Add 2021 advent - day 3.

2021/day3/p.rkt

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+#lang racket
+
+(require gtp-util)
+
+; max supported length
+(define len 16)
+
+; part 1 vars
+(define onecount (make-vector len))
+; part 2 vars
+(define slist null)
+
+(define binary-len 0)
+(for ([t (in-lines)])
+	  (define l (string->list t))
+	  (for ([v (enumerate (reverse l))])
+			 (set! binary-len (+ 1 (car v))) ; will take the last index
+			 (when (equal? #\1 (cdr v))
+				(define index (- (- len (car v)) 1))
+				(vector-set! onecount index (add1 (vector-ref onecount index)))))
+	  (set! slist (cons (list->vector l) slist)))
+
+(display " binary-len: ")
+(display binary-len)
+(display " v: ")
+(display onecount)
+
+; utility function
+(define (binary-vector-to-num v)
+  (define s (list->string (vector->list v)))
+  (string->number s 2))
+
+; part 2 utility functions
+(define (bit-criteria lines pos bias)
+  (define count (for/sum ([t lines] #:when (equal? (vector-ref t pos) #\1)) 1))
+  (if (>= (* 2 count) (length lines))
+	 (if (equal? 1 bias) #\1 #\0)
+	 (if (equal? 1 bias) #\0 #\1)))
+(define (select-lines lines pos value)
+  (filter (lambda (x) (equal? value (vector-ref x pos))) lines))
+  ;(for/list ([t lines] #:when (equal? value (vector-ref t pos))) t))
+
+; part 1 variables
+(define gamma (make-vector binary-len #\0))
+(define epsilon (make-vector binary-len #\0))
+; part 2 variables
+(define oxygen slist)
+(define co2 slist)
+
+(for ([index (in-range binary-len)])
+	  (define v (vector-ref onecount (+ index (- len binary-len))))
+	  (if (>= (* 2 v) (length slist))
+		 (vector-set! gamma index #\1)
+		 (vector-set! epsilon index #\1))
+	  (when (> (length oxygen) 1)
+		 (define c (bit-criteria oxygen index 1))
+		 (set! oxygen (select-lines oxygen index c)))
+	  (when (> (length co2) 1)
+		 (define c (bit-criteria co2 index 0))
+		 (set! co2 (select-lines co2 index c))))
+
+(display " res1: ")
+(display (* (binary-vector-to-num gamma) (binary-vector-to-num epsilon)))
+(display " res2: ")
+(display (* (binary-vector-to-num (car oxygen)) (binary-vector-to-num (car co2))))

2021/day3/test1.txt

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