getStartOfWeek

gistfile1.coffee

  DAY = 86400000
  getStartOfWeek = (t) ->
    UTC3_OFFSET = 60*60*3
    kDays4Years = [0, 365, 2 * 365, 3 * 365 + 1]
    kDaysIn4Years = 4 * 365 + 1
    kDaysIn100Years = 25 * kDaysIn4Years - 1
    kDaysIn400Years = 4 * kDaysIn100Years + 1
    kDays1970to2000 = 30 * 365 + 7
    kDaysOffset = 1000 * kDaysIn400Years + 5 * kDaysIn400Years - kDays1970to2000
    kYearsOffset = 400000
    kDaysInMonths = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
    monthCodes = [5,1,1,4,6,2,4,0,3,5,1,3]

    t = Date.now() unless t
    saveDays = days = Math.floor(t / DAY)

    year = 0
    day = 0
    month = 0

    days += kDaysOffset
    year = 400 * Math.floor(days / kDaysIn400Years) - kYearsOffset
    days %= kDaysIn400Years

    days--
    yd1 = Math.floor(days / kDaysIn100Years)
    days %= kDaysIn100Years
    year += 100 * yd1

    days++
    yd2 = Math.floor(days / kDaysIn4Years)
    days %= kDaysIn4Years
    year += 4 * yd2

    days--
    yd3 = Math.floor(days / 365)
    days %= 365
    year += yd3

    is_leap = (!yd1 or yd2) and !yd3

    if days >= 31 + 28 + is_leap
      days -= 31 + 28 + is_leap
      for i in [2...12]
        if days < kDaysInMonths[i]
          month = i
          day = days + 1
          break
        days -= kDaysInMonths[i]
    else
      if days < 31
        month = 0
        day = days + 1
      else
        month = 1
        day = days - 31 + 1

    m = month + 1
    d = day + 1
    y = year % 100
    c = Math.floor(year/100)
    if (year%4 == 0) && (year%100 != 0 || year%400 == 0) && (m==1 || m==2)
      isLeapJanFeb = 1
    else
      isLeapJanFeb = 0
    w = Math.floor(1*d + 1*monthCodes[m-1] + 1*y + 1*Math.floor(y/4) - 2*(c%4) - 1*isLeapJanFeb) % 7
    return (saveDays - w) * DAY