1 files changed, 46 insertions, 0 deletions

diff --git a/2023/09/9b.c b/2023/09/9b.c
new file mode 100644
index 0000000..1b539c1
--- /dev/null
+++ b/2023/09/9b.c
@@ -0,0 +1,46 @@
+#include <inttypes.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define N 1000
+
+#define isnum(c) (c == '-' || (c >= '0' && c <= '9'))
+
+int64_t readl(int64_t nums[], char *buf) {
+        int64_t i;
+        for (i = 0; *buf; buf++) {
+                if (!isnum(*buf)) continue;
+                nums[i++] = atoll(buf);
+                while (isnum(*buf)) buf++;
+        }
+        return i;
+}
+
+bool isconstant(int64_t a[], int n) {
+        for (int i = 0; i < n-1; i++)
+                if (a[i] != a[i+1])
+                        return false;
+        return true;
+}
+
+int64_t next(int64_t a[], int64_t n) {
+        int64_t x = a[0];
+        if (isconstant(a, n))
+                return x;
+        for (int i = 0; i < n-1; i++)
+                a[i] = a[i+1] - a[i];
+        return x - next(a, n-1);
+}
+
+int main() {
+        char line[N];
+        int64_t s, a[N];
+
+        for (s = 0; fgets(line, N, stdin) != NULL; )
+                s += next(a, readl(a, line));
+
+        printf("%" PRId64 "\n", s);
+        return 0;
+}