From 4ced0399644ff2b75b53ce26d6afb0688e78b264 Mon Sep 17 00:00:00 2001
From: Oscar Najera <hi@oscarnajera.com>
Date: Thu, 14 Dec 2023 09:11:34 +0100
Subject: solve part2

---
 AoC2023/day10/solver.lisp | 86 ++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 67 insertions(+), 19 deletions(-)

diff --git a/AoC2023/day10/solver.lisp b/AoC2023/day10/solver.lisp
index fcc1eed..d93617a 100644
--- a/AoC2023/day10/solver.lisp
+++ b/AoC2023/day10/solver.lisp
@@ -3,10 +3,10 @@
 (ql:quickload '(fiveam))
 
 (defparameter directions
-  '((up 0 -1)
-    (dw 0  1)
-    (lf -1 0)
-    (rt  1 0)))
+  '((up -1 0)
+    (dw  1 0)
+    (lf 0 -1)
+    (rt 0  1)))
 
 (defparameter pipes
   '((#\| up dw)
@@ -32,7 +32,7 @@ LJ.LJ")
    (cdr (assoc point pipes))))
 
 (fiveam:test parts
-  (fiveam:is (equal '((-1 0) (1 0))
+  (fiveam:is (equal '((0 -1) (0 1))
                     (get-neighbors #\-))))
 
 
@@ -52,7 +52,7 @@ LJ.LJ")
                         (< -1 x xmax))
                (list y x)))))
     (destructuring-bind (y x) point
-      (loop for (dx dy) in (get-neighbors (aref (aref map y) x))
+      (loop for (dy dx) in (get-neighbors (aref (aref map y) x))
             when (bounds (+ y dy) (+ x dx)) collect it))))
 
 (defun start-neighbors (map start)
@@ -67,21 +67,69 @@ LJ.LJ")
         (cadr next)
         (car next))))
 
-(defun solver1 (rows)
-  (let* ((map (make-array (length rows) :initial-contents rows))
-         (start (find-start map))
+(defun get-loop (map)
+  (let* ((start (find-start map))
          (options (start-neighbors map start))
          (last start)
-         (cur (car options))
-         )
-    (loop
-      for next = (advance-next map cur last)
-      for steps from 1
-      until (equal next start)
-      do (setf last cur
-               cur next)
-      finally (return (floor (+ steps 1) 2)))))
+         (cur (car options)))
+    (cons cur
+          (loop
+            for next = (advance-next map cur last)
+            do (setf last cur
+                     cur next)
+            collect next
+            until (equal next start)))))
+;; (get-loop
+;;  (uiop:split-string eg-input :separator '(#\Newline)))
+
+(defun solver1 (rows)
+  (let ((map (make-array (length rows) :initial-contents rows)))
+    (floor (length (get-loop map)) 2)))
+
+;; To know if a point is inside a loop, I need to count how often a ray crosses
+;; until it reaches the boundary. I can just count to the right.
+;; In this case only the vertical edges count, and element that change direction.
+;; Meaning LJ is a u, same as F7 an n, that means the ray passes tangent across
+;; those 2 cells taking the point a bit lower, would mean only counting pipes
+;; with down component thus F,|,7. The rest L,-,J are irrelevant.
+
+(defun in-loop-p (point loop)
+  (member point loop :test #'equal))
+
+(defun rassoc-get (item list)
+  (car (rassoc item list :test #'equal)))
+
+(defun correct-start (map)
+  (let ((start (find-start map)))
+    (setf (aref (aref map (car start)) (cadr start))
+          (rassoc-get
+           (loop for (y x) in (start-neighbors map start)
+                 collect
+                 (rassoc-get (list (- y (first start))
+                                   (- x (second start)))
+                             directions))
+           pipes))))
+
+(defun solver2 (rows)
+  (let* ((map (make-array (length rows) :initial-contents rows))
+         (loop-tiles (get-loop map))
+         insidep)
+    (correct-start map)
+    (loop for row across map
+          for y from 0 do
+            (setf insidep nil)
+          sum
+          (loop for col across row
+                for x from 0
+                for point = (list y x)
+                when (and (in-loop-p point loop-tiles)
+                          (member col '(#\F #\| #\7)))
+                  do (setf insidep (not insidep))
+                count (and (not (in-loop-p point loop-tiles))
+                           insidep)))))
 
 (fiveam:test solution
   (fiveam:is (= 8 (solver1 (uiop:split-string eg-input :separator '(#\Newline)))))
-  (fiveam:is (= 6812 (solver1 (uiop:read-file-lines "input")))))
+  (fiveam:is (= 6812 (solver1 (uiop:read-file-lines "input"))))
+  (fiveam:is (= 1 (solver2 (uiop:split-string eg-input :separator '(#\Newline)))))
+  (fiveam:is (= 527 (solver2 (uiop:read-file-lines "input")))))
-- 
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