import totalDaysInMonth from './totalDaysInMonth';
import totalDaysInYear from './totalDaysInYear';
import convert from '../time/convert';

/**
 * calculate the difference between dates (range)
 */
function diff(start, end, unitName) {
    // sort the dates to make it easier to process (specially year/month)
    if (start > end) {
        const swap = start;
        start = end;
        end = swap;
    }

    let output;

    if (unitName === 'month') {
        output = getMonthsDiff(start, end);
    } else if (unitName === 'year') {
        output = getYearsDiff(start, end);
    } else if (unitName != null) {
        if (unitName === 'day') {
            // ignore timezone difference because of daylight savings time
            start = toUtc(start);
            end = toUtc(end);
        }
        output = convert(end - start, 'ms', unitName);
    } else {
        output = end - start;
    }

    return output;
}

function toUtc(d) {
    // we ignore timezone differences on purpose because of daylight
    // savings time, otherwise it would return fractional days/weeks even
    // if a full day elapsed. eg:
    // Wed Feb 12 2014 00:00:00 GMT-0200 (BRST)
    // Sun Feb 16 2014 00:00:00 GMT-0300 (BRT)
    // diff should be 4 days and not 4.041666666666667
    return Date.UTC(
        d.getUTCFullYear(),
        d.getUTCMonth(),
        d.getUTCDate(),
        d.getHours(),
        d.getMinutes(),
        d.getSeconds(),
        d.getMilliseconds()
    );
}

function getMonthsDiff(start, end) {
    return (
        getElapsedMonths(start, end) +
        getElapsedYears(start, end) * 12 +
        getFractionalMonth(start, end)
    );
}

function getYearsDiff(start, end) {
    const elapsedYears = getElapsedYears(start, end);
    return elapsedYears + getFractionalYear(start, end, elapsedYears);
}

function getElapsedMonths(start, end) {
    let monthDiff = end.getMonth() - start.getMonth();
    if (monthDiff < 0) {
        monthDiff += 12;
    }
    // less than a full month
    if (start.getDate() > end.getDate()) {
        monthDiff -= 1;
    }
    return monthDiff;
}

function getElapsedYears(start, end) {
    let yearDiff = end.getFullYear() - start.getFullYear();
    // less than a full year
    if (start.getMonth() > end.getMonth()) {
        yearDiff -= 1;
    }
    return yearDiff;
}

function getFractionalMonth(start, end) {
    let fractionalDiff = 0;
    const startDay = start.getDate();
    const endDay = end.getDate();

    if (startDay !== endDay) {
        const startTotalDays = totalDaysInMonth(start);
        const endTotalDays = totalDaysInMonth(end);
        let totalDays;
        let daysElapsed;

        if (startDay > endDay) {
            // eg: Jan 29 - Feb 27 (29 days elapsed but not a full month)
            const baseDay = startTotalDays - startDay;
            daysElapsed = endDay + baseDay;
            // total days should be relative to 1st day of next month if
            // startDay > endTotalDays
            totalDays = startDay > endTotalDays ? endTotalDays + baseDay + 1 : startDay + baseDay;
        } else {
            // fractional is only based on endMonth eg: Jan 12 - Feb 18
            // (6 fractional days, 28 days until next full month)
            daysElapsed = endDay - startDay;
            totalDays = endTotalDays;
        }

        fractionalDiff = daysElapsed / totalDays;
    }

    return fractionalDiff;
}

function getFractionalYear(start, end, elapsedYears) {
    const base = elapsedYears
        ? new Date(end.getFullYear(), start.getMonth(), start.getDate())
        : start;
    const elapsedDays = diff(base, end, 'day');
    return elapsedDays / totalDaysInYear(end);
}

export default diff;