One last time with threads.

p1 isolated: 12ms

p2 isolated: 13ms

combined run: 25ms

typedef struct job_s {
  Mode mode;
  Point rng;
  usize total;
} Job;

static void*
worker(void* a) {
  Job* job = a;
  (job->mode == MODE_ONE) ? one(job->rng, &job->total)
                          : two(job->rng, &job->total);
  return nullptr;
}

static void
solve(Mode mode) {
  FILE* input = fopen("input", "r");
  c8 line[LINE_BUFSZ] = {};
  fgets(line, sizeof(line), input);
  line[strcspn(line, "\n")] = 0;
  fclose(input);
  usize nrng = 1;
  for (strc s = line; *s; s++) {
    if (*s == ',') {
      nrng++;
    }
  }
  Job* jobs = calloc(nrng, sizeof(*jobs));
  pthread_t* threads = calloc(nrng, sizeof(*threads));
  usize idx = 0;
  strc tok = strtok(line, ",");
  while (tok) {
    sscanf(tok, "%zu-%zu", &jobs[idx].rng.x, &jobs[idx].rng.y);
    jobs[idx].mode = mode;
    tok = strtok(nullptr, ",");
    idx++;
  }
  for (usize i = 0; i < nrng; i++) {
    pthread_create(&threads[i], nullptr, worker, &jobs[i]);
  }
  usize total = 0;
  for (usize i = 0; i < nrng; i++) {
    pthread_join(threads[i], nullptr);
    total += jobs[i].total;
  }
  free(threads);
  free(jobs);
  printf("%zu\n", total);
}

i32
main(void) {
  solve(MODE_ONE);
  solve(MODE_TWO);
}