Day23.c

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

// https://adventofcode.com/2020/day/23

int lut[1000001];
int current = 0;

void setup(const char *numbers) {
    for (int i = 0; i < 1000001; i++) {
        lut[i] = i + 1;
    }

    int nf = (int)strlen(numbers);

    current = 3;
    for (int i = 0; i < nf; i++) {
        int value1 = numbers[i] - 48;
        int value2 = numbers[i + 1] - 48;
        if (value2 == -48) {
            lut[value1] = nf + 1;
            lut[1000000] = numbers[0] - 48;
        } else {
            lut[value1] = value2;
        }
    }

}

void playRound() {
    int numberToRelocate1 = lut[current];
    int numberToRelocate2 = lut[numberToRelocate1];
    int numberToRelocate3 = lut[numberToRelocate2];
    int modulo = 1000000;
    int destination = (current + modulo - 2) % modulo + 1;

    while (numberToRelocate1 == destination ||
           numberToRelocate2 == destination ||
           numberToRelocate3 == destination) {
        destination = (destination + modulo - 2) % modulo + 1;
    }

    lut[current] = lut[numberToRelocate3];
    lut[numberToRelocate3] = lut[destination];
    lut[destination] = numberToRelocate1;
    current = lut[current];
}

int main(int argc, const char * argv[]) {
    if (argc < 2) {
        printf("usage: %s SINGLE_DIGIT_CUP_LABELS\n", *argv);
        return 1;
    }

    setup(argv[1]);

    for (int i = 0; i < 10000000; i++) {
        playRound();
    }

    long number1 = lut[1];
    long number2 = lut[number1];

    printf("Part 2: %ld\n", number1 * number2);
    return 0;
}