Day 23: Added a C version of the vector version of the Racket solution. It doesn't work.

src/23-array.c

@@ -0,0 +1,51 @@
+#include <stdlib.h>
+#include <stdio.h>
+#include <stdbool.h>
+
+unsigned long CUPS = 1000000;
+unsigned long MOVES = 10000000;
+
+// Precondition: 1 <= cup <= CUPS
+unsigned long prevCup(unsigned long cup) {
+    if (cup == 1) {
+        return CUPS;
+    } else {
+        return cup - 1;
+    }
+}
+
+unsigned long play(unsigned long* cups, unsigned long cup) {
+    unsigned long first = cups[cup];
+    unsigned long second = cups[first];
+    unsigned long third = cups[second];
+    unsigned long fourth = cups[third];
+    unsigned long dest = prevCup(cup);
+    while (dest == first || dest == second || dest == third) {
+        dest = prevCup(dest);
+    }
+    cups[cup] = fourth;
+    cups[third] = cups[dest];
+    cups[dest] = first;
+    return fourth;
+}
+
+int main() {
+    int input[9] = {3, 6, 2, 9, 8, 1, 7, 5, 4};
+    int arr[9] = {7, 9, 6, 10, 4, 2, 5, 1, 8};
+    unsigned long* cups = malloc((CUPS + 1) * sizeof(*cups));
+    unsigned long i;
+    unsigned long cup = 3;
+    cups[0] = 0;
+    for (i = 1; i <= 9; i++) {
+        cups[i] = arr[i - 1];
+    }
+    for (i = 10; i <= CUPS; i++) {
+        cups[i] = i + 1;
+    }
+    for (i = 0; i < MOVES; i++) {
+        cup = play(cups, cup);
+    }
+    unsigned long first = cups[1];
+    unsigned long second = cups[first];
+    printf("Product of two cups after cup 1 is %lu * %lu = %lu\n", first, second, first * second);
+}