export class Calendar {
  firstWeekDay: number;

  constructor(firstWeekDay = 0) {
    this.firstWeekDay = firstWeekDay; // 0 = Sunday
  }

  weekStartDate(date: Date): Date {
    const startDate = new Date(date.getTime());
    while (startDate.getDay() !== this.firstWeekDay) {
      startDate.setDate(startDate.getDate() - 1);
    }
    return startDate;
  }

  monthDates<T = Date, W = T[]>(
    year: number,
    month: number,
    dayFormatter?: (date: Date) => T,
    weekFormatter?: (week: T[]) => W
  ): W[] {
    if (typeof year !== "number" || year < 1970) {
      throw Error("year must be a number >= 1970");
    }
    if (typeof month !== "number" || month < 0 || month > 11) {
      throw Error("month must be a number (Jan is 0)");
    }
    const weeks: W[] = [];
    let week: T[] = [],
      i = 0,
      date = this.weekStartDate(new Date(year, month, 1));
    do {
      for (i = 0; i < 7; i++) {
        week.push(dayFormatter ? dayFormatter(date) : (date as unknown as T));
        date = new Date(date.getTime());
        date.setDate(date.getDate() + 1);
      }
      weeks.push(weekFormatter ? weekFormatter(week) : (week as unknown as W));
      week = [];
    } while (date.getMonth() <= month && date.getFullYear() === year);
    return weeks;
  }

  monthDays(year: number, month: number): (Date | number)[][] {
    const getDayOrZero = (date: Date): Date | number => {
      return date.getMonth() === month ? date : 0;
    };
    return this.monthDates(year, month, getDayOrZero);
  }

  monthText(year?: number, month?: number): string {
    if (typeof year === "undefined") {
      const now = new Date();
      year = now.getFullYear();
      month = now.getMonth();
    }
    const getDayOrBlank = (date: Date): string => {
      let s = date.getMonth() === month ? date.getDate().toString() : "  ";
      while (s.length < 2) {
        s = " " + s;
      }
      return s;
    };
    const weeks = this.monthDates(
      year,
      month,
      getDayOrBlank,
      function (week: string[]) {
        return week.join(" ");
      }
    );
    return weeks.join("\n");
  }
}

const months = "JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC".split(" ");
for (let i = 0; i < months.length; i++) {
  (Calendar as unknown as Record<string, number>)[months[i]] = i;
}