1 files changed, 21 insertions(+), 20 deletions(-)

M day09.lisp

@@ 20,22 20,20 @@
 (defparameter +width+ (array-dimension +input+ 1))
 (defparameter +height+ (array-dimension +input+ 0))
 
-(defun adjacents (x y)
-  (flet ((access (x y)
-           (if (and (>= x 0)
-                    (>= y 0)
-                    (< x +width+)
-                    (< y +height+))
-               (aref +input+ y x)
-               9999)))
-    (list (access x (1- y))
-          (access x (1+ y))
-          (access (1- x) y)
-          (access (1+ x) y))))
+(defun height (x y)
+  (if (and (>= x 0)
+           (>= y 0)
+           (< x +width+)
+           (< y +height+))
+      (aref +input+ y x)
+      9))
 
 (defun low-point? (x y)
-  (loop :with point := (aref +input+ y x)
-        :for adjacent :in (adjacents x y)
+  (loop :with point := (height x y)
+        :for adjacent :in (list (height (1- x) y)
+                                (height (1+ x) y)
+                                (height x (1- y))
+                                (height x (1+ y)))
         :always (< point adjacent)))
 
 (defun solve-part-1 ()


@@ 45,12 43,12 @@
                      :sum (1+ (aref +input+ y x)))))
 
 (defun fill-basin (x y touch-array)
-  (when (or (< x 0) (< y 0)
-            (>= x +width+) (>= y +height+)
-            (= 1 (aref touch-array y x))
-            (= 9 (aref +input+ y x)))
+  "Fill the basin located at the given point, marking it as already visited on the touch-array and returning the baisn's size."
+  (when (or (= 9 (height x y))
+            (= 1 (aref touch-array y x)))
     (return-from fill-basin 0))
 
+  ;; Good ol' Fill Algorithm
   (setf (aref touch-array y x) 1)
   (+ 1
      (fill-basin (1- x) y touch-array)


@@ 59,13 57,16 @@
      (fill-basin x (1+ y) touch-array)))
 
 (defun basin-sizes ()
+  "Return an unordered list of all basin sizes"
   (loop :with touch-array := (make-array (list +height+ +width+)
                                          :element-type 'bit)
         :for y :below +height+
         :append (loop :for x :below +width+
                       :for score := (fill-basin x y touch-array)
                       :when (> score 0)
-                      :collect score)))
+                        :collect score)))
 
 (defun solve-part-2 ()
-  (reduce #'* (subseq (sort (basin-sizes) #'>) 0 3)))
+  "Multiply the largest 3 basin sizes."
+  (reduce #'* (subseq (sort (basin-sizes) #'>)
+                      0 3)))