commit b3ea9384b0dc206a41beeb11c04a585c7835b0e8 · treybastian.com/advent-of-code

+97
2024/day_06_part_2.f90
···

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       program day_06_part_2

     

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          implicit none

     

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          character(len=130) :: line, lines(130), temp_lines(130)

     

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          integer:: io, x,y, xd,yd, i, j, ct, o_ct, init_x, init_y, init_xd, init_yd

     

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          open(newunit=io, file='./day_06_input.txt', status='old', action='read')

     

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          read(io, '(a)') lines

     

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          ct = 0

     

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          ! find starting position

     

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          do i = 1, size(lines)

     

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             if (index(lines(i), '^') > 0) then

     

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                y = i

     

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                x = index(lines(i), '^')

     

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                lines(i)(x:x) = '.' ! replace with a dot to not confuse this part

     

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                xd = 0

     

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                yd = -1

     

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                init_x = x

     

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                init_y = y

     

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                init_xd = xd

     

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                init_yd = yd

     

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                exit

     

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             end if

     

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          end do

     

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          do i = 1, size(lines)

     

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             do j = 1, len(lines(i))

     

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                o_ct = 0

     

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                ! reset positions

     

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                x = init_x

     

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                y = init_y

     

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                xd = init_xd

     

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                yd = init_yd

     

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                temp_lines = lines ! fresh board every run

     

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                if(temp_lines(i)(j:j) == '.') then

     

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                   temp_lines(i)(j:j) = 'O'  ! place obstacle to test

     

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                   ! move until we leave the bounds when we hit a valid spot mark it with an X and increase count

     

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                   do

     

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                      ! are we going out of bounds?

     

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                      if (x+xd < 1 .or. x+xd > len(temp_lines(y)) .or. y+yd < 1 .or. y+yd > size(temp_lines)) then

     

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                         exit

     

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                      end if

     

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                      ! try to move

     

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                      select case(temp_lines(y+yd)(x+xd:x+xd))

     

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                       case('.')

     

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                         ! valid move

     

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                         x = x + xd

     

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                         y = y + yd

     

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                       case('O')

     

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                         ! we are blocked lets rotate 90 degrees and count obstacle

     

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                         o_ct = o_ct + 1

     

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                         if(xd == 0 .and. yd == -1) then

     

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                            xd = 1

     

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                            yd = 0

     

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                         else if (xd == 1 .and. yd == 0) then

     

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                            xd = 0

     

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                            yd = 1

     

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                         else if (xd == 0 .and. yd == 1) then

     

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                            xd = -1

     

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                            yd = 0

     

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                         else if (xd == -1 .and. yd == 0) then

     

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                            xd = 0

     

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                            yd = -1

     

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                         end if

     

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                       case('#')

     

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                         ! we are blocked lets rotate 90 degrees

     

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                         ! change this to an O to try and detect when stuck

     

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                         temp_lines(y+yd)(x+xd:x+xd) = 'O'

     

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                         if(xd == 0 .and. yd == -1) then

     

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                            xd = 1

     

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                            yd = 0

     

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                         else if (xd == 1 .and. yd == 0) then

     

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                            xd = 0

     

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                            yd = 1

     

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                         else if (xd == 0 .and. yd == 1) then

     

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                            xd = -1

     

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                            yd = 0

     

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                         else if (xd == -1 .and. yd == 0) then

     

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                            xd = 0

     

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                            yd = -1

     

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                         end if

     

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                       case default

     

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                         exit

     

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                      end select

     

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                      if(o_ct > 100) then

     

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                         exit ! probably stuck

     

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                      end if

     

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                   end do

     

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                end if

     

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                if(o_ct > 100) then ! we got stuck probably 100 is so arbitrary

     

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                   ct = ct + 1

     

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                end if

     

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             end do

     

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          end do

     

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          print*, "block points: ", ct

     

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       end program day_06_part_2