// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'dart:convert';
import 'dart:ffi';
import 'dart:typed_data';

import 'package:ffi/ffi.dart';

const int _kMaxSmi64 = (1 << 62) - 1;
const int _kMaxSmi32 = (1 << 30) - 1;
final int _maxSize = sizeOf<IntPtr>() == 8 ? _kMaxSmi64 : _kMaxSmi32;

/// [Utf8] implements conversion between Dart strings and null-terminated
/// Utf8-encoded "char*" strings in C.
///
/// [Utf8] is respresented as a struct so that `Pointer<Utf8>` can be used in
/// native function signatures.
//
// TODO(https://github.com/dart-lang/ffi/issues/4): No need to use
// 'asExternalTypedData' when Pointer operations are performant.
class Utf8 extends Struct {
  /// Returns the length of a null-terminated string -- the number of (one-byte)
  /// characters before the first null byte.
  static int strlen(Pointer<Utf8> string) {
    final Pointer<Uint8> array = string.cast<Uint8>();
    final Uint8List nativeString = array.asExternalTypedData(count: _maxSize);
    return nativeString.indexWhere((char) => char == 0);
  }

  /// Creates a [String] containing the characters UTF-8 encoded in [string].
  ///
  /// The [string] must be a zero-terminated byte sequence of valid UTF-8
  /// encodings of Unicode code points. It may also contain UTF-8 encodings of
  /// unpaired surrogate code points, which is not otherwise valid UTF-8, but
  /// which may be created when encoding a Dart string containing an unpaired
  /// surrogate. See [Utf8Decoder] for details on decoding.
  ///
  /// Returns a Dart string containing the decoded code points.
  static String fromUtf8(Pointer<Utf8> string) {
    final int length = strlen(string);
    return utf8.decode(Uint8List.view(
        string.cast<Uint8>().asExternalTypedData(count: length).buffer,
        0,
        length));
  }

  /// Convert a [String] to a Utf8-encoded null-terminated C string.
  ///
  /// If 'string' contains NULL bytes, the converted string will be truncated
  /// prematurely. Unpaired surrogate code points in [string] will be preserved
  /// in the UTF-8 encoded result. See [Utf8Encoder] for details on encoding.
  ///
  /// Returns a malloc-allocated pointer to the result.
  static Pointer<Utf8> toUtf8(String string) {
    final units = utf8.encode(string);
    final Pointer<Uint8> result = allocate<Uint8>(count: units.length + 1);
    final Uint8List nativeString =
        result.asExternalTypedData(count: units.length + 1);
    nativeString.setAll(0, units);
    nativeString[units.length] = 0;
    return result.cast();
  }

  String toString() => fromUtf8(addressOf);
}