'use strict';

const EVENT_VERSION = '1';
const MAX_LENGTH = 100;

function pubSubMetrics (pubSubMetricsBackend) {
    if (!pubSubMetricsBackend.isEnabled()) {
        return function pubSubMetricsDisabledMiddleware (req, res, next) {
            next();
        };
    }

    return function pubSubMetricsMiddleware (req, res, next) {
        res.on('finish', () => {
            const { event, attributes } = getEventData(req, res);

            if (event) {
                pubSubMetricsBackend.sendEvent(event, attributes);
            }
        });

        return next();
    };
}

function getEventData (req, res) {
    const event = normalizedField(req.get('Carto-Event'));
    const eventSource = normalizedField(req.get('Carto-Event-Source'));
    const eventGroupId = normalizedField(req.get('Carto-Event-Group-Id'));

    if (!event || !eventSource) {
        return { event: undefined, attributes: undefined };
    }

    const attributes = {
        event_source: eventSource,
        user_id: res.locals.userId,
        response_code: res.statusCode.toString(),
        source_domain: req.hostname,
        event_time: new Date().toISOString(),
        event_version: EVENT_VERSION
    };

    if (eventGroupId) {
        attributes.event_group_id = eventGroupId;
    }

    const responseTime = getResponseTime(res);

    if (responseTime) {
        attributes.response_time = responseTime.toString();
    }

    return { event, attributes };
}

function normalizedField (field) {
    if (!field) {
        return undefined;
    }

    return field.toString().trim().substr(0, MAX_LENGTH);
}

function getResponseTime (res) {
    const profiler = res.get('X-Tiler-Profiler');
    let stats;

    try {
        stats = JSON.parse(profiler);
    } catch (e) {
        return undefined;
    }

    return stats.total;
}

module.exports = pubSubMetrics;