solve day 6

day6.c

@@ -0,0 +1,56 @@
+#include <stdbool.h>
+#include <stdio.h>
+#include <string.h>
+
+#include "aoc.h"
+
+#define MESSAGE_LENGTH 14
+
+static void *day6_parse(char **lines, int line_count)
+{
+	return lines[0];
+}
+
+static void day6_part1(void *p)
+{
+	char *msg = p;
+	int n = strlen(msg);
+	for (int i = 3; i < n; i++) {
+		char c1 = msg[i-3];
+		char c2 = msg[i-2];
+		char c3 = msg[i-1];
+		char c4 = msg[i-0];
+		if (c1 != c2 && c1 != c3 && c1 != c4 &&
+				c2 != c3 && c2 != c4 &&
+				c3 != c4) {
+			printf("%d\n", i+1);
+			break;
+		}
+	}
+}
+
+static bool is_start_of_message(char *str)
+{
+	for (int i=0; i < MESSAGE_LENGTH - 1; i++) {
+		for (int j=i+1; j < MESSAGE_LENGTH; j++) {
+			if (str[i] == str[j]) {
+				return false;
+			}
+		}
+	}
+	return true;
+}
+
+static void day6_part2(void *p)
+{
+	char *msg = p;
+	int n = strlen(msg);
+	for (int i = 0; i < n-13; i++) {
+		if (is_start_of_message(msg+i)) {
+			printf("%d\n", i+14);
+			break;
+		}
+	}
+}
+
+ADD_SOLUTION(6, day6_parse, day6_part1, day6_part2);

main.c

@@ -13,6 +13,7 @@
 #include "day3.c"
 #include "day4.c"
 #include "day5.c"
+#include "day6.c"
 
 Solution *find_solution(int day)
 {