2 files changed, 65 insertions, 0 deletions
diff --git a/2021/day3/03a.rkt b/2021/day3/03a.rkt
new file mode 100644
index 0000000..bb42d83
--- /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)
diff --git a/2021/day3/03b.rkt b/2021/day3/03b.rkt
new file mode 100644
index 0000000..aea9c86
--- /dev/null
+++ b/2021/day3/03b.rkt
@@ -0,0 +1,35 @@
+#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))
+  (cond [(= (hash-ref ht #\0 0) (hash-ref ht #\1 0)) #f]
+        [else max-key]))
+
+(define (flip x)
+  (cond [(char=? x #\0) #\1]
+                    [else #\0]))
+
+(define (bit-criteria lines least [fallback (cond [least #\0] [else #\1])] [n 0])
+  (cond [(= (length lines) 1) (car lines)]
+        [else
+         (define bits (map (curryr string-ref n) lines))
+         (define bMode (mode bits))
+         (define criteria (cond [(not bMode) fallback]
+                                [least (flip bMode)]
+                                [else bMode]))
+         (bit-criteria (filter (lambda (x) (char=? criteria (string-ref x n)))
+                               lines)
+                       least fallback (+ n 1))]))
+
+(define input (open-input-file "input"))
+(define lines (port->lines input))
+
+(define oxy (string->number (bit-criteria lines #f) 2))
+(define co2 (string->number (bit-criteria lines #t) 2))
+
+(* oxy co2)