {
            return '-';
        }
    });

    //
    // Define an Apache Combined Log equivalent format that uses our user id
    // rather than default `basic-auth` user value. See:
    //    https://github.com/expressjs/morgan#user-content-combined
    // for details of the base format.
    //
    morgan.format(
        'bridge-combined',
        ':remote-addr - :user-id [:date[clf]] ":method :url HTTP/:http-version" :status :res[content-length] ":referrer" ":user-agent"'
    );

    initialised = true;
}

/**
 * Function to create a new record for storing in the database.
 *
 * @param {string} record   - the record value from Morgan
 * @returns {Object}        - an object suitable for storing in MongoDB
 */
function entry(record) {
    return {
        timestamp: new Date(),
        request: record
    };
}

/**
 * Returns a new Writeable stream which can be used to log Morgan entries to
 * the database via Morgan's `stream` parameter.
 *
 * @returns {Writeable}     - A Writeable stream for use with Morgan logging
 */
function writeableStream() {
    let buffer = [];
    let writePending = false;
    const writeable = new Writeable({
        objectMode: true,
        highWaterMark: 1,
        write: function write(record, encoding, next) {
            // Always log to stdout immediately
            process.stdout.write(record + '\n');

            if (writePending || !mainDBP.mainDB.dbOnline) {
                // DB write in progress, or the DB is offline, so just buffer
                if (buffer.length < MAX_BUFFER) {
                    buffer.push(entry(record));
                    debug('Buffered log message:', buffer.length, writePending);
                } else {
                    process.stderr.write('Activity log buffer exceeded. MESSAGES WILL BE LOST!\n');
                }
            } else {
                // Online so try to send entries to the db.
                // There may be buffered entries, so swap them into pending array
                // so more can buffer while we wait for the DB to confirm.
                const pending = buffer.slice();
                buffer = [];

                // Add our new entry to the pending array
                pending.push(entry(record));

                // Try to upload them to mongo
                debug('WRITE Started:', pending.length);
                writePending = true;
                mainDBP.addMany(mainDBP.mainDB.collectionActivityLog, pending, {}, false)
                    .then((result) => {
                        // The request ran, but may not have inserted everything.
                        // If it didn't we can't really know which ones were and
                        // were not inserted, so just notify the error.
                        if (result.result.ok) {
                            debug('   - WRITE OK:', result.result.n);
                        } else {
                            process.stderr.write('Some activity log entries may have failed to save to the db!\n');
                            debug('   - Write partial failure:', result.result.n);
                        }

                        writePending = false;
                        return result;
                    })
                    .catch((error) => {
                        debug('   - WRITE ERROR received:', error);

                        // The request didn't run for some reason; likely that the
                        // database went down.  Add them back into the buffer,
                        // up to our max buffer size.
                        // Note: keep the original items as they are likely to be
                        //       closer to the cause of the outage.

                        // Add any new entries to the back of our pending list
                        const temp = pending.concat(buffer);

                        // And copy up to MAX_BUFFER items back over to the buffer
                        buffer = temp.slice(0, MAX_BUFFER);

                        writePending = false;
                        return null; // We handled the error, so no need to pass on
                    });
            }

            //
            // Allow the server to continue without waiting for the result of the write to db
            //
            next();
        }
    });
    return writeable;
}