Day 23: Attempt with C, but I don't think it's working...

src/23.c

@@ -0,0 +1,121 @@
+#include <stdlib.h>
+#include <stdio.h>
+#include <stdbool.h>
+
+int CUPS = 1000000;
+int MOVES = 10000000;
+
+struct link {
+    int cup;
+    struct link* next;
+};
+
+// Precondition: head is not NULL
+void freeLoop(struct link* head) {
+    struct link* curr = head;
+    do {
+        struct link* next = curr->next;
+        free(curr);
+        curr = next;
+    } while (curr && curr != head);
+}
+
+// Precondition: head is not NULL
+void printLoop(struct link* head) {
+    struct link* curr = head;
+    do {
+        printf("%d", curr->cup);
+        curr = curr->next;
+    } while (curr && curr != head);
+    printf("\n");
+}
+
+// Builds a chain starting from the given link,
+// with each subsequent link containing ints start to end
+// Returns the last link in the chain
+struct link* buildChain(struct link* curr, int start, int end) {
+    for (int i = start; i <= end; i++) {
+        struct link* next = malloc(sizeof(struct link));
+        next->cup = i;
+        next->next = NULL;
+        curr->next = next;
+        curr = next;
+    }
+    return curr;
+}
+
+// Builds a chain from the given ints in the array,
+// setting the given link address to point to the first link
+// Returns the last link in the chain
+// Precondition: arr has at least one int
+struct link* buildArrayChain(struct link** head, int* arr, int length) {
+    *head = malloc(sizeof(struct link));
+    struct link* curr = *head;
+    curr->cup = arr[0];
+    for (int i = 1; i < length; i++) {
+        struct link* next = malloc(sizeof(struct link));
+        next->cup = arr[i];
+        next->next = NULL;
+        curr->next = next;
+        curr = next;
+    }
+    return curr;
+}
+
+// Returns the link in the chain with the given cup
+// Precondition: The cup must exist in the chain
+struct link* findLink(struct link* curr, int cup) {
+    while (curr->cup != cup) {
+        curr = curr->next;
+    }
+    return curr;
+}
+
+// Precondition: 1 <= cup <= CUPS
+int prevCup(int cup) {
+    if (cup == 1) {
+        return CUPS;
+    } else {
+        return cup - 1;
+    }
+}
+
+struct link* move(struct link* head) {
+    struct link* firstLink = head->next;
+    struct link* secondLink = head->next->next;
+    struct link* thirdLink = head->next->next->next;
+    int first = firstLink->cup;
+    int second = secondLink->cup;
+    int third = thirdLink->cup;
+    head->next = thirdLink->next;
+
+    int destCup = prevCup(head->cup);
+    while (destCup == first || destCup == second || destCup == third) {
+        destCup = prevCup(destCup);
+    }
+
+    struct link* dest = findLink(head, destCup);
+    thirdLink->next = dest->next;
+    dest->next = firstLink;
+    return head->next;
+}
+
+int main() {
+    int arr[9] = {3, 6, 2, 9, 8, 1, 7, 5, 4};
+    int length = 9;
+    struct link* head;
+    struct link* tail = buildArrayChain(&head, arr, length);
+    tail = buildChain(tail, 10, 1000000);
+    tail->next = head;
+    printf("Loop has been built.\n");
+
+    for (int i = 0; i < MOVES; i++) {
+        head = move(head);
+    }
+    head = findLink(head, 1);
+
+    int first = head->next->cup;
+    int second = head->next->next->cup;
+    printf("Product of two cups after cup 1 is %d * %d = %d\n", first, second, first * second);
+    freeLoop(head);
+}