Added quick and dirty C solution for b

2 files changed, 55 insertions, 0 deletions

diff --git a/2020/23/Makefile b/2020/23/Makefile
index dad052e..197dba2 100644
--- a/2020/23/Makefile
+++ b/2020/23/Makefile
@@ -1,2 +1,5 @@
+c:
+	@gcc -Ofast solve.c && ./a.out
+
 run:
 	@node solve.js
diff --git a/2020/23/solve.c b/2020/23/solve.c
new file mode 100644
index 0000000..ab7e878
--- /dev/null
+++ b/2020/23/solve.c
@@ -0,0 +1,52 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define SZ 9
+
+int *play(int *I, int C, int M)
+{
+	int *c = malloc((C + 1) * sizeof(*c));
+	if (!c)
+		exit(1);
+
+	for (int i = 0; i < C; i++)
+		c[i] = i + 1;
+
+	for (int i = 0; i < C + 1; i++)
+		c[I[i]] = I[(i + 1) % SZ];
+
+	c[0] = I[0];
+	if (C > SZ) {
+		c[I[SZ - 1]] = SZ + 1;
+		c[C] = I[0];
+	}
+
+	for (int m = 1; m <= M; m++) {
+		int p1 = c[c[0]];
+		int p2 = c[c[c[0]]];
+		int p3 = c[c[c[c[0]]]];
+		int d = c[0] - 1 ? c[0] - 1 : C;
+		while (p1 == d || p2 == d || p3 == d)
+			d = d - 1 ? d - 1 : C;
+		c[c[0]] = c[p3];
+		c[p3] = c[d];
+		c[d] = p1;
+		c[0] = c[c[0]];
+	}
+
+	return c;
+}
+
+int main(int argc, char *argv[])
+{
+	int data[] = { 4, 6, 3, 5, 2, 8, 1, 7, 9 };
+	int *d = malloc((1e6 + 1) * sizeof(*d));
+	memcpy(d, data, 9 * 4);
+	int *c = play(d, 1e6, 1e7);
+	long res = ((long)c[1]) * ((long)c[c[1]]);
+	printf("%lu\n", res);
+	free(d);
+	free(c);
+	return 0;
+}