1 files changed, 78 insertions, 0 deletions
diff --git a/2024/day_04.f90 b/2024/day_04.f90
new file mode 100644
index 0000000..3be12da
--- /dev/null
+++ b/2024/day_04.f90
@@ -0,0 +1,78 @@
+module day_04_utils
+   implicit none
+contains
+   recursive function find_xmas(arr, row, col,row_dir, col_dir, search_letter) result(found)
+      implicit none
+      character(len=140), intent(in), allocatable :: arr(:)
+      character(len=1), intent(in) :: search_letter
+      integer, intent(in) :: row, col, row_dir, col_dir
+      integer :: new_row, new_col
+      logical :: found
+
+      if(arr(row)(col:col) == search_letter) then
+         if(search_letter == 'S') then
+            found = .true.
+         else
+            new_row = row + row_dir
+            new_col = col + col_dir
+            if(new_row > len(arr(row)) .or. new_row < 1 .or. new_col > size(arr) .or. new_col < 1) then
+               found = .false.
+            else
+               select case(search_letter)
+                case('M')
+                  found = find_xmas(arr, new_row, new_col, row_dir, col_dir, 'A')
+                case('A')
+                  found = find_xmas(arr, new_row, new_col, row_dir, col_dir, 'S')
+               end select
+            end if
+         end if
+      else
+         found = .false.
+      end if
+   end function find_xmas
+end module day_04_utils
+program day_04
+   use day_04_utils
+   implicit none
+   character(len=140) , allocatable :: lines(:)
+   integer :: io, row, col, score, z
+   logical :: found
+
+   score = 0
+   open(newunit=io, file='./day_04_input.txt', status='old', action='read')
+   allocate(lines(140))
+   read(io, '(a)') lines
+   do row = 1, size(lines)
+      do col = 1, len(lines(row))
+         if (lines(row)(col:col) == 'X') then
+            do z = 1, 8
+               found = .false.
+               select case (z)
+                case(1)
+                  found = find_xmas(lines, row, col-1, 0, -1, 'M')
+                case(2)
+                  found = find_xmas(lines, row-1, col-1, -1, -1, 'M')
+                case(3)
+                  found = find_xmas(lines, row-1, col, -1, 0, 'M')
+                case(4)
+                  found = find_xmas(lines, row-1, col+1,  -1, 1, 'M')
+                case(5)
+                  found = find_xmas(lines, row, col+1, 0, 1, 'M')
+                case(6)
+                  found = find_xmas(lines, row+1, col+1, 1, 1, 'M')
+                case(7)
+                  found = find_xmas(lines, row+1, col, 1, 0, 'M')
+                case(8)
+                  found = find_xmas(lines, row+1, col-1, 1, -1, 'M')
+               end select
+               if(found) then
+                  score = score + 1
+               end if
+            end do
+         end if
+      end do
+   end do
+   deallocate(lines)
+   print*, score
+
+end program day_04