[Project Euler] Problem 19

Problem 19

難しくは無いはずなんだが、1900を1990と入力していたtypoの為でなかなか正解に辿り着けなかった。

object P019 {
  def isLeap(y:Int):Boolean = y match {
    case _ if y%400==0 => true
    case _ if y%100==0 => false
    case _ if y%4==0   => true
    case _             => false
  }
  def daysOfMonth(y:Int, m:Int):Int = m match {
    case 9 => 30
    case 4 => 30
    case 6 => 30
    case 11 => 30
    case 2 => if (isLeap(y)) 29 else 28
    case _ => 31
  }
  def daysInYear(y:Int):Int = if (isLeap(y)) 366 else 365
  def daysFrom1Jan1900(year:Int, month:Int, day:Int):Int =
    List.range(1900,year).map{y => daysInYear(y)}.foldLeft(0)(_+_) +
    List.range(1,month).map{m => daysOfMonth(year,m)}.foldLeft(0)(_+_) +
    (day-1)
  val day0Jan1900:Int = daysFrom1Jan1900(1900,1,0) // Sunday
  def dayOfTheWeek(year:Int, month:Int, day:Int):Int = (daysFrom1Jan1900(year,month,day)-day0Jan1900)%7
  def main(args:Array[String]) {
    println(dayOfTheWeek(1901,1,1))
    println(dayOfTheWeek(2000,12,1))
    println(
      (for(y <- List.range(1901,2001); m <- List.range(1,13))
       yield dayOfTheWeek(y,m,1)).filter{_==0}.size)
  }
}