1 files changed, 80 insertions, 0 deletions
diff --git a/2020/10/solve.c b/2020/10/solve.c
new file mode 100644
index 0000000..83c5535
--- /dev/null
+++ b/2020/10/solve.c
@@ -0,0 +1,80 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+static int LENGTH = 0;
+
+int part_one(int *data)
+{
+	int prev = 0, three = 1, one = 0;
+
+	for (int i = 0; i < LENGTH; i++) {
+		if (data[i] == 0)
+			continue;
+		if (data[i] - prev == 3)
+			three++;
+		else
+			one++;
+		prev = data[i];
+	}
+
+	return one * three;
+}
+
+long part_two(int *data)
+{
+	long a, b, c;
+	a = 1;
+	b = 0;
+	c = 0;
+	for (int i = LENGTH - 2; i > 0; i--) {
+		long next = (i + 1 < LENGTH && data[i + 1] - data[i] <= 3 ? a : 0) +
+			    (i + 2 < LENGTH && data[i + 2] - data[i] <= 3 ? b : 0) +
+			    (i + 3 < LENGTH && data[i + 3] - data[i] <= 3 ? c : 0);
+		c = b;
+		b = a;
+		a = next;
+	}
+
+	return a;
+}
+
+int cmp(const void *a, const void *b)
+{
+	int int_a = *((int *)a);
+	int int_b = *((int *)b);
+
+	if (int_a == int_b)
+		return 0;
+	else if (int_a < int_b)
+		return -1;
+	else
+		return 1;
+}
+
+int *to_int(FILE *fp, int *arr)
+{
+	int d = 0, i = 0;
+	while (fscanf(fp, "%d\n", &d) != EOF)
+		arr[i++] = d;
+	LENGTH = i + 2;
+
+	return arr;
+}
+
+int main()
+{
+	FILE *fp = fopen("input", "r");
+	if (!fp)
+		exit(EXIT_FAILURE);
+
+	int data[128] = { 0 };
+	to_int(fp, data);
+	qsort(data, LENGTH, sizeof(int), cmp);
+	data[LENGTH] = data[LENGTH - 1] + 3;
+
+	printf("%d\n", part_one(data));
+	printf("%lu\n", part_two(data));
+
+	fclose(fp);
+	return 0;
+}