1 files changed, 101 insertions, 0 deletions
diff --git a/06.c b/06.c
new file mode 100644
index 0000000..44f7dc9
--- /dev/null
+++ b/06.c
@@ -0,0 +1,101 @@
+#include <stdlib.h>
+#include <stdio.h>
+#include <err.h>
+#include <stdbool.h>
+#include "input.h"
+#include "list.h"
+
+#define INPUT "input/06.txt"
+#define INPUT_SIZE 300L
+#define EXPECTED1 380758L
+#define EXPECTED2 1710623015163L
+
+void part1(struct input_str* input) {
+	char* start = input->lines[0];
+	char* end = start;
+	struct long_list* lanterns = long_list_init(INPUT_SIZE);
+
+	// parse input
+	while(*end != '\n') {
+		long timer = strtol(start, &end, 0);
+		start = end;
+		if(*end == ',')
+			start++;
+
+		long_list_add(lanterns, timer);
+	}
+
+	// time flies
+	for(int day = 0; day < 80; day++) {
+		size_t init_length = lanterns->length; // we should leave alone the new lanterns
+		for(size_t i = 0; i < init_length; i++) {
+			long timer = long_list_get(lanterns, i);
+			if(timer == 0) {
+				timer = 7;
+				long_list_add(lanterns, 8);
+			}
+			timer--;
+			long_list_set(lanterns, i, timer);
+		}
+	}
+
+	CHECK(lanterns->length, EXPECTED1);
+	long_list_free(lanterns);
+}
+
+void part2(struct input_str* input) {
+	// simulating 256 days with the naive approach from part1 is too long and uses up too much memory
+	// we need to count lanternfishes based on their timer
+	long* lantern_count_by_timer = calloc(9, sizeof(long));
+	char* start = input->lines[0];
+	char* end = start;
+
+	// parse input
+	while(*end != '\n') {
+		long timer = strtol(start, &end, 0);
+		start = end;
+		if(*end == ',')
+			start++;
+
+		lantern_count_by_timer[timer]++;
+	}
+
+	long* lantern_count_2 = calloc(9, sizeof(long));
+	for(int day = 0; day < 256; day++) {
+		// simulate the timers decreasing
+		for(int i = 8; i > 0; i--) {
+			lantern_count_2[i-1] = lantern_count_by_timer[i];
+		}
+		// handle special cases of lanternfishes with timer 0
+		lantern_count_2[8] = lantern_count_by_timer[0];
+		lantern_count_2[6] += lantern_count_by_timer[0];
+
+		// switch arrays for the next day
+		long* lantern_count_tmp = lantern_count_by_timer;
+		lantern_count_by_timer = lantern_count_2;
+		lantern_count_2 = lantern_count_tmp;
+	}
+
+	// compute total of lanternfish
+	long sum = 0;
+	for(int i = 0; i < 9; i++)
+		sum += lantern_count_by_timer[i];
+
+	free(lantern_count_by_timer);
+	free(lantern_count_2);
+	CHECK(sum, EXPECTED2);
+}
+
+int main() {
+	// read input
+	struct input_str input;
+	input_str_read(&input, INPUT);
+
+	// do stuff
+	part1(&input);
+	part2(&input);
+
+	// cleanup & exit
+	input_str_free(&input);
+	return 0;
+}