Initial commit

2021/day03/day03.bqn

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+in←-⟜'0'¨•FLines"input"
+D2B←2⊸×⊸+˜´∘⌽
+•Show ×´D2B¨(1⊸-⋈⊢)(+´>≠÷2˙)in  # part 1

2021/day03/day03.c

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+#include <stdio.h>
+
+#define N 1000
+#define M 12
+
+int main() {
+  char c = 0;
+  int i = 0, j = 0;
+  char in[N][M] = {0};
+
+  while ((c = getchar()) != EOF) {
+    switch (c) {
+    case '\n': i++; j = 0; break;
+    case '1': in[i][j] = 1; j++; break;
+    case '0': j++; break;
+    default: fprintf(stderr, "unknown digit %c", c); return 1;
+    }
+  }
+
+  // Part 1
+  int counts[M] = {0};
+  for (i = 0; i < N; ++i) {
+    for (j = 0; j < M; ++j) {
+      counts[j] += in[i][j];
+    }
+  }
+  int gamma = 0, epsilon = 0;
+  for (j = 0; j < M; ++j) {
+    gamma = (gamma << 1) + (counts[j] > N/2);
+    epsilon = (epsilon << 1) + (counts[j] <= N/2);
+  }
+  printf("%d\n", gamma * epsilon);
+
+  return 0;
+}

2021/day03/day03.scm

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+(import (chicken io)
+	(chicken bitwise)
+	srfi-1
+	srfi-13)
+
+(define (binary-string->list s)
+  (map (lambda (c) (- (char->integer c) (char->integer #\0)))
+       (string->list s)))
+
+(define (list->binary-string l)
+  (string-concatenate (map number->string l)))
+
+(define (read-input #!optional (port (current-input-port)))
+  (map binary-string->list (read-lines port)))
+
+(define (part1 l)
+  (let* ((n (/ (length l) 2))
+	 (counts (apply map + l))
+	 (most-common (map (lambda (x) (if (> x n) 1 0)) counts))
+	 (gamma (string->number (list->binary-string most-common) 2))
+	 (epsilon (string->number (list->binary-string (map (lambda (d) (- 1 d)) most-common)) 2)))
+    (* gamma epsilon)))
+
+(define (o2-rating numbers i)
+  (if (= 1 (length numbers))
+      (car numbers)
+      (let* ((digits (map (lambda (l) (list-ref l i)) numbers))
+	     (most-common (if (>= (apply + digits) (/ (length numbers) 2)) 1 0))
+	     (new-numbers (filter (lambda (n) (= (list-ref n i) most-common)) numbers)))
+	(o2-rating new-numbers (+ 1 i)))))
+
+(define (co2-rating numbers i)
+  (if (= 1 (length numbers))
+      (car numbers)
+      (let* ((digits (map (lambda (l) (list-ref l i)) numbers))
+	     (least-common (if (>= (apply + digits) (/ (length numbers) 2)) 0 1))
+	     (new-numbers (filter (lambda (n) (= (list-ref n i) least-common)) numbers)))
+	(co2-rating new-numbers (+ 1 i)))))
+
+(define (part2 l)
+  (let ((o2 (string->number (list->binary-string (o2-rating l 0)) 2))
+	(co2 (string->number (list->binary-string (co2-rating l 0)) 2)))
+    (* o2 co2)))
+
+(let ((in (read-input)))
+  (print (part1 in))
+  (print (part2 in)))