; SPDX-License-Identifier: MIT


Import "DateMath.rgr"
Import "IsoDateParse.rgr"

; ISO week numbers, day-of-week, and calendar offsets (no timezone).
class IsoCalendar {

  sfn intDiv:int (numerator:int denominator:int) {
    if (denominator <= 0) {
      return 0
    }
    def num:double (int2double numerator)
    def den:double (int2double denominator)
    return (floor (num / den))
  }

  sfn sakamotoMonthShift:int (month:int) {
    if (month == 1) {
      return 0
    }
    if (month == 2) {
      return 3
    }
    if (month == 3) {
      return 2
    }
    if (month == 4) {
      return 5
    }
    if (month == 5) {
      return 0
    }
    if (month == 6) {
      return 3
    }
    if (month == 7) {
      return 5
    }
    if (month == 8) {
      return 1
    }
    if (month == 9) {
      return 4
    }
    if (month == 10) {
      return 6
    }
    if (month == 11) {
      return 2
    }
    return 4
  }

  sfn sakamotoDow:int (year:int month:int day:int) {
    def y:int year
    if (month < 3) {
      y = year - 1
    }
    def shift:int (IsoCalendar.sakamotoMonthShift(month))
    def y4:int (IsoCalendar.intDiv(y 4))
    def y100:int (IsoCalendar.intDiv(y 100))
    def y400:int (IsoCalendar.intDiv(y 400))
    def q:int (y + y4 - y100 + y400 + shift + day)
    def r:int (q % 7)
    if (r < 0) {
      r = r + 7
    }
    return r
  }

  sfn dayOfWeekIso:int (year:int month:int day:int) {
    def dow:int (IsoCalendar.sakamotoDow(year month day))
    if (dow == 0) {
      return 7
    }
    return dow
  }

  sfn addDaysOffset:string (iso:string days:int) {
    return (DateMath.addDays(iso days))
  }

  sfn daysBetween:int (fromIso:string toIso:string) {
    def count:int 0
    def current:string fromIso
    while ((DateMath.compareIso(current toIso)) < 0) {
      current = (DateMath.addDays(current 1))
      count = count + 1
      if (count > 4000) {
        return 0
      }
    }
    if (current == toIso) {
      return count
    }
    return 0
  }

  sfn pad2:string (n:int) {
    if (n < 10) {
      return ("0" + (DateMath.intToString(n)))
    }
    return (DateMath.intToString(n))
  }

  sfn isoWeekKey:string (iso:string) {
    def dateOnly:string (IsoDateParse.toIsoDateOnly(iso))
    if ((strlen dateOnly) == 0) {
      return ""
    }
    def week:int (IsoCalendar.getIsoWeekNumber(dateOnly))
    if (week <= 0) {
      return ""
    }
    def p:DateParts (DateParts.fromIso(dateOnly))
    def isoDow:int (IsoCalendar.dayOfWeekIso(p.year p.month p.day))
    def toThursday:int (4 - isoDow)
    def thursdayIso:string (IsoCalendar.addDaysOffset(dateOnly toThursday))
    def tp:DateParts (DateParts.fromIso(thursdayIso))
    def isoYear:string (DateMath.intToString(tp.year))
    return (isoYear + "-W" + (IsoCalendar.pad2(week)))
  }

  sfn getIsoWeekNumber:int (iso:string) {
    def p:DateParts (DateParts.fromIso(iso))
    def isoDow:int (IsoCalendar.dayOfWeekIso(p.year p.month p.day))
    def toThursday:int (4 - isoDow)
    def thursdayIso:string (IsoCalendar.addDaysOffset(iso toThursday))
    def tp:DateParts (DateParts.fromIso(thursdayIso))
    def jan4:string ((DateMath.intToString(tp.year)) + "-01-04")
    def jan4Dow:int (IsoCalendar.dayOfWeekIso(tp.year 1 4))
    def week1Monday:string (IsoCalendar.addDaysOffset(jan4 (1 - jan4Dow)))
    def days:int (IsoCalendar.daysBetween(week1Monday thursdayIso))
    def weeks:int (IsoCalendar.intDiv(days 7))
    return (weeks + 1)
  }
}