From 6c8340752cb55d49bf514dc0fda90e54fa571967 Mon Sep 17 00:00:00 2001
From: Alan Daniels <alan.daniels10@gmail.com>
Date: Wed, 10 Dec 2025 20:19:45 +1100
Subject: [PATCH] day 4 part 1

---
 04a.lisp      | 53 +++++++++++++++++++++++++++++++++++++++++++++++++++
 04example.txt | 10 ++++++++++
 2 files changed, 63 insertions(+)
 create mode 100644 04a.lisp
 create mode 100644 04example.txt

diff --git a/04a.lisp b/04a.lisp
new file mode 100644
index 0000000..95ac703
--- /dev/null
+++ b/04a.lisp
@@ -0,0 +1,53 @@
+(require :uiop)
+(load (sb-ext:posix-getenv "ASDF"))
+(asdf:load-system 'str)
+
+(defun read-diagram (lines)
+  (let* (
+         (dim (length lines))
+         (diagram (make-array (list dim dim))))
+    (loop for line in lines
+          for y from 0
+          do (loop for char across line
+                   for x from 0
+                   do (if (char-equal char #\@)
+                          (setf (aref diagram y x) 1))))
+    diagram))
+(read-diagram (uiop:read-file-lines "04example.txt"))
+
+(defun make-kernel (diagram -x -y)
+  (let ( (kernel (make-array '(3 3))))
+    (loop for y from -1 to 1
+          do (loop for x from -1 to 1
+                   do (let ((-x (+ -x x))
+                            (-y (+ -y y)))
+                        (if (array-in-bounds-p diagram -y -x)
+                          (setf (aref kernel (+ y 1) (+ x 1)) (aref diagram -y -x))
+                          (setf (aref kernel (+ y 1) (+ x 1)) 0)))))
+    kernel))
+(make-kernel (read-diagram (uiop:read-file-lines "04example.txt")) 0 2)
+
+(defun sum-kernel (kernel)
+  (if (= (aref kernel 1 1) 1)
+    (if
+      (<
+        (-
+          (loop for y from 0 to 2
+            sum (loop for x from 0 to 2
+                      sum (aref kernel y x)))
+          1)
+        4)
+      1
+      0)
+    0))
+(sum-kernel (make-kernel (read-diagram (uiop:read-file-lines "04example.txt")) 2 0))
+
+(defun sum-diagram (lines)
+  (let ( (dim (length lines))
+        (diagram (read-diagram lines)))
+    (loop for y from 0 to dim
+          sum (loop for x from 0 to dim
+                    sum (sum-kernel (make-kernel diagram x y))))))
+
+(sum-diagram (uiop:read-file-lines "04example.txt"))
+(sum-diagram (uiop:read-file-lines "04data.txt"))
diff --git a/04example.txt b/04example.txt
new file mode 100644
index 0000000..8209399
--- /dev/null
+++ b/04example.txt
@@ -0,0 +1,10 @@
+..@@.@@@@.
+@@@.@.@.@@
+@@@@@.@.@@
+@.@@@@..@.
+@@.@@@@.@@
+.@@@@@@@.@
+.@.@.@.@@@
+@.@@@.@@@@
+.@@@@@@@@.
+@.@.@@@.@.