// this file:
//   https://dataswamp.org/~incal/emacs-init/random-urandom/random-urandom.c

#include "random-urandom.h"

#include <emacs-module.h>

#include <errno.h>
#include <fcntl.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
#include <unistd.h>

int plugin_is_GPL_compatible;

int emacs_module_init(struct emacs_runtime* runtime) {
  if (runtime->size < (long int)sizeof(*runtime)) {
    return 1; // incompatible Emacs binary
  }

  emacs_env* env = runtime->get_environment(runtime);
  if (env->size < (long int)sizeof(*env)) {
    return 2; // incompatible module API
  }

  int __attribute__((unused)) emacs_version;
  if ((long int)sizeof(struct emacs_env_29) <= env->size) {
    emacs_version = 29;
  }
  else {
    return 3; // too old Emacs
  }

  emacs_value rur_func   = env->make_function(env, 0, 0, random_urandom, NULL, NULL);
  emacs_value rur_symbol = env->intern(env, "random-urandom");
  emacs_value args[]     = {rur_symbol, rur_func};
  env->funcall(env, env->intern(env, "defalias"), 2, args);

  return 0;
}

// originally from pwgen(1) version 2.08, Debian 11
// https://github.com/tytso/pwgen
static int get_random_fd() {
   struct timeval tv;
   static int fd = -2;
   if (fd == -2) {
      gettimeofday(&tv, 0);
      fd = open("/dev/urandom", O_RDONLY);
      if (fd == -1) {
         fd = open("/dev/random", O_RDONLY | O_NONBLOCK);
      }
   }
   return fd;
}

unsigned int pw_random_number() {
  unsigned int rnd_num;
  unsigned int max_num = UINT_MAX;

  int fd                   = get_random_fd();
  int lose_counter         = 0;
  long unsigned int nbytes = sizeof(rnd_num);
  char* cp                 = (char*)&rnd_num;

  int i;
  if (fd >= 0) {
    while (nbytes > 0) {
      i = read(fd, cp, nbytes);
      if ((i < 0) && ((errno == EINTR) || (errno == EAGAIN))) {
        continue;
      }
      if (i <= 0) {
        if (lose_counter++ == 8) {
          break;
        }
        continue;
      }
      nbytes -= i;
      cp += i;
      lose_counter = 0;
    }
  }

  if (close(fd) != 0) {
    fprintf(stderr, "Error closing file.\n");
    exit(1);
  }

  if (nbytes == 0) {
    return (rnd_num % max_num);
  } else {
    fprintf(stderr, "No entropy available.\n");
    exit(1);
  }
}

emacs_value random_urandom(emacs_env*   env,
                           ptrdiff_t    nargs __attribute__((unused)),
                           emacs_value* args  __attribute__((unused)),
                           void*        data  __attribute__((unused))) {
  emacs_value rnd_num = env->make_integer(env, pw_random_number());
  return rnd_num;
}