3 files changed, 62 insertions, 0 deletions
diff --git a/2024/18/.gitignore b/2024/18/.gitignore
new file mode 100644
index 0000000..5fcaefb
--- /dev/null
+++ b/2024/18/.gitignore
@@ -0,0 +1 @@
+puzzle-[12].lisp
+\ No newline at end of file
diff --git a/2024/18/Makefile b/2024/18/Makefile
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--- /dev/null
+++ b/2024/18/Makefile
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+include ../../Makefiles/lisp.mk
+\ No newline at end of file
diff --git a/2024/18/puzzles.lisp b/2024/18/puzzles.lisp
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--- /dev/null
+++ b/2024/18/puzzles.lisp
@@ -0,0 +1,60 @@
+#!/usr/bin/sbcl --script
+
+(defconstant +end-pos+ #C(70 70))
+
+(load "../heap.lisp")
+
+(defun main (filename)
+  ;; START PART 1
+  (dijkstra (parse filename 1024))
+  ;; END PART 1 START PART 2
+  (loop for i upfrom 0
+        while (dijkstra (parse filename i))
+        finally (return (nth-line i filename)))
+  ;; END PART 2
+  )
+
+(defun dijkstra (corrupted)
+  (let ((to-visit (heap:make-heap :priority-function #'car))
+        (seen (make-hash-table)))
+    (flet ((enqueue (dist pos)
+             (setf (gethash pos seen) t)
+             (heap:enqueue (cons dist pos) to-visit)))
+      (enqueue 0 0)
+      (loop until (heap:emptyp to-visit) do
+        (destructuring-bind (dist . pos) (heap:dequeue to-visit)
+          (when (= pos +end-pos+)
+            (return-from dijkstra dist))
+          (dolist (new-pos (list (+ pos #C(0 +1)) (+ pos #C(0 -1))
+                                 (+ pos #C(+1 0)) (+ pos #C(-1 0))))
+            (when (valid-step-p new-pos corrupted seen)
+              (enqueue (1+ dist) new-pos))))))))
+
+(defun valid-step-p (pos corrupted seen)
+  (and (<= 0 (realpart pos) (realpart +end-pos+))
+       (<= 0 (imagpart pos) (imagpart +end-pos+))
+       (null (gethash pos seen))
+       (null (gethash pos corrupted))))
+
+(defun parse (filename limit)
+  (let ((corrupted (make-hash-table)))
+    (with-open-file (stream filename)
+      (loop repeat limit
+            for line = (read-line stream nil)
+            for pos = (parse-pos line)
+            do (setf (gethash pos corrupted) t)))
+    corrupted))
+
+(defun parse-pos (string)
+  (let* ((comma (position #\, string))
+         (x (parse-integer (subseq string 0 comma)))
+         (y (parse-integer (subseq string (1+ comma)))))
+    (complex x y)))
+
+(defun nth-line (n filename)
+  (with-open-file (stream filename)
+    (dotimes (i (1- n))
+      (read-line stream))
+    (read-line stream)))
+
+(format t "~a~%" (main "input"))
+\ No newline at end of file