day 3

day3.rkt

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+#lang racket
+
+; utility functions
+
+(define (char->number char)
+  (if (equal? char #\0) 0 1))
+
+(define (number->char num)
+  (if (= num 0) #\0 #\1))
+
+(define (string->numbers str)
+  (map char->number (string->list str)))
+
+(define (transpose xss)
+  (apply map list xss))
+
+; IO
+
+(define lines
+  (file->lines "day3.txt"))
+
+(define chars
+  (map string->numbers lines))
+
+; part 1
+
+(define (count-ones-in-list xss)
+  (foldl + 0 (filter positive? xss)))
+
+(define counts
+  (map count-ones-in-list (transpose chars)))
+
+(define gamma-bin-list
+  (map (λ (count)
+         (if (> count (/ (length chars) 2)) 1 0))
+       counts))
+
+(define (flip-bits num)
+  (map (λ (n) (modulo (+ 1 n) 2)) num))
+
+(define epsilon-bin-list (flip-bits gamma-bin-list))
+
+(define (binary-list->decimal num)
+  (string->number (list->string (map number->char num)) 2))
+
+(define gamma-dec (binary-list->decimal gamma-bin-list))
+(define epsilon-dec (binary-list->decimal epsilon-bin-list))
+(define part1 (* gamma-dec epsilon-dec))
+
+; part 2
+; sounds like a fold to me
+
+(define (most-common-digit column xs type)
+  (define col-values (map (λ (x) (list-ref x column)) xs))
+  (define ones (count-ones-in-list col-values))
+  (define op (if (equal? type 'o2) >= <)) ; 'o2 uses >=, 'co2 uses <
+  (if (op ones (/ (length xs) 2)) 1 0))
+
+(define (get-rating xs type)
+  (flatten
+   (foldl (λ (column remaining)
+            (define most-common (most-common-digit column remaining type))
+            (if (= (length remaining) 1) ; if there's only one left
+                remaining ; return it
+                (filter (λ (ys)
+                          (= (list-ref ys column) most-common))
+                        remaining)))
+          xs 
+          (range (length (first xs))))))
+
+(define o2-rating (binary-list->decimal (get-rating chars 'o2)))
+(define co2-rating (binary-list->decimal (get-rating chars 'co2)))
+(define part2 (* o2-rating co2-rating))

day3.test.txt

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+00100
+11110
+10110
+10111
+10101
+01111
+00111
+11100
+10000
+11001
+00010
+01010