Collatz問題

Collatz.f95

module m_collatz
  implicit none
contains
  recursive function icollatz(n) result(result)
    integer, intent(in) :: n
    integer, allocatable :: result(:)
    if (n == 1) then
      result = [1]
    else if (mod(n, 2) == 0) then ! even
      result = [n, icollatz(n / 2)]
    else                          ! odd
      result = [n, icollatz(3 * n + 1)]
    end if
    return
  end function icollatz
end module m_collatz
!===========================
program main
  use m_collatz
  implicit none
  integer :: i, target, maxStep
  integer, allocatable :: m(:)
  integer t1, t2, t_rate, t_max, diff
  call system_clock(t1)   ! 開始時を記録
  target = 1
  maxStep = 1
!$omp parallel shared(maxStep), private(m)
!$omp do
  do i = 1, 10**5
    m = icollatz(i)
    if (maxStep < size(m)) then
      maxStep = max(maxStep, size(m))
      target = i
    end if
  end do
!$omp end do
!$omp end parallel
  print "(2i10, ' step')", target, maxStep
  call system_clock(t2, t_rate, t_max)   ! 終了時を記録
  if ( t2 < t1 ) then
    diff = t_max - t1 + t2
  else
    diff = t2 - t1
  endif
  print "(a, f10.3)", "time it took was:", diff/dble(t_rate)
  stop
end program main

 77031       351 step

time it took was: 3.073