// Copyright (c) 2015, 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:async';
import 'dart:io';
import 'dart:isolate';
import 'dart:typed_data';

import 'package:front_end/src/byte_store/byte_store.dart';
import 'package:front_end/src/byte_store/fletcher16.dart';
import 'package:path/path.dart';

/**
 * The request that is sent from the main isolate to the clean-up isolate.
 */
class CacheCleanUpRequest {
  final String cachePath;
  final int maxSizeBytes;
  final SendPort replyTo;

  CacheCleanUpRequest(this.cachePath, this.maxSizeBytes, this.replyTo);
}

/**
 * [ByteStore] that stores values as files and performs cache eviction.
 *
 * Only the process that manages the cache, e.g. Analysis Server, should use
 * this class. Other processes, e.g. Analysis Server plugins, should use
 * [FileByteStore] instead and let the main process to perform eviction.
 */
class EvictingFileByteStore implements ByteStore {
  static bool _cleanUpSendPortShouldBePrepared = true;
  static SendPort _cleanUpSendPort;

  final String _cachePath;
  final int _maxSizeBytes;
  final FileByteStore _fileByteStore;

  int _bytesWrittenSinceCleanup = 0;
  bool _evictionIsolateIsRunning = false;

  EvictingFileByteStore(this._cachePath, this._maxSizeBytes)
      : _fileByteStore = new FileByteStore(_cachePath) {
    _requestCacheCleanUp();
  }

  @override
  List<int> get(String key) => _fileByteStore.get(key);

  @override
  void put(String key, List<int> bytes) {
    _fileByteStore.put(key, bytes);
    // Update the current size.
    _bytesWrittenSinceCleanup += bytes.length;
    if (_bytesWrittenSinceCleanup > _maxSizeBytes ~/ 8) {
      _requestCacheCleanUp();
    }
  }

  /**
   * If the cache clean up process has not been requested yet, request it.
   */
  Future<Null> _requestCacheCleanUp() async {
    if (_cleanUpSendPortShouldBePrepared) {
      _cleanUpSendPortShouldBePrepared = false;
      ReceivePort response = new ReceivePort();
      await Isolate.spawn(_cacheCleanUpFunction, response.sendPort);
      _cleanUpSendPort = await response.first as SendPort;
    } else {
      while (_cleanUpSendPort == null) {
        await new Future.delayed(new Duration(milliseconds: 100), () {});
      }
    }

    if (!_evictionIsolateIsRunning) {
      _evictionIsolateIsRunning = true;
      try {
        ReceivePort response = new ReceivePort();
        _cleanUpSendPort.send(new CacheCleanUpRequest(
            _cachePath, _maxSizeBytes, response.sendPort));
        await response.first;
      } finally {
        _evictionIsolateIsRunning = false;
        _bytesWrittenSinceCleanup = 0;
      }
    }
  }

  /**
   * This function is started in a new isolate, receives cache folder clean up
   * requests and evicts older files from the folder.
   */
  static void _cacheCleanUpFunction(message) {
    SendPort initialReplyTo = message;
    ReceivePort port = new ReceivePort();
    initialReplyTo.send(port.sendPort);
    port.listen((request) {
      if (request is CacheCleanUpRequest) {
        _cleanUpFolder(request.cachePath, request.maxSizeBytes);
        // Let the client know that we're done.
        request.replyTo.send(true);
      }
    });
  }

  static void _cleanUpFolder(String cachePath, int maxSizeBytes) {
    // Prepare the list of files and their statistics.
    List<File> files = <File>[];
    Map<File, FileStat> fileStatMap = {};
    int currentSizeBytes = 0;
    List<FileSystemEntity> resources = new Directory(cachePath).listSync();
    for (FileSystemEntity resource in resources) {
      if (resource is File) {
        try {
          final FileStat fileStat = resource.statSync();
          // Make sure that the file was not deleted out from under us (a return
          // value of FileSystemEntityType.notFound).
          if (fileStat.type == FileSystemEntityType.file) {
            files.add(resource);
            fileStatMap[resource] = fileStat;
            currentSizeBytes += fileStat.size;
          }
        } catch (_) {}
      }
    }
    files.sort((a, b) {
      return fileStatMap[a].accessed.millisecondsSinceEpoch -
          fileStatMap[b].accessed.millisecondsSinceEpoch;
    });

