Added solution for 13

1 files changed, 52 insertions, 0 deletions

diff --git a/2023/13/13b.c b/2023/13/13b.c
new file mode 100644
index 0000000..090a895
--- /dev/null
+++ b/2023/13/13b.c
@@ -0,0 +1,52 @@
+#include <inttypes.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define N 64
+
+#define MAX(a,b) ((a)>(b)?(a):(b))
+
+char *buf, line[N];
+int64_t i, s, nc, nr, r[N], c[N];
+
+int64_t onebit(int64_t x) {
+        while (x % 2 == 0) x >>= 1;
+        return !(x-1);
+}
+
+int64_t issmudgedmirror(int64_t i, int64_t a[], int64_t n) {
+        int64_t smudge = 0;
+        for (int64_t j = MAX(0, 2*i-n+2); j <= i && 2*i-j+1 < n; j++) {
+                if (a[j] == a[2*i-j+1]) continue;
+                if (onebit(a[j] ^ a[2*i-j+1]))
+                        smudge++;
+                else
+                        return 0;
+        }
+        return smudge == 1;
+}
+
+int main() {
+        do {
+                buf = fgets(line, N, stdin);
+                if (buf == NULL || line[0] == '\n') {
+                        for (i = 0; i < nc-1; i++)
+                                s += (i + 1) * issmudgedmirror(i, c, nc);
+                        for (i = 0; i < nr-1; i++)
+                                s += 100 * (i + 1) * issmudgedmirror(i, r, nr);
+                        nr = nc = 0;
+                        memset(r, 0, N * sizeof(int64_t));
+                        memset(c, 0, N * sizeof(int64_t));
+                } else {
+                        for (nc = 0; line[nc] != '\n'; nc++) {
+                                r[nr] = (r[nr] << 1) + (line[nc] == '#');
+                                c[nc] = (c[nc] << 1) + (line[nc] == '#');
+                        }
+                        nr++;
+                }
+        } while (buf != NULL);
+
+        printf("%" PRId64 "\n", s);
+        return 0;
+}