/*
Copyright (c) 2014, Yahoo! Inc. All rights reserved.
Copyrights licensed under the New BSD License.
See the accompanying LICENSE file for terms.
*/

/* jslint esnext: true */

var round = Math.round;

function daysToYears(days) {
    // 400 years have 146097 days (taking into account leap year rules)
    return days * 400 / 146097;
}

// Thanks to date-fns
// https://github.com/date-fns/date-fns
// MIT © Sasha Koss

var MILLISECONDS_IN_MINUTE = 60000;
var MILLISECONDS_IN_DAY = 86400000;

function startOfDay (dirtyDate) {
    var date = new Date(dirtyDate);
    date.setHours(0, 0, 0, 0);
    return date;
}

function differenceInCalendarDays (dirtyDateLeft, dirtyDateRight) {
    var startOfDayLeft = startOfDay(dirtyDateLeft);
    var startOfDayRight = startOfDay(dirtyDateRight);

    var timestampLeft = startOfDayLeft.getTime() -
        startOfDayLeft.getTimezoneOffset() * MILLISECONDS_IN_MINUTE;
    var timestampRight = startOfDayRight.getTime() -
        startOfDayRight.getTimezoneOffset() * MILLISECONDS_IN_MINUTE;

    // Round the number of days to the nearest integer
    // because the number of milliseconds in a day is not constant
    // (e.g. it's different in the day of the daylight saving time clock shift)
    return Math.round((timestampLeft - timestampRight) / MILLISECONDS_IN_DAY);
}

export default function (from, to) {
    // Convert to ms timestamps.
    from = +from;
    to   = +to;

    var millisecond = round(to - from),
        second      = round(millisecond / 1000),
        minute      = round(second / 60),
        hour        = round(minute / 60);

        // We expect a more precision in rounding when dealing with
        // days as it feels wrong when something happended 13 hours ago and
        // is regarded as "yesterday" even if the time was this morning.

    var day = differenceInCalendarDays(to, from);
    var week = round(day / 7);

    var rawYears = daysToYears(day),
        month    = round(rawYears * 12),
        year     = round(rawYears);

    return {
        millisecond    : millisecond,
        second         : second,
        'second-short' : second,
        minute         : minute,
        'minute-short' : minute,
        hour           : hour,
        'hour-short'   : hour,
        day            : day,
        'day-short'    : day,
        week           : week,
        'week-short'   : week,
        month          : month,
        'month-short'  : month,
        year           : year,
        'year-short'   : year
    };
}