    // Delete files until the current size is less than the max.
    for (File file in files) {
      if (currentSizeBytes < maxSizeBytes) {
        break;
      }
      try {
        file.deleteSync();
      } catch (_) {}
      currentSizeBytes -= fileStatMap[file].size;
    }
  }
}

/**
 * [ByteStore] that stores values as files.
 */
class FileByteStore implements ByteStore {
  static final FileByteStoreValidator _validator = new FileByteStoreValidator();

  final String _cachePath;
  final String _tempSuffix;
  final Map<String, List<int>> _writeInProgress = {};
  final FuturePool _pool = new FuturePool(20);

  /**
   * If the same cache path is used from more than one isolate of the same
   * process, then a unique [tempNameSuffix] must be provided for each isolate.
   */
  FileByteStore(this._cachePath, {String tempNameSuffix: ''})
      : _tempSuffix =
            '-temp-${pid}${tempNameSuffix.isEmpty ? '' : '-$tempNameSuffix'}';

  @override
  List<int> get(String key) {
    List<int> bytes = _writeInProgress[key];
    if (bytes != null) {
      return bytes;
    }

    try {
      final File file = _getFileForKey(key);
      if (!file.existsSync()) {
        return null;
      }
      return _validator.getData(file.readAsBytesSync());
    } catch (_) {
      // ignore exceptions
      return null;
    }
  }

  @override
  void put(String key, List<int> bytes) {
    _writeInProgress[key] = bytes;

    final List<int> wrappedBytes = _validator.wrapData(bytes);

    // We don't wait for the write and rename to complete.
    _pool.execute(() {
      final File tempFile = _getFileForKey('$key$_tempSuffix');
      return tempFile.writeAsBytes(wrappedBytes).then((_) {
        return tempFile.rename(join(_cachePath, key));
      }).catchError((_) {
        // ignore exceptions
      }).whenComplete(() {
        if (_writeInProgress[key] == bytes) {
          _writeInProgress.remove(key);
        }
      });
    });
  }

  File _getFileForKey(String key) => new File(join(_cachePath, key));
}

/**
 * Generally speaking, we cannot guarantee that any data written into a file
 * will stay the same - there is always a chance of a hardware problem, file
 * system problem, truncated data, etc.
 *
 * So, we need to embed some validation into data itself. This class append the
 * version and the checksum to data.
 */
class FileByteStoreValidator {
  static const List<int> _VERSION = const [0x01, 0x00];

  /**
   * If the [rawBytes] have the valid version and checksum, extract and
   * return the data from it. Otherwise return `null`.
   */
  List<int> getData(List<int> rawBytes) {
    // There must be at least the version and the checksum in the raw bytes.
    if (rawBytes.length < 4) {
      return null;
    }
    int len = rawBytes.length - 4;

    // Check the version.
    if (rawBytes[len + 0] != _VERSION[0] || rawBytes[len + 1] != _VERSION[1]) {
      return null;
    }

    // Check the checksum of the data.
    List<int> data = rawBytes.sublist(0, len);
    int checksum = fletcher16(data);
    if (rawBytes[len + 2] != checksum & 0xFF ||
        rawBytes[len + 3] != (checksum >> 8) & 0xFF) {
      return null;
    }

    // OK, the data is probably valid.
    return data;
  }

  /**
   * Return bytes that include the given [data] plus the current version and
   * the checksum of the [data].
   */
  List<int> wrapData(List<int> data) {
    int len = data.length;
    var bytes = new Uint8List(len + 4);

    // Put the data.
    bytes.setRange(0, len, data);

    // Put the version.
    bytes[len + 0] = _VERSION[0];
    bytes[len + 1] = _VERSION[1];

    // Put the checksum of the data.
    int checksum = fletcher16(data);
    bytes[len + 2] = checksum & 0xFF;
    bytes[len + 3] = (checksum >> 8) & 0xFF;

    return bytes;
  }
}

class FuturePool {
  int _available;
  List waiting = [];

  FuturePool(this._available);

  void execute(Future Function() fn) {
    if (_available > 0) {
      _run(fn);
    } else {
      waiting.add(fn);
    }
  }

  void _run(Future Function() fn) {
    _available--;

    fn().whenComplete(() {
      _available++;

      if (waiting.isNotEmpty) {
        _run(waiting.removeAt(0));
      }
    });
  }
}