From 2564751363804f38200eab42fcb43fa77c690ec0 Mon Sep 17 00:00:00 2001
From: Thomas Voss <mail@thomasvoss.com>
Date: Fri, 13 Dec 2024 13:29:26 +0100
Subject: Add 2024 day 13 solutions

---
 2024/13/.gitignore   |  1 +
 2024/13/Makefile     |  1 +
 2024/13/puzzles.lisp | 60 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 62 insertions(+)
 create mode 100644 2024/13/.gitignore
 create mode 100644 2024/13/Makefile
 create mode 100644 2024/13/puzzles.lisp

diff --git a/2024/13/.gitignore b/2024/13/.gitignore
new file mode 100644
index 0000000..5fcaefb
--- /dev/null
+++ b/2024/13/.gitignore
@@ -0,0 +1 @@
+puzzle-[12].lisp
\ No newline at end of file
diff --git a/2024/13/Makefile b/2024/13/Makefile
new file mode 100644
index 0000000..5a21270
--- /dev/null
+++ b/2024/13/Makefile
@@ -0,0 +1 @@
+include ../../Makefiles/lisp.mk
\ No newline at end of file
diff --git a/2024/13/puzzles.lisp b/2024/13/puzzles.lisp
new file mode 100644
index 0000000..bec30f4
--- /dev/null
+++ b/2024/13/puzzles.lisp
@@ -0,0 +1,60 @@
+#!/usr/bin/sbcl --script
+
+(defun main (filename)
+  (loop for numbers in (parse-input filename)
+        sum (solve numbers)))
+
+(defun parse-input (filename)
+  (with-open-file (stream filename)
+    (let ((contents (make-string (file-length stream))))
+      (read-sequence contents stream)
+      (loop for paragraph in (split-string contents (format nil "~%~%"))
+            collect (extract-numbers-from-string paragraph)))))
+
+(defun extract-numbers-from-string (string)
+  (loop with number-count = 6
+        with numbers = (make-array number-count)
+        for i from 0 below number-count
+        for start = (position-if #'digit-char-p string)
+        for end = (position-if-not #'digit-char-p string :start start)
+        do (setf (aref numbers i) (parse-integer (subseq string start end))
+                 string (subseq string (or end 0)))
+        finally (return numbers)))
+
+(defun split-string (string delimiter)
+  (loop with delimiter-length = (length delimiter)
+        for end = (search delimiter string)
+        unless (eq end 0)
+          collect (subseq string 0 end) into parts
+        unless end
+          return parts
+        do (setq string (subseq string (+ end delimiter-length)))))
+
+(defun solve (numbers)
+  ;; ⎡ a0 b0 ⎢ c0 ⎤
+  ;; ⎣ a1 b1 ⎢ c1 ⎦
+  (let* ((a0 (aref numbers 0))
+         (a1 (aref numbers 1))
+         (b0 (aref numbers 2))
+         (b1 (aref numbers 3))
+         (c0 (aref numbers 4))
+         (c1 (aref numbers 5))
+         ;; START PART 2
+         (c0 (+ 10000000000000 c0))
+         (c1 (+ 10000000000000 c1))
+         ;; END PART 2
+         (D-bar  (determinant a0 b0 a1 b1))
+         (Dx-bar (determinant c0 b0 c1 b1))
+         (Dy-bar (determinant a0 c0 a1 c1))
+         (x (/ Dx-bar D-bar))
+         (y (/ Dy-bar D-bar)))
+    (if (and (integerp x)
+             (integerp y))
+        (+ (* x 3) y)
+        0)))
+
+(defun determinant (a b c d)
+  (- (* a d)
+     (* b c)))
+
+(format t "~d~%" (main "input"))
\ No newline at end of file
-- 
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