module mscore_f
implicit none

private
integer, parameter :: A97 = 2 * iachar( '0', 1 ) + 1    ! 97

integer, parameter :: CHUNK = 25000
integer * 1        :: a( CHUNK )            ! scratch-pad #1
integer            :: cum( CHUNK )          ! scratch-pad #2

public :: f, c2f

contains

subroutine c2f( n, s, ret )

    ! skipped, the same code

end subroutine c2f

subroutine f( n, s, ret )
    integer,     intent( in )  :: n
    integer * 1, intent( in )  :: s( n )
    integer,     intent( out ) :: ret
    
    integer :: i, j, m, pre, L
    integer :: rprev                    ! "running previous"
    integer :: rmin                     ! "running minimum"
    
    ret = count( s == iachar( '1', 1 ))
    
    m = n / CHUNK
    if ( mod( n, CHUNK ) .ne. 0 ) then
        m = m + 1
    end if
    
    rprev = 0;
    rmin = 2;
    do j = 1, m
        pre = ( j - 1 ) * CHUNK
        L = min( CHUNK, ubound( s, 1 ) - pre - 1 )
        
        a( 1 : L ) = 2 * s( pre + 1 : pre + L ) - A97
        
        cum( 1 ) = rprev + a( 1 )
        do i = 2, L
            cum( i ) = cum( i - 1 ) + a( i )
        end do
        
        rmin = min( rmin, minval( cum( 1 : L )))
        rprev = cum( L )
    end do
    
    ret = ret - rmin
end subroutine f

end module mscore_f