Day 16 AoC 2023

program day16
    implicit none
    integer, parameter :: dim = 110
    character, dimension(dim,dim) :: contraption
    integer, dimension(dim,dim) :: visited
    integer :: i, j, ecode, solution, x, y
    integer, parameter :: in = 50

    open(in, file="day16.txt", action="read", iostat=ecode)
    if ( ecode .eq. 0 ) then
        do j = 1, dim
            read (in, '(110(a))', iostat=ecode) contraption(:,j)
            if ( ecode .ne. 0 ) exit
        end do
        close(in)

        call visit(visited, 0, 1, 0)
        solution = viscount(visited)
        write (*, '("Part 1: ", i0)') solution

        solution = 0
        do i = 1, dim
            call visit(visited, 0, i, 0)
            solution = max(solution, viscount(visited))
            call visit(visited, dim + 1, i, 3)
            solution = max(solution, viscount(visited))
        end do
        do i = 1, dim
            call visit(visited, i, dim + 1, 1)
            solution = max(solution, viscount(visited))
            call visit(visited, i, 0, 3)
            solution = max(solution, viscount(visited))
        end do
        write (*, '("Part 2: ", i0)') solution

    else
        print '(a)', "Error opening file"
    end if

contains

    integer function viscount(visited)
        integer, dimension(dim,dim), intent(in) :: visited
        integer :: x, y
        viscount = 0
        do x = 1, dim
            do y = 1, dim
                if ( visited(x,y) .gt. 0 ) viscount = viscount + 1
            end do
        end do
    end function viscount

    subroutine visit(visited, startx, starty, startd)
        integer, dimension(dim,dim), intent(inout) :: visited
        integer, intent(in) :: startx, starty, startd
        integer, dimension(0:100) :: visitorsx, visitorsy, direction
        integer :: next_visitor, currx, curry, currd
        visited = 0; next_visitor = 0
        visitorsx(next_visitor) = startx
        visitorsy(next_visitor) = starty
        direction(next_visitor) = startd
        next_visitor = next_visitor + 1

        do while ( next_visitor .gt. 0 )
            next_visitor = next_visitor - 1
            currx = visitorsx(next_visitor)
            curry = visitorsy(next_visitor)
            currd = direction(next_visitor)

            if ( 0 .eq. currd ) then ! RIGHT
                currx = currx + 1
            else if ( 1 .eq. currd ) then ! UP
                curry = curry - 1
            else if ( 2 .eq. currd ) then ! LEFT
                currx = currx - 1
            else if ( 3 .eq. currd ) then ! DOWN
                curry = curry + 1
            else ! INVALID DIRECTION
                exit
            end if

            if ( currx .eq. 0 .or. curry .eq. 0 .or. currx .eq. (dim + 1) .or. curry .eq. (dim + 1)) cycle

            if ( iand(visited(currx,curry), ishft(1, currd)) .gt. 0 ) cycle
            visited(currx, curry) = ior(visited(currx, curry), ishft(1, currd))

            if ( contraption(currx, curry) .eq. '.' ) then
                visitorsx(next_visitor) = currx
                visitorsy(next_visitor) = curry
                next_visitor = next_visitor + 1
            else if ( contraption(currx, curry) .eq. '|' ) then
                if ( currd .eq. 1 .or. currd .eq. 3 ) then ! UP or DOWN
                    visitorsx(next_visitor) = currx
                    visitorsy(next_visitor) = curry
                    next_visitor = next_visitor + 1
                else
                    ! push UP
                    visitorsx(next_visitor) = currx
                    visitorsy(next_visitor) = curry
                    direction(next_visitor) = 1
                    next_visitor = next_visitor + 1
                    ! push DOWN
                    visitorsx(next_visitor) = currx
                    visitorsy(next_visitor) = curry
                    direction(next_visitor) = 3
                    next_visitor = next_visitor + 1
                end if
            else if ( contraption(currx, curry) .eq. '-' ) then
                if ( currd .eq. 0 .or. currd .eq. 2 ) then ! LEFT or RIGHT
                    visitorsx(next_visitor) = currx
                    visitorsy(next_visitor) = curry
                    next_visitor = next_visitor + 1
                else
                    ! push RIGHT
                    visitorsx(next_visitor) = currx
                    visitorsy(next_visitor) = curry
                    direction(next_visitor) = 0
                    next_visitor = next_visitor + 1
                    ! push LEFT
                    visitorsx(next_visitor) = currx
                    visitorsy(next_visitor) = curry
                    direction(next_visitor) = 2
                    next_visitor = next_visitor + 1
                end if
            else if ( contraption(currx, curry) .eq. '/' ) then
                if ( 0 .eq. currd ) then ! RIGHT
                    visitorsx(next_visitor) = currx
                    visitorsy(next_visitor) = curry
                    direction(next_visitor) = 1
                    next_visitor = next_visitor + 1
                else if ( 1 .eq. currd ) then ! UP
                    visitorsx(next_visitor) = currx
                    visitorsy(next_visitor) = curry
                    direction(next_visitor) = 0
                    next_visitor = next_visitor + 1
                else if ( 2 .eq. currd ) then ! LEFT
                    visitorsx(next_visitor) = currx
                    visitorsy(next_visitor) = curry
                    direction(next_visitor) = 3
                    next_visitor = next_visitor + 1
                else if ( 3 .eq. currd ) then ! DOWN
                    visitorsx(next_visitor) = currx
                    visitorsy(next_visitor) = curry
                    direction(next_visitor) = 2
                    next_visitor = next_visitor + 1
                end if
            else if ( contraption(currx, curry) .eq. '\' ) then
                if ( 0 .eq. currd ) then ! RIGHT
                    visitorsx(next_visitor) = currx
                    visitorsy(next_visitor) = curry
                    direction(next_visitor) = 3
                    next_visitor = next_visitor + 1
                else if ( 1 .eq. currd ) then ! UP
                    visitorsx(next_visitor) = currx
                    visitorsy(next_visitor) = curry
                    direction(next_visitor) = 2
                    next_visitor = next_visitor + 1
                else if ( 2 .eq. currd ) then ! LEFT
                    visitorsx(next_visitor) = currx
                    visitorsy(next_visitor) = curry
                    direction(next_visitor) = 1
                    next_visitor = next_visitor + 1
                else if ( 3 .eq. currd ) then ! DOWN
                    visitorsx(next_visitor) = currx
                    visitorsy(next_visitor) = curry
                    direction(next_visitor) = 0
                    next_visitor = next_visitor + 1
                end if
            end if
        end do
    end subroutine visit
end program day16