day09.c

#include <stddef.h>
#include <malloc.h>
#include <stdio.h>
#include <time.h>

typedef struct linked_t {
    struct linked_t* previous;
    struct linked_t* next;
    int value;
} linked_t;

void insert_after(linked_t* list, int i) {
    linked_t* new = malloc(sizeof(linked_t));
    new->previous = list;
    new->next = list->next;
    new->value = i;
    list->next = new;
    new->next->previous = new;
}

int remove_current(linked_t* list) {
    list->previous->next = list->next;
    list->next->previous = list->previous;
    int v = list->value;
    free(list);
    return v;
}

int main(void) {

    const clock_t start = clock();

    const int players = 412;
    const int points = 7164600;

    unsigned long* scores = malloc(players * sizeof(long));
    for(int i = 0; i < players; i++) {
        scores[i] = 0;
    }

    linked_t* list = malloc(sizeof(linked_t));
    list->previous = list;
    list->next = list;
    list->value = 0;

    int player = 0;
    for(int i = 1; i <= points; i++) {

        if(i % 23 != 0) {
            list = list->next;
            insert_after(list, i);
            list = list->next;
        } else {
            for(int j = 0; j < 6; j++) {
                list = list->previous;
            }
            int v = remove_current(list->previous);
            scores[player] += v + i;
        }


        player = (player + 1) % players;
    }

    long max = 0;
    for(int i = 0; i < players; i++) {
        if(scores[i] > max) {
            max = scores[i];
        }
    }

    printf("%lu\n", max);

    const clock_t stop = clock();
    printf("(result in %f seconds)\n", (stop - start) / (double) CLOCKS_PER_SEC);
}