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program day_06_part_2
implicit none
character(len=130) :: line, lines(130), temp_lines(130)
integer:: io, x,y, xd,yd, i, j, ct, o_ct, init_x, init_y, init_xd, init_yd
open(newunit=io, file='./day_06_input.txt', status='old', action='read')
read(io, '(a)') lines
ct = 0
! find starting position
do i = 1, size(lines)
if (index(lines(i), '^') > 0) then
y = i
x = index(lines(i), '^')
lines(i)(x:x) = '.' ! replace with a dot to not confuse this part
xd = 0
yd = -1
init_x = x
init_y = y
init_xd = xd
init_yd = yd
exit
end if
end do
do i = 1, size(lines)
do j = 1, len(lines(i))
o_ct = 0
! reset positions
x = init_x
y = init_y
xd = init_xd
yd = init_yd
temp_lines = lines ! fresh board every run
if(temp_lines(i)(j:j) == '.') then
temp_lines(i)(j:j) = 'O' ! place obstacle to test
! move until we leave the bounds when we hit a valid spot mark it with an X and increase count
do
! are we going out of bounds?
if (x+xd < 1 .or. x+xd > len(temp_lines(y)) .or. y+yd < 1 .or. y+yd > size(temp_lines)) then
exit
end if
! try to move
select case(temp_lines(y+yd)(x+xd:x+xd))
case('.')
! valid move
x = x + xd
y = y + yd
case('O')
! we are blocked lets rotate 90 degrees and count obstacle
o_ct = o_ct + 1
if(xd == 0 .and. yd == -1) then
xd = 1
yd = 0
else if (xd == 1 .and. yd == 0) then
xd = 0
yd = 1
else if (xd == 0 .and. yd == 1) then
xd = -1
yd = 0
else if (xd == -1 .and. yd == 0) then
xd = 0
yd = -1
end if
case('#')
! we are blocked lets rotate 90 degrees
! change this to an O to try and detect when stuck
temp_lines(y+yd)(x+xd:x+xd) = 'O'
if(xd == 0 .and. yd == -1) then
xd = 1
yd = 0
else if (xd == 1 .and. yd == 0) then
xd = 0
yd = 1
else if (xd == 0 .and. yd == 1) then
xd = -1
yd = 0
else if (xd == -1 .and. yd == 0) then
xd = 0
yd = -1
end if
case default
exit
end select
if(o_ct > 100) then
exit ! probably stuck
end if
end do
end if
if(o_ct > 100) then ! we got stuck probably 100 is so arbitrary
ct = ct + 1
end if
end do
end do
print*, "block points: ", ct
end program day_06_part_2
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