/**
 * library to play around with date, year, month.
 * */
import {weekDayOfDate, countDaysOfMonth} from "./chronos.js";


/**
 * represents a monthly calendar-sheet, contains required data to generate a view of a Calendar-month.
 * */
export const Calendar = {
    /**
     * Define the first day of a week, which is shown in a Calendar View
     * Days of week are coded as follows:
     * 0 → Sunday, 1 → Monday, 2 → Tuesday, ... , 6 → Saturday
     * we set firstDayOfWeek by default to 1.
     * It is Monday, as convenient in Germany.
     * */
    _firstDayOfWeek: 1,
    get firstDayOfWeek() {
        return this._firstDayOfWeek;
    },
    set firstDayOfWeek(firstDayOfWeek) {
        this._firstDayOfWeek = firstDayOfWeek;
    },
    /**
     * return an object with following structure:
     *
     * ```{
     *     month: 1,
     *     year: 1970,
     *     weekdays: [1, 2, 3, 4, 5, 6, 0],
     *     weeks: [
     *         ["","","", 1, 2, 3, 4],
     *         [5,  6, 7, 8, 9,10,11],
     *         [12,13,14,15,16,17,18],
     *         [19,20,21,22,23,24,25],
     *         [26,27,28,29,30,31,""]
     *     ]
     * }```
     *
     * this structure is intended to be used to create a View of Calendar without
     * further calculation.
     *
     *  @param {number} month
     *  @param {number} year
     *
     * */
    buildCalendar : function(month, year) {
        let cal = {
            month: month,
            year: year,
            weekDays: [],
            weeks:[]
        };
        const DAYS_IN_WEEK = 7;
        for(let i = 0; i < DAYS_IN_WEEK; ++i) {
            let d = (i + Calendar._firstDayOfWeek) % DAYS_IN_WEEK;
            cal.weekDays.push(d);
        }
        // calculate how many days of last month get place in the first week
        // of this month
        const dayOfFirstDate = weekDayOfDate(1, month, year);
        const diff = (DAYS_IN_WEEK + (dayOfFirstDate - Calendar._firstDayOfWeek)) % DAYS_IN_WEEK;
        let countSpace = diff;
        let firstWeek = [];
        while(countSpace-- > 0) {
            firstWeek.push("");
        }
        // append days of first week
        let d = 1;
        for (; d + diff <= 7; d++) {
            firstWeek.push(d);
        }
        // append the first week to result
        cal.weeks.push(firstWeek);
        // append the further weeks (2. 3. 4. maybe 5.) to result
        let daysOfMonth = countDaysOfMonth(month, year);
        let week = [];
        for(;d <= daysOfMonth; ++d) {
            week.push(d);
            if ( (d + diff) % 7 === 0 ) {
                cal.weeks.push(week);
                week = [];
            }
        }
        if (week.length > 0) {
            cal.weeks.push(week);
        }
        return cal;
    }
}