/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.juneau.config.store;

import static java.nio.file.StandardOpenOption.*;
import static java.nio.file.StandardWatchEventKinds.*;
import static org.apache.juneau.commons.utils.FileUtils.*;
import static org.apache.juneau.commons.utils.Shorts.*;
import static org.apache.juneau.commons.utils.StringUtils.*;
import static org.apache.juneau.commons.utils.SystemUtils.*;
import static org.apache.juneau.commons.utils.ThrowableUtils.*;

import java.io.*;
import java.nio.*;
import java.nio.channels.*;
import java.nio.charset.*;
import java.nio.file.*;
import java.nio.file.attribute.*;
import java.util.concurrent.*;

import org.apache.juneau.commons.collections.*;

/**
 * Filesystem-based storage location for configuration files.
 *
 * <p>
 * Points to a file system directory containing configuration files.
 *
 * <h2 class='section'>Notes:</h2><ul>
 *    <li class='note'>This class is thread safe and reusable.
 * </ul>
 */
@SuppressWarnings({
   "java:S1192", // Duplicated literals (argument/property names) read more clearly inline than as constants
   "java:S3776", // read() and write() combine cache lookup, file locking and compare-and-swap of contents in one flow
   "javabugs:S2259", // The nullable watcher and the conditional FileChannel lock resource in read() are guarded by nn()/ternary checks that flow analysis does not track
   "resource" // Resources are managed by caller
})
public class FileStore extends ConfigStore {

   private static final String MSG_PATH_ESCAPE = "Name escapes configured root directory.";

   /**
    * Builder class.
    */
   public static class Builder extends ConfigStore.Builder<Builder> {

      private Charset charset;
      private boolean allowAbsoluteNames;
      private boolean enableWatcher;
      private boolean updateOnWrite;
      private String directory;
      private String extensions;
      private WatcherSensitivity watcherSensitivity;

      /**
       * Constructor, default settings.
       */
      protected Builder() {
         charset = env("ConfigFileStore.charset").map(Charset::forName).orElse(Charset.defaultCharset());
         directory = env("ConfigFileStore.directory", ".");
         allowAbsoluteNames = env("ConfigFileStore.allowAbsoluteNames", false);
         enableWatcher = env("ConfigFileStore.enableWatcher", false);
         extensions = env("ConfigFileStore.extensions", "cfg,yml,yaml");
         updateOnWrite = env("ConfigFileStore.updateOnWrite", false);
         watcherSensitivity = env("ConfigFileStore.watcherSensitivity", WatcherSensitivity.MEDIUM);
      }

      /**
       * Copy constructor.
       *
       * @param copyFrom The builder to copy from.
       *    <br>Cannot be <jk>null</jk>.
       */
      protected Builder(Builder copyFrom) {
         super(reqnn("copyFrom", copyFrom));
         charset = copyFrom.charset;
         directory = copyFrom.directory;
         allowAbsoluteNames = copyFrom.allowAbsoluteNames;
         enableWatcher = copyFrom.enableWatcher;
         extensions = copyFrom.extensions;
         updateOnWrite = copyFrom.updateOnWrite;
         watcherSensitivity = copyFrom.watcherSensitivity;
      }

      /**
       * Copy constructor.
       *
       * @param copyFrom The bean to copy from.
       *    <br>Cannot be <jk>null</jk>.
       */
      protected Builder(FileStore copyFrom) {
         super(reqnn("copyFrom", copyFrom));
         type(copyFrom.getClass());
         charset = copyFrom.charset;
         directory = copyFrom.directory;
         allowAbsoluteNames = copyFrom.allowAbsoluteNames;
         enableWatcher = copyFrom.enableWatcher;
         extensions = copyFrom.extensions;
         updateOnWrite = copyFrom.updateOnWrite;
         watcherSensitivity = copyFrom.watcherSensitivity;
      }

      @Override /* Overridden from Context.Builder<?> */
      public FileStore build() {
         return build(FileStore.class);
      }

      /**
       * Charset for external files.
       *
       * <p>
       * Identifies the charset of external files.
       *
       * @param value
       *    The new value for this property.
       *    <br>The default is the first value found:
       *    <ul>
       *       <li>System property <js>"ConfigFileStore.charset"</js>
       *       <li>Environment variable <js>"CONFIGFILESTORE_CHARSET"</js>
       *       <li>{@link Charset#defaultCharset()}
       *    </ul>
       *    <br>Cannot be <jk>null</jk>.
       * @return This object.
       */
      public Builder charset(Charset value) {
         charset = reqnn("value", value);
         return this;
      }

      @Override /* Overridden from Context.Builder<?> */
      public Builder copy() {
         return new Builder(this);
      }

      /**
       * Local file system directory.
       *
       * <p>
       * Identifies the path of the directory containing the configuration files.
       *
       * @param value
       *    The new value for this property.
       *    <br>The default is the first value found:
       *    <ul>
       *       <li>System property <js>"ConfigFileStore.directory"</js>
       *       <li>Environment variable <js>"CONFIGFILESTORE_DIRECTORY"</js>
       *       <li><js>"."</js>.
       *    </ul>
       *    <br>Cannot be <jk>null</jk>.
       * @return This object.
       */
      public Builder directory(File value) {
         directory = reqnn("value", value).getAbsolutePath();
         return this;
      }

      /**
       * Local file system directory.
       *
       * <p>
       * Identifies the path of the directory containing the configuration files.
       *
       * @param value
       *    The new value for this property.
       *    <br>The default is the first value found:
       *    <ul>
       *       <li>System property <js>"ConfigFileStore.directory"</js>
       *       <li>Environment variable <js>"CONFIGFILESTORE_DIRECTORY"</js>
       *       <li><js>"."</js>
       *    </ul>
       *    <br>Cannot be <jk>null</jk>.
       * @return This object.
       */
      public Builder directory(String value) {
         directory = reqnn("value", value);
         return this;
      }

      /**
       * Use watcher.
       *
       * <p>
       * Use a file system watcher for file system changes.
       *
       * <h4 class='section'>Notes:</h4><ul>
       *    <li class='note'>Calling {@link FileStore#close()} closes the watcher.
       * </ul>
       *
       * <p>
       *    The default is the first value found:
       *    <ul>
       *       <li>System property <js>"ConfigFileStore.enableWatcher"</js>
       *       <li>Environment variable <js>"CONFIGFILESTORE_ENABLEWATCHER"</js>
       *       <li><jk>false</jk>.
       *    </ul>
       *
       * @return This object.
       */
      public Builder enableWatcher() {
         enableWatcher = true;
         return this;
      }

      /**
       * Allows absolute file names (e.g. <js>"/etc/app/app.cfg"</js>) to be read and written outside the store directory.
       *
       * <p>
       * By default, every name must resolve inside the configured directory.
       *
       * @return This object.
       */
      public Builder allowAbsoluteNames() {
         allowAbsoluteNames = true;
         return this;
      }

      /**
       * File extensions.
       *
       * <p>
       * Defines what file extensions to search for when the config name does not have an extension.
       *
       * @param value
       *    The new value for this property.
       *    The default is the first value found:
       *    <ul>
       *       <li>System property <js>"ConfigFileStore.extensions"</js>
       *       <li>Environment variable <js>"CONFIGFILESTORE_EXTENSIONS"</js>
       *       <li><js>"cfg"</js>
       *    </ul>
       *    <br>Cannot be <jk>null</jk>.
       * @return This object.
       */
      public Builder extensions(String value) {
         extensions = reqnn("value", value);
         return this;
      }

      /**
       * Update-on-write.
       *
       * <p>
       * When enabled, the {@link FileStore#update(String, String)} method will be called immediately following
       * calls to {@link FileStore#write(String, String, String)} when the contents are changing.
       * <br>This allows for more immediate responses to configuration changes on file systems that use
       * polling watchers.
       * <br>This may cause double-triggering of {@link ConfigStoreListener ConfigStoreListeners}.
       *
       * <p>
       *    The default is the first value found:
       *    <ul>
       *       <li>System property <js>"ConfigFileStore.updateOnWrite"</js>
       *       <li>Environment variable <js>"CONFIGFILESTORE_UPDATEONWRITE"</js>
       *       <li><jk>false</jk>.
       *    </ul>
       *
       * @return This object.
       */
      public Builder updateOnWrite() {
         updateOnWrite = true;
         return this;
      }

      /**
       * Watcher sensitivity.
       *
       * <p>
       * Determines how frequently the file system is polled for updates.
       *
       * <h4 class='section'>Notes:</h4><ul>
       *    <li class='note'>This relies on internal Sun packages and may not work on all JVMs.
       * </ul>
       *
       * @param value
       *    The new value for this property.
       *    <br>The default is the first value found:
       *    <ul>
       *       <li>System property <js>"ConfigFileStore.watcherSensitivity"</js>
       *       <li>Environment variable <js>"CONFIGFILESTORE_WATCHERSENSITIVITY"</js>
       *       <li>{@link WatcherSensitivity#MEDIUM}
       *    </ul>
       *    <br>Cannot be <jk>null</jk>.
       * @return This object.
       */
      public Builder watcherSensitivity(WatcherSensitivity value) {
         watcherSensitivity = reqnn("value", value);
         return this;
      }
   }

   @SuppressWarnings({
      "java:S112" // throws Exception intentional - callback/lifecycle method
   })
   class WatcherThread extends Thread {
      private final WatchService watchService;

      WatcherThread(File dir, WatcherSensitivity s) throws Exception {
         watchService = FileSystems.getDefault().newWatchService();
         var kinds = a(ENTRY_CREATE, ENTRY_DELETE, ENTRY_MODIFY);
         var modifier = lookupModifier(s);
         dir.toPath().register(watchService, kinds, modifier);
      }

      @Override /* Overridden from Thread */
      public void interrupt() {
         try {
            watchService.close();
         } catch (IOException e) {
            throw toRex(e); // HTT - watchService.close() doesn't throw in normal operation
         } finally {
            super.interrupt();
         }
      }

      private void handleEvent(WatchEvent<Path> event) {
         try {
            FileStore.this.onFileEvent(event);
         } catch (@SuppressWarnings("unused") IOException | IllegalArgumentException e) {
            // The file or directory vanished (or became unreadable) between the event and its handling,
            // e.g. while the watched directory is being deleted.  Skip the event; don't kill the watcher.
         }
      }

      @SuppressWarnings({
         "unchecked" // Type erasure requires unchecked casts
      })
      @Override /* Overridden from Thread */
      public void run() {
         try {
            WatchKey key;
            while (nn(key = watchService.take())) {
               for (var event : key.pollEvents()) {
                  var kind = event.kind();
                  if (kind != OVERFLOW) // HTT - OVERFLOW events are rare in tests
                     handleEvent((WatchEvent<Path>)event);
               }
               if (! key.reset()) // HTT - key always resets in test environment
                  break;
            }
         } catch (@SuppressWarnings("unused") InterruptedException e) {
            Thread.currentThread().interrupt();
         } catch (@SuppressWarnings("unused") ClosedWatchServiceException e) {
            // Expected on shutdown: interrupt() closes the WatchService while take() is blocked in this loop.
         } catch (Exception e) {
            throw toRex(e); // HTT - unexpected exception from watchService.take()
         }
      }

      @SuppressWarnings({
         "java:S5738" // com.sun.nio.file.SensitivityWatchEventModifier is deprecated-for-removal but has no public JDK replacement for WatchService poll sensitivity; required for the watcher-sensitivity feature on JVMs that still honor it.
      })
      private WatchEvent.Modifier lookupModifier(WatcherSensitivity s) {
         try {
            return switch (s) {
               case LOW -> com.sun.nio.file.SensitivityWatchEventModifier.LOW;
               case MEDIUM -> com.sun.nio.file.SensitivityWatchEventModifier.MEDIUM;
               case HIGH -> com.sun.nio.file.SensitivityWatchEventModifier.HIGH;
            };
         } catch (@SuppressWarnings("unused") Exception e) {
            /* Ignore */ // HTT - SensitivityWatchEventModifier exists on all supported JVMs
         }
         return null; // HTT - return null is only reached if SensitivityWatchEventModifier is unavailable
      }
   }

   /** Default file store, all default values.*/
   public static final FileStore DEFAULT = FileStore.create().build();

   /**
    * Creates a new builder for this object.
    *
    * @return A new builder.
    */
   public static Builder create() {
      return new Builder();
   }

   protected final boolean allowAbsoluteNames;
   protected final boolean enableWatcher;
   protected final boolean updateOnWrite;
   protected final Charset charset;
   protected final String directory;
   protected final String extensions;
   protected final WatcherSensitivity watcherSensitivity;

   private final ConcurrentHashMap<String,String> cache = new ConcurrentHashMap<>();
   private final ConcurrentHashMap<String,String> nameCache = new ConcurrentHashMap<>();
   private final File dir;
   private final String[] exts;
   private final WatcherThread watcher;

   /**
    * Constructor.
    *
    * @param builder The builder for this object.
    *    <br>Cannot be <jk>null</jk>.
    */
   public FileStore(Builder builder) {
      super(builder);
      directory = builder.directory;
      extensions = builder.extensions;
      charset = builder.charset;
      allowAbsoluteNames = builder.allowAbsoluteNames;
      enableWatcher = builder.enableWatcher;
      updateOnWrite = builder.updateOnWrite;
      watcherSensitivity = builder.watcherSensitivity;
      try {
         dir = new File(directory).getCanonicalFile();
         dir.mkdirs();
         exts = split(extensions).toArray(String[]::new);
         watcher = enableWatcher ? new WatcherThread(dir, watcherSensitivity) : null;
         if (nn(watcher))
            watcher.start();
      } catch (Exception e) {
         throw toRex(e); // HTT - getCanonicalFile() IOException is not normally triggered
      }
   }

   @Override /* Overridden from Closeable */
   public synchronized void close() {
      if (nn(watcher))
         watcher.interrupt();
   }

   @Override /* Overridden from Context */
   public Builder copy() {
      return new Builder(this);
   }

   @Override /* Overridden from ConfigStore */
   public synchronized boolean exists(String name) {
      try {
         return Files.exists(resolveFile(name));
      } catch (@SuppressWarnings("unused") IllegalArgumentException e) {
         return false; // Relative names that escape the store directory are treated as not found.
      }
   }

   @Override /* Overridden from ConfigStore */
   public synchronized String read(String name) throws IOException {
      name = resolveName(name);

      var p = resolveFile(name);
      var pName = p.getFileName();
      if (n(pName))
         throw ioex("Could not resolve a file name from path '%s'.", p); // Real NPE guard: getFileName() can return null when p resolves to a root path (e.g. a caller-supplied absolute name).
      name = pName.toString();
      var key = p.toAbsolutePath().normalize().toString();

      var s = cache.get(key);
      if (nn(s))
         return s;

      dir.mkdirs();

      // If file doesn't exist, don't trigger creation.
      if (! Files.exists(p))
         return "";

      var isWritable = isWritable(p);
      var oo = isWritable ? a(READ, WRITE, CREATE) : a(READ); // HTT - read-only files require special OS setup
      try (var fc = FileChannel.open(p, oo)) {
         try (var lock = isWritable ? fc.lock() : null) { // HTT - null lock path requires read-only file
            var buf = ByteBuffer.allocate(1024);
            var sb = new StringBuilder();
            while (fc.read(buf) != -1) {
               sb.append(charset.decode((buf.flip()))); // Fixes Java 11 issue involving overridden flip method.
               buf.clear();
            }
            s = sb.toString();
            cache.put(key, s);
         }
      }

      return cache.get(key);
   }

   @Override /* Overridden from ConfigStore */
   public synchronized FileStore update(String name, String newContents) {
      cache.put(cacheKey(name), newContents);
      super.update(name, newContents);
      return this;
   }

   @Override /* Overridden from ConfigStore */
   public synchronized String write(String name, String expectedContents, String newContents) throws IOException {
      name = resolveName(name);

      // This is a no-op.
      if (eq(expectedContents, newContents))
         return null;

      dir.mkdirs();

      var p = resolveFile(name);
      var pName = p.getFileName();
      if (n(pName))
         throw ioex("Could not resolve a file name from path '%s'.", p); // Real NPE guard: getFileName() can return null when p resolves to a root path (e.g. a caller-supplied absolute name).
      name = pName.toString();
      var key = p.toAbsolutePath().normalize().toString();

      var exists = Files.exists(p);

      // Don't create the file if we're not going to match.
      if ((! exists) && ine(expectedContents))
         return "";

      if (isWritable(p)) {
         if (newContents == null)
            Files.delete(p);
         else {
            try (var fc = FileChannel.open(p, READ, WRITE, CREATE)) {
               try (var lock = fc.lock()) {
                  var currentContents = "";
                  if (exists) {
                     var buf = ByteBuffer.allocate(1024);
                     var sb = new StringBuilder();
                     while (fc.read(buf) != -1) {
                        sb.append(charset.decode(buf.flip()));
                        buf.clear();
                     }
                     currentContents = sb.toString();
                  }
                  if (nn(expectedContents) && neq(currentContents, expectedContents)) {
                     if (currentContents == null)
                        cache.remove(key);
                     else
                        cache.put(key, currentContents);
                     return currentContents;
                  }
                  fc.position(0);
                  fc.write(charset.encode(newContents));
                  fc.truncate(fc.position()); // Drop leftovers when the new contents are shorter.
               }
            }
         }
      }

      if (updateOnWrite)
         update(name, newContents);
      else
         cache.remove(key);  // Invalidate the cache.

      return null;
   }

   /*
    * Creates a new empty file readable and writable by its owner only, where the file system supports POSIX permissions.
    */
   private static boolean createOwnerOnly(Path p) throws IOException {
      try {
         Files.createFile(p, PosixFilePermissions.asFileAttribute(PosixFilePermissions.fromString("rw-------")));
         return true;
      } catch (@SuppressWarnings("unused") UnsupportedOperationException e) {
         return p.toFile().createNewFile(); // Non-POSIX file system.
      } catch (@SuppressWarnings("unused") FileAlreadyExistsException e) {
         return true;
      }
   }

   private synchronized boolean isWritable(Path p) {
      try {
         if (! Files.exists(p)) {
            Files.createDirectories(p.getParent());
            if (! Files.exists(p) && ! createOwnerOnly(p)) { // HTT - createNewFile() failure requires OS-level file system restrictions
               throw ioex("Could not create file: %s", p);
            }
         }
      } catch (@SuppressWarnings("unused") IOException e) {
         return false; // HTT - IOException from createDirectories/createNewFile requires OS-level restrictions
      }
      return Files.isWritable(p);
   }

   /*
    * Rejects relative names that resolve (lexically) outside the configured directory, e.g. "../x".
    * Absolute names are rejected unless the builder opted in with allowAbsoluteNames().
    * Done before any file-system probe so escaping names are never even tested for existence.
    */
   private void checkContained(String name) {
      try {
         if (Path.of(name).isAbsolute()) {
            if (allowAbsoluteNames)
               return;
            throw new IllegalArgumentException(MSG_PATH_ESCAPE);
         }
         var root = dir.toPath().toAbsolutePath().normalize();
         var target = root.resolve(name).normalize();
         if (! target.startsWith(root))
            throw new IllegalArgumentException(MSG_PATH_ESCAPE);
         // Symlinks: if the target (or its nearest existing ancestor) resolves outside the real root, reject.
         var existing = target;
         while (existing != null && ! Files.exists(existing))
            existing = existing.getParent();
         if (existing != null && Files.exists(root) && ! existing.toRealPath().startsWith(root.toRealPath()))
            throw new IllegalArgumentException(MSG_PATH_ESCAPE);
      } catch (@SuppressWarnings("unused") NoSuchFileException e) {
         // The path vanished between the existence probe and toRealPath() (e.g. concurrent delete).  Nothing left to
         // resolve through a symlink, so this is not an escape; the lexical check above already passed.
      } catch (@SuppressWarnings("unused") InvalidPathException | IOException e) {
         throw new IllegalArgumentException(MSG_PATH_ESCAPE);
      }
   }

   private String cacheKey(String name) {
      return resolveFile(name).toAbsolutePath().normalize().toString();
   }

   private Path resolveFile(String name) {
      return dir.toPath().resolve(resolveName(name));
   }

   /**
    * Gets called when the watcher service on this store is triggered with a file system change.
    *
    * @param e The file system event.
    * @throws IOException Thrown by underlying stream.
    */
   protected synchronized void onFileEvent(WatchEvent<Path> e) throws IOException {
      var fn = e.context().getFileName().toString();

      var key = cacheKey(fn);
      var oldContents = cache.get(key);
      cache.remove(key);
      var newContents = read(fn);

      if (neq(oldContents, newContents)) { // HTT - requires actual file system watcher event with content change
         update(fn, newContents);
      }
   }

   @Override /* Overridden from ConfigStore */
   protected FluentMap<String,Object> properties() {
      return super.properties()
         .a("charset", charset)
         .a("extensions", extensions)
         .a("updateOnWrite", updateOnWrite);
   }

   @Override
   protected String resolveName(String name) {
      checkContained(name);
      if (! nameCache.containsKey(name)) {
         String n = null;

         // Does file exist as-is?
         if (fileExists(dir, name))
            n = name;

         // Does name already have an extension?
         if (n == null) {
            for (var ext : exts) {
               if (hasExtension(name, ext)) {
                  n = name;
                  break;
               }
            }
         }

         // Find file with the correct extension.
         if (n == null) {
            for (var ext : exts) {
               if (fileExists(dir, name + '.' + ext)) {
                  n = name + '.' + ext;
                  break;
               }
            }
         }

         // If file not found, use the default which is the name with the first extension.
         if (n == null)
            n = exts.length == 0 ? name : (name + "." + exts[0]);

         nameCache.put(name, n);
      }
      return nameCache.get(name);
   }
}