1 files changed, 78 insertions, 0 deletions
diff --git a/2020/09/part2.c b/2020/09/part2.c
new file mode 100644
index 0000000..28c335e
--- /dev/null
+++ b/2020/09/part2.c
@@ -0,0 +1,78 @@
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+#define INPUT_LEN 1000
+#define SEARCH_LEN 25
+
+bool has_sum(uint64_t *cur_num)
+{
+    uint64_t search = *cur_num;
+    uint64_t *p1 = cur_num - SEARCH_LEN;
+    do {
+        uint64_t *p2 = cur_num - SEARCH_LEN;
+        do {
+            if (*p1 + *p2 == search) {
+                return true;
+            }
+        } while (++p2 < cur_num);
+    } while (++p1 < cur_num);
+
+    return false;
+}
+
+int exe_program(const char *filename)
+{
+    FILE *file = fopen(filename, "r");
+
+    // Include space for newline and string terminator
+    char buffer[24] = { 0 };
+
+    uint64_t nums[INPUT_LEN] = { 0 };
+    size_t num_size = 0;
+
+    while (fgets(buffer, 24, file)) {
+        nums[num_size++] = strtoull(buffer, NULL, 10);
+    }
+
+    fclose(file);
+
+    uint64_t *cur_num = nums + SEARCH_LEN;
+    uint64_t search = -1;
+    do {
+        if (!has_sum(cur_num)) {
+            search = *cur_num;
+            break;
+        }
+    } while (++cur_num < nums + num_size);
+
+    cur_num = nums;
+    do {
+        uint64_t min = __UINT64_MAX__;
+        uint64_t max = 0;
+        uint64_t sum = 0;
+        uint64_t *p1 = cur_num;
+        do {
+            if (*p1 < min) {
+                min = *p1;
+            }
+            else if (*p1 > max) {
+                max = *p1;
+            }
+
+            sum += *p1;
+
+            if (sum == search) {
+                return min + max;
+            }
+        } while (++p1 < nums + num_size);
+    } while (++cur_num < nums + num_size);
+
+    return -1;
+}
+
+int main(int argc, char *argv[])
+{
+    printf("%i\n", exe_program(argv[argc - 1]));
+}