rlm@0:       program collatz
rlm@0:       implicit none
rlm@0:       character (len = 100) :: file_name  
rlm@0:       integer, parameter :: big = selected_int_kind(11)
rlm@0:       integer  :: n
rlm@0:       call getarg( 1, file_name )  
rlm@0:       OPEN(unit = 7, file = file_name)
rlm@0:       read (7,*) n
rlm@0:       write (unit = *, fmt = "(I0)") search_collatz(n)
rlm@0: 
rlm@0:       contains
rlm@0: 
rlm@0:       function collatz_next (n) result (r)
rlm@0:       implicit none
rlm@0:       integer (kind = big)  :: r,n
rlm@0:       if (0 == mod(n,2)) then
rlm@0:          r = n/2
rlm@0:       else
rlm@0:          r = 1 + 3*n
rlm@0:       end if 
rlm@0:       end function collatz_next
rlm@0:       
rlm@0:       function collatz_length (n) result (r)
rlm@0:       implicit none
rlm@0:       integer (kind = big)  :: t
rlm@0:       integer :: n,r
rlm@0:       r = 1
rlm@0:       t = n
rlm@0:       do 
rlm@0:          if (1 == t) then
rlm@0:          exit
rlm@0:          end if
rlm@0:          r = r + 1
rlm@0:          t = collatz_next(t)
rlm@0:       end do
rlm@0:       end function collatz_length
rlm@0:       
rlm@0:       function search_collatz (n) result (r)
rlm@0:       implicit none
rlm@0:       integer n,r,i,max,max_val,temp_val
rlm@0:       integer (kind = big) :: t
rlm@0:       t = n
rlm@0:       do i = 1,n-1
rlm@0:          temp_val = collatz_length(i)
rlm@0:          if (temp_val > max_val) then
rlm@0:          max = i
rlm@0:          max_val = temp_val
rlm@0:       end if
rlm@0:       end do
rlm@0:       r = max
rlm@0:       end function search_collatz
rlm@0:       end program collatz