// Part of the Wasmtime Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://github.com/bytecodealliance/wasmtime/blob/master/LICENSE for license information.
//
// Significant parts of this file are derived from cloudabi-utils. See
// https://github.com/bytecodealliance/wasmtime/blob/master/lib/wasi/sandboxed-system-primitives/src/LICENSE
// for license information.
//
// The upstream file contains the following copyright notice:
//
// Copyright (c) 2016 Nuxi, https://nuxi.nl/

#include "config.h"

#include <fcntl.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>

#include "random.h"

#if CONFIG_HAS_ARC4RANDOM_BUF

void random_buf(void *buf, size_t len) {
  arc4random_buf(buf, len);
}

#elif CONFIG_HAS_GETRANDOM

#include <sys/random.h>

void random_buf(void *buf, size_t len) {
  for (;;) {
     ssize_t x = getrandom(buf, len, 0);
     if (x < 0) {
         if (errno == EINTR)
             continue;
         fprintf(stderr, "getrandom failed: %s", strerror(errno));
         abort();
     }
     if (x == len)
         return;
     buf = (void *)((unsigned char *)buf + x);
     len -= x;
  }
}

#else

static int urandom;

static void open_urandom(void) {
  urandom = open("/dev/urandom", O_RDONLY);
  if (urandom < 0) {
    fputs("Failed to open /dev/urandom\n", stderr);
    abort();
  }
}

void random_buf(void *buf, size_t len) {
  static pthread_once_t open_once = PTHREAD_ONCE_INIT;
  pthread_once(&open_once, open_urandom);

  if (read(urandom, buf, len) != len) {
    fputs("Short read on /dev/urandom\n", stderr);
    abort();
  }
}

#endif

// Calculates a random number within the range [0, upper - 1] without
// any modulo bias.
//
// The function below repeatedly obtains a random number from
// arc4random() until it lies within the range [2^k % upper, 2^k). As
// this range has length k * upper, we can safely obtain a number
// without any modulo bias.
uintmax_t random_uniform(uintmax_t upper) {
  // Compute 2^k % upper
  //      == (2^k - upper) % upper
  //      == -upper % upper.
  uintmax_t lower = -upper % upper;
  for (;;) {
    uintmax_t value;
    random_buf(&value, sizeof(value));
    if (value >= lower)
      return value % upper;
  }
}