co.easimart.LocalIdManager Maven / Gradle / Ivy

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package co.easimart;

import java.io.File;
import java.io.IOException;
import java.util.Random;

import org.json.JSONException;
import org.json.JSONObject;

/**
 * Manages a set of local ids and possible mappings to global Easimart objectIds. This class is
 * thread-safe.
 */
/** package */ class LocalIdManager {

  /**
   * Internal class representing all the information we know about a local id.
   */
  private static class MapEntry {
    String objectId;
    int retainCount;
  }

  // Path to the local id storage on disk.
  private final File diskPath;

  // Random generator for inventing new ids.
  private final Random random;

  /**
   * Creates a new LocalIdManager with default options.
   */
  /* package for tests */ LocalIdManager(File root) {
    diskPath = new File(root, "LocalId");
    random = new Random();
  }

  /**
   * Returns true if localId has the right basic format for a local id.
   */
  private boolean isLocalId(String localId) {
    if (!localId.startsWith("local_")) {
      return false;
    }
    for (int i = 6; i < localId.length(); ++i) {
      char c = localId.charAt(i);
      if (!(c >= '0' && c <= '9') && !(c >= 'a' && c <= 'f')) {
        return false;
      }
    }
    return true;
  }

  /**
   * Grabs one entry in the local id map off the disk.
   */
  private synchronized MapEntry getMapEntry(String localId) {
    if (!isLocalId(localId)) {
      throw new IllegalStateException("Tried to get invalid local id: \"" + localId + "\".");
    }

    try {
      JSONObject json = EasimartFileUtils.readFileToJSONObject(new File(diskPath, localId));

      MapEntry entry = new MapEntry();
      entry.retainCount = json.optInt("retainCount", 0);
      entry.objectId = json.optString("objectId", null);
      return entry;
    } catch (IOException | JSONException e) {
      return new MapEntry();
    }
  }

  /**
   * Writes one entry to the local id map on disk.
   */
  private synchronized void putMapEntry(String localId, MapEntry entry) {
    if (!isLocalId(localId)) {
      throw new IllegalStateException("Tried to get invalid local id: \"" + localId + "\".");
    }

    JSONObject json = new JSONObject();
    try {
      json.put("retainCount", entry.retainCount);
      if (entry.objectId != null) {
        json.put("objectId", entry.objectId);
      }
    } catch (JSONException je) {
      throw new IllegalStateException("Error creating local id map entry.", je);
    }

    File file = new File(diskPath, localId);
    if (!diskPath.exists()) {
      diskPath.mkdirs();
    }

    try {
      EasimartFileUtils.writeJSONObjectToFile(file, json);
    } catch (IOException e) {
      //TODO (grantland): We should do something if this fails...
    }
  }

  /**
   * Removes an entry from the local id map on disk.
   */
  private synchronized void removeMapEntry(String localId) {
    if (!isLocalId(localId)) {
      throw new IllegalStateException("Tried to get invalid local id: \"" + localId + "\".");
    }
    File file = new File(diskPath, localId);
    EasimartFileUtils.deleteQuietly(file);
  }

  /**
   * Creates a new local id.
   */
  synchronized String createLocalId() {
    long localIdNumber = random.nextLong();
    String localId = "local_" + Long.toHexString(localIdNumber);

    if (!isLocalId(localId)) {
      throw new IllegalStateException("Generated an invalid local id: \"" + localId + "\". "
          + "This should never happen. Contact us at https://parse.com/help");
    }

    return localId;
  }

  /**
   * Increments the retain count of a local id on disk.
   */
  synchronized void retainLocalIdOnDisk(String localId) {
    MapEntry entry = getMapEntry(localId);
    entry.retainCount++;
    putMapEntry(localId, entry);
  }

  /**
   * Decrements the retain count of a local id on disk. If the retain count hits zero, the id is
   * forgotten forever.
   */
  synchronized void releaseLocalIdOnDisk(String localId) {
    MapEntry entry = getMapEntry(localId);
    entry.retainCount--;

    if (entry.retainCount > 0) {
      putMapEntry(localId, entry);
    } else {
      removeMapEntry(localId);
    }
  }

  /**
   * Returns the objectId associated with a given local id. Returns null if no objectId is yet known
   * for the local id.
   */
  synchronized String getObjectId(String localId) {
    MapEntry entry = getMapEntry(localId);
    return entry.objectId;
  }

  /**
   * Sets the objectId associated with a given local id.
   */
  synchronized void setObjectId(String localId, String objectId) {
    MapEntry entry = getMapEntry(localId);
    if (entry.retainCount > 0) {
      if (entry.objectId != null) {
        throw new IllegalStateException(
            "Tried to set an objectId for a localId that already has one.");
      }
      entry.objectId = objectId;
      putMapEntry(localId, entry);
    }
  }

  /**
   * Clears all local ids from the map. Returns true is the cache was already empty.
   */
  synchronized boolean clear() throws IOException {
    String[] files = diskPath.list();
    if (files == null) {
      return false;
    }
    if (files.length == 0) {
      return false;
    }
    for (String fileName : files) {
      File file = new File(diskPath, fileName);
      if (!file.delete()) {
        throw new IOException("Unable to delete file " + fileName + " in localId cache.");
      }
    }
    return true;
  }
}