From c5a03887370dda0d142dee7b6812bfe7b47ccedc Mon Sep 17 00:00:00 2001
From: 4903000 <4903000@chiru.no>
Date: Sat, 4 Dec 2021 01:06:49 +0000
Subject: [PATCH] Day 3

---
 aoc21.lisp | 46 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 46 insertions(+)

diff --git a/aoc21.lisp b/aoc21.lisp
index 0e023ee..07c2ab0 100644
--- a/aoc21.lisp
+++ b/aoc21.lisp
@@ -67,3 +67,49 @@
   (setup-solution-2 2)
   (eval (cons 'progn data))
   (reduce #'* (result)))
+
+;;; Day 3
+
+(defun vector+ (&rest vectors)
+  (labels ((internal (old new)
+	     (map 'vector (lambda (n m) (+ n m)) old new)))
+    (reduce #'internal vectors)))
+
+(defun read-3 (file)
+  (with-open-file (stream file)
+    (loop for line = (read-line stream nil)
+	  while line
+	  collect (map 'vector (lambda (c) (- (char-code c) 48))
+		       line))))
+
+(defun solve-3-1 (data)
+  (let* ((max (/ (length data) 2))
+	 (sums (reduce #'vector+ data))
+	 (gamma (map 'string (lambda (i) (digit-char (if (< 0 (- i max)) 1 0))) sums))
+	 (epsilon (map 'string (lambda (c) (if (char= c #\1) #\0 #\1)) gamma)))
+    (* (parse-integer gamma :radix 2)
+       (parse-integer epsilon :radix 2))))
+
+(defun rec (data acc head bias)
+  (if data
+      (rec (cdr data)
+	   (if (= (elt (first data) head) bias)
+	       (list (cons (first data) (first acc)) (second acc))
+	       (list (first acc) (cons (first data) (second acc))))
+	   head bias)
+      (if (funcall ((lambda () (if (= bias 1) #'< #'>)))
+		   (length (first acc)) (length (second acc)))
+	  (second acc)
+	  (first acc))))
+
+(defun recrec (data head bias)
+  (let ((new-data (rec data (list nil nil) head bias)))
+    (if (= (length new-data) 1)
+	(first new-data)
+	(recrec new-data (+ 1 head) bias))))
+
+(defun solve-3-2 (data)
+  (let ((oxygen (map 'string (lambda (n) (digit-char n)) (recrec data 0 1)))
+	(co2    (map 'string (lambda (n) (digit-char n)) (recrec data 0 0))))
+    (* (parse-integer oxygen :radix 2)
+       (parse-integer co2 :radix 2))))