initial commit

1 files changed, 30 insertions, 0 deletions

diff --git a/2021/day3/03a.rkt b/2021/day3/03a.rkt
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--- /dev/null
+++ b/2021/day3/03a.rkt
@@ -0,0 +1,30 @@
+#lang racket
+(define (mode xs)
+  (define ht (make-hash))
+  (define max-key '0)
+  (for ([x xs])
+    (define new-val (+ 1 (hash-ref ht x 0)))
+    (hash-set! ht x new-val)
+    (if (> new-val (hash-ref ht max-key 0)) (set! max-key x) 3))
+  max-key)
+
+(define (transpose xss)
+  (apply map list xss))
+
+(define (flip x)
+  (cond [(string=? x "0") "1"]
+        [else "0"]))
+
+(define input (open-input-file "input"))
+(define bit-positions
+  (transpose (map (lambda (xs)
+                    (map (curry make-string 1) (string->list xs)))
+                  (port->lines input))))
+
+(define gamma-str (foldr string-append "" (map mode bit-positions)))
+(define epsilon-str (foldr string-append "" (map (compose flip mode) bit-positions)))
+
+(define gamma (string->number gamma-str 2))
+(define epsilon (string->number epsilon-str 2))
+
+(* gamma epsilon)