const erl = @cImport({
  @cInclude("erl_nif.h");
});


pub export fn hello(env: ?*erl.ErlNifEnv, argc: c_int, argv: [*c]const erl.ERL_NIF_TERM) erl.ERL_NIF_TERM{
  _ = argv;
  _ = argc;
  //const ERL_NIF_LATIN1 = @as(c_uint, 1);
  return erl.enif_make_string(env, "Hello world!", 1); // ERL_NIF_LATIN1);
}


fn u128_to_buf(value: u128, buffer: *align(1) [40]u8) usize{
  if(value == 0){
    buffer[0] = '0';
    return 1;
  }
  
  var i: usize = 0;
  var n = value;
  while(n > 0){
    buffer[i] = @as(u8, @intCast('0' + (n % 10)) );
    n /= 10;
    i += 1;
  }
  
  var j: usize = 0;
  while(j < i / 2) : (j += 1){
    const temp = buffer[j];
    buffer[j] = buffer[i - 1 - j];
    buffer[i - 1 - j] = temp;
  }
  
  return i;
}


pub export fn factorial(env: ?*erl.ErlNifEnv, argc: c_int, argv: [*c]const erl.ERL_NIF_TERM) erl.ERL_NIF_TERM{
  if(argc != 1){
    return erl.enif_make_badarg(env);
  }
  
  var n: c_long = undefined;
  if(erl.enif_get_long(env, argv[0], &n) == 0 or n < 0){
    return erl.enif_make_badarg(env);
  }
  
  if(n > 34){ // max n for max factorial for u128
    return erl.enif_make_tuple2(env,
      erl.enif_make_atom(env, "error"),
      erl.enif_make_string(env, "input too large, max is 34", 1)
    );
  }
  
  var result: u128 = 1;
  var i = @as(u128, @intCast(n) );
  //var i = n;
  while(i > 1) : (i -= 1){
    result *= i;
  }
  
  var buf: [40]u8 = undefined;
  const len = u128_to_buf(result, &buf);
  
  return erl.enif_make_string_len(env, buf[0..len].ptr, len, 1); // , ERL_NIF_LATIN1);
}


const nif_funcs = [_]erl.ErlNifFunc{
  .{
    .name = "hello",
    .arity = 0,
    .fptr = hello,
    .flags = 0,
  },
  .{
    .name = "factorial",
    .arity = 1,
    .fptr = factorial,
    .flags = 0,
  },
};


export fn nif_init() ?*erl.ErlNifEntry{
  return &nif_entry;
}


var nif_entry: erl.ErlNifEntry = .{
  .major = 2,
  .minor = 16,
  .name = "niftest",
  .num_of_funcs = nif_funcs.len,
  .funcs = @constCast(&nif_funcs),
  .load = null,
  .reload = null,
  .upgrade = null,
  .unload = null,
  .vm_variant = "beam.vanilla",
  .options = 1, // 0,
  .sizeof_ErlNifResourceTypeInit = @sizeOf(erl.ErlNifResourceTypeInit),
  .min_erts = "erts-12.0", // .entry_version = 3,
};