1 files changed, 67 insertions, 0 deletions
diff --git a/puzzle-3.lisp b/puzzle-3.lisp
new file mode 100644
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--- /dev/null
+++ b/puzzle-3.lisp
@@ -0,0 +1,67 @@
+(defvar input (uiop:read-file-string "puzzle-3-input.txt"))
+
+(defun split-string-in-two (s)
+  (let ((len (length s)))
+    (list (subseq s 0 (/ len 2)) (subseq s (/ len 2)))))
+
+(defvar inputs (with-input-from-string (s input)
+                 (loop
+                   for line = (read-line s nil)
+                   until (null line)
+                   collect (split-string-in-two line))))
+
+(defun string-to-clist (str)
+  (loop for char across str collect char))
+
+(defun common-types (s1 s2)
+  (car (intersection
+        (string-to-clist s1)
+        (string-to-clist s2))))
+
+(defvar shared (mapcar (lambda (x)
+                    (destructuring-bind (s1 s2) x
+                      (common-types s1 s2)))
+                  inputs))
+
+(defun priority-map (c)
+  (if (upper-case-p c)
+      (+ 27 (- (char-code c) (char-code #\A)))
+      (+ 1 (- (char-code c) (char-code #\a)))))
+
+(defvar round-1-answer (reduce #'+ (mapcar #'priority-map shared)))
+
+;; Round 2
+
+;; Simple recursive algorithm which produces consecutive groups of 3 elements
+(defun group-by-3 (lst)
+  (if (null lst)
+      nil
+      (cons
+       (list (car lst) (car (cdr lst)) (car (cdr (cdr lst))))
+       (group-by-3 (cdr (cdr (cdr lst)))))))
+
+;; Note the use of group-by-3 here
+(defvar inputs (group-by-3
+                (with-input-from-string (s input)
+                  (loop
+                    for line = (read-line s nil)
+                    until (null line)
+                    collect line))))
+
+;; Extend intersection to three
+(defun common-types-3 (s1 s2 s3)
+  (car
+   (intersection
+    (string-to-clist s1)
+    (intersection
+     (string-to-clist s2)
+     (string-to-clist s3)))))
+
+;; Extend the destructuring bind and use of common-types-3
+(defvar shared (mapcar (lambda (x)
+                    (destructuring-bind (s1 s2 s3) x
+                      (common-types-3 s1 s2 s3)))
+                  inputs))
+
+;; Same as before
+(defvar round-2-answer (reduce #'+ (mapcar #'priority-map shared)))