1 files changed, 54 insertions, 0 deletions

diff --git a/2023/21/21a.c b/2023/21/21a.c
new file mode 100644
index 0000000..a4009d9
--- /dev/null
+++ b/2023/21/21a.c
@@ -0,0 +1,54 @@
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define N 200
+
+typedef struct { int i, j; } point_t;
+
+char map[N][N];
+int n, nq, nnext;
+point_t s, q[N*N], nextq[N*N];
+
+bool bad(point_t p) {
+        int i = p.i, j = p.j;
+        return i < 0 || j < 0 || i >= n || j >= n || map[i][j] == '#';
+}
+
+void add(point_t p) {
+        if (bad(p)) return;
+        for (int i = 0; i < nnext; i++)
+                if (nextq[i].i == p.i && nextq[i].j == p.j)
+                        return;
+        nextq[nnext++] = p;
+}
+
+point_t up(point_t p) { return (point_t) {.i = p.i-1, .j = p.j}; }
+point_t down(point_t p) { return (point_t) {.i = p.i+1, .j = p.j}; }
+point_t right(point_t p) { return (point_t) {.i = p.i, .j = p.j+1}; }
+point_t left(point_t p) { return (point_t) {.i = p.i, .j = p.j-1}; }
+
+int main() {
+        for (n = 0; fgets(map[n], N, stdin) != NULL; n++)
+                for (int i = 0; map[n][i] != '\n'; i++)
+                        if (map[n][i] == 'S')
+                                s = (point_t) {.i = n, .j = i};
+
+        int d = 64;
+        add(s);
+        for (int i = 0; i < d; i++) {
+                memcpy(q, nextq, nnext * sizeof(point_t));
+                nq = nnext;
+                nnext = 0;
+                for (int j = 0; j < nq; j++) {
+                        add(up(q[j]));
+                        add(down(q[j]));
+                        add(right(q[j]));
+                        add(left(q[j]));
+                }
+        }
+
+        printf("%d\n", nnext);
+        return 0;
+}