// Copyright 2021 Jeisson Hidalgo-Cespedes <jeisson.hidalgo@ucr.ac.cr> CC-BY-4
// Creates an arbitrary amount of threads that greet in stdout

#include <assert.h>
#include <errno.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>


typedef struct {
  size_t thread_count;
  size_t next_thread;
} shared_data_t;

typedef struct  {
  size_t thread_number;
  shared_data_t* shared_data;
} private_data_t;

int create_threads(shared_data_t* shared_data);
void* run(void* data);


int main(int argc, char* argv[]) {
  int error = EXIT_SUCCESS;

  shared_data_t* shared_data = (shared_data_t*)
    calloc(1, sizeof(shared_data_t));

  if (shared_data) {
    shared_data->next_thread = 0;
    shared_data->thread_count = sysconf(_SC_NPROCESSORS_ONLN);
    if (argc == 2) {
      if (sscanf(argv[1], "%zu", &shared_data->thread_count) != 1 || errno) {
        fprintf(stderr, "error: invalid thread count\n");
        return EXIT_FAILURE;
      }
    }

    struct timespec start_time;
    clock_gettime(/*clk_id*/CLOCK_MONOTONIC, &start_time);

    error = create_threads(shared_data);

    struct timespec finish_time;
    clock_gettime(/*clk_id*/CLOCK_MONOTONIC, &finish_time);

    double elapsed = (finish_time.tv_sec - start_time.tv_sec) +
      (finish_time.tv_nsec - start_time.tv_nsec) * 1e-9;
    printf("execution time: %.9lfs\n", elapsed);

    free(shared_data);
  }

  return error;
}

int create_threads(shared_data_t* shared_data) {
  assert(shared_data);
  int error = EXIT_SUCCESS;
  pthread_t* threads = (pthread_t*) calloc(shared_data->thread_count
    , sizeof(pthread_t));
  private_data_t* private_data = (private_data_t*)
    calloc(shared_data->thread_count, sizeof(private_data_t));
  if (threads && private_data) {
    for (size_t index = 0; index < shared_data->thread_count; ++index) {
      private_data[index].thread_number = index;
      private_data[index].shared_data = shared_data;
      if (pthread_create(&threads[index], /*attr*/ NULL, run
        , &private_data[index]) == EXIT_SUCCESS) {
      } else {
        fprintf(stderr, "error: could not create thread %zu\n", index);
        error = 21;
        break;
      }
    }

    printf("Hello from main thread\n");

    for (size_t index = 0; index < shared_data->thread_count; ++index) {
      pthread_join(threads[index], /*value_ptr*/ NULL);
    }
    free(threads);
    free(private_data);
  } else {
    fprintf(stderr, "error: could not allocate memory for %zu threads\n"
      , shared_data->thread_count);
    error = 22;
  }
  
  return error;
}

void* run(void* data) {
  const private_data_t* private_data = (private_data_t*)data;
  const size_t my_thread_id = private_data->thread_number;
  const size_t thread_count = private_data->shared_data->thread_count;

  // Wait until it is my turn
  while (private_data->shared_data->next_thread < my_thread_id) {
    // busy-waiting
  }

  // Do the ordered-task here
  printf("Hello from secondary thread %zu of %zu\n", my_thread_id
    , thread_count);

  // Allow subsequent thread to do the task
  ++private_data->shared_data->next_thread;

  return NULL;
}