Van Eck's sequence with gnuplot https://youtu.be/etMJxB-igrc https://oeis.org/A181391 https://codegolf.stackexchange.com/questions/186654/nth-term-of-van-eck-sequence

eck.c

#include <stdio.h>
#include <inttypes.h>

#define N 100000

uint_fast16_t f()
{
    static uint_fast16_t last = 0, indxs[N] = {0}, i = 0;
    i++;
    uint_fast16_t tmp = last;
    if (indxs[last] == 0)
        last = 0;
    else
        last = i - indxs[tmp];
    indxs[tmp] = i;
    return last;
}

int main(int argc, char **argv)
{
    printf("USAGE:\n\t%s | gnuplot --persist\n", argv[0]);
    FILE *out = fopen("out.csv", "w+");
    fputs("0, 0\n", out);
    for (int i = 0; i < 100000; i++)
        fprintf(out, "%i, %lu\n", i + 1, f());
    puts("plot 'out.csv' with boxes");
    fclose(out);
    
}