// ***************************************************************************************************************************
// * 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.internal;

import static org.apache.juneau.internal.StringUtils.*;
import static org.apache.juneau.config.event.ConfigEventType.*;

import java.io.*;
import java.util.*;
import java.util.concurrent.*;
import java.util.concurrent.locks.*;

import org.apache.juneau.*;
import org.apache.juneau.collections.*;
import org.apache.juneau.config.event.*;
import org.apache.juneau.config.store.*;
import org.apache.juneau.internal.*;

/**
 * Represents the parsed contents of a configuration.
 */
public class ConfigMap implements ConfigStoreListener {

   private final ConfigStore store;         // The store that created this object.
   private volatile String contents;        // The original contents of this object.
   final String name;                       // The name  of this object.

   private final static AsciiSet MOD_CHARS = AsciiSet.create("#$%&*+^@~");

   // Changes that have been applied since the last load.
   private final List<ConfigEvent> changes = Collections.synchronizedList(new ConfigEvents());

   // Registered listeners listening for changes during saves or reloads.
   private final Set<ConfigEventListener> listeners = Collections.synchronizedSet(new HashSet<ConfigEventListener>());

   // The parsed entries of this map with all changes applied.
   final Map<String,ConfigSection> entries = Collections.synchronizedMap(new LinkedHashMap<String,ConfigSection>());

   // The original entries of this map before any changes were applied.
   final Map<String,ConfigSection> oentries = Collections.synchronizedMap(new LinkedHashMap<String,ConfigSection>());

   // Import statements in this config.
   final List<Import> imports = new CopyOnWriteArrayList<>();

   private final ReadWriteLock lock = new ReentrantReadWriteLock();

   /**
    * Constructor.
    *
    * @param store The config store.
    * @param name The config name.
    * @throws IOException Thrown by underlying stream.
    */
   public ConfigMap(ConfigStore store, String name) throws IOException {
      this.store = store;
      this.name = name;
      load(store.read(name));
   }

   ConfigMap(ConfigStore store, String name, String contents) throws IOException {
      this.store = store;
      this.name = name;
      load(contents);
   }

   private ConfigMap load(String contents) throws IOException {
      if (contents == null)
         contents = "";
      this.contents = contents;

      entries.clear();
      oentries.clear();
      for (Import ir : imports)
         ir.unregisterAll();
      imports.clear();

      Map<String,ConfigMap> imports = new LinkedHashMap<>();

      List<String> lines = new LinkedList<>();
      try (Scanner scanner = new Scanner(contents)) {
         while (scanner.hasNextLine()) {
            String line = scanner.nextLine();
            char c = firstChar(line);
            int c2 = StringUtils.lastNonWhitespaceChar(line);
            if (c == '[') {
               String l = line.trim();
               if (c2 != ']' || ! isValidNewSectionName(l.substring(1, l.length()-1)))
                  throw new ConfigException("Invalid section name found in configuration:  {0}", line);
            } else if (c == '<') {
               String l = line.trim();
               int i = l.indexOf('>');
               if (i != -1) {
                  String l2 = l.substring(1, i);
                  if (! isValidConfigName(l2))
                     throw new ConfigException("Invalid import config name found in configuration:  {0}", line);
                  String l3 = l.substring(i+1);
                  if (! (isEmpty(l3) || firstChar(l3) == '#'))
                     throw new ConfigException("Invalid import config name found in configuration:  {0}", line);
                  String importName = l2.trim();
                  try {
                     if (! imports.containsKey(importName))
                        imports.put(importName, store.getMap(importName));
                  } catch (StackOverflowError e) {
                     throw new IOException("Import loop detected in configuration '"+name+"'->'"+importName+"'");
                  }
               }
            }
            lines.add(line);
         }
      }

      List<Import> irl = new ArrayList<>(imports.size());
      for (ConfigMap ic : AList.of(imports.values()).riterable())
         irl.add(new Import(ic).register(listeners));
      this.imports.addAll(irl);

      // Add [blank] section.
      boolean inserted = false;
      boolean foundComment = false;
      for (ListIterator<String> li = lines.listIterator(); li.hasNext();) {
         String l = li.next();
         char c = firstNonWhitespaceChar(l);
         if (c != '#') {
            if (c == 0 && foundComment) {
               li.set("[]");
               inserted = true;
            }
            break;
         }
         foundComment = true;
      }
      if (! inserted)
         lines.add(0, "[]");

      // Collapse any multi-lines.
      ListIterator<String> li = lines.listIterator(lines.size());
      String accumulator = null;
      while (li.hasPrevious()) {
         String l = li.previous();
         char c = firstChar(l);
         if (c == '\t') {
            c = firstNonWhitