'use strict';

function formatRelativeTime(timeString) {
    if (!timeString) {
        return 'never';
    }

    const then = Date.parse(timeString);
    const now = Date.now();
    const difference = Math.abs(now - then) / 1000.0;
    const future = now < then;

    const descriptions = [
        [60,                        'a few seconds',    null],
        [60*2,                      'a minute',         null],
        [60*60,                     '% minutes',        60],
        [60*60*2,                   'an hour',          null],
        [60*60*24,                  '% hours',          60*60],
        [60*60*24*2,                'a day',            null],
        [60*60*24*30.42,            '% days',           60*60*24],
        [60*60*24*30.42*2,          'a month',          null],
        [60*60*24*30.42*12,         '% months',         60*60*24*30.42],
        [60*60*24*30.42*12*2,       'a year',           null],
        [8640000000000000/*max*/,   '% years',          60*60*24*30.42*12],
    ];

    let text = null;
    for (let kv of descriptions) {
        const multiplier = kv[0];
        const template = kv[1];
        const divider = kv[2];
        if (difference < multiplier) {
            text = template.replace(/%/, Math.round(difference / divider));
            break;
        }
    }

    if (text === 'a day') {
        return future ? 'tomorrow' : 'yesterday';
    }
    return future ? 'in ' + text : text + ' ago';
}

module.exports = {formatRelativeTime: formatRelativeTime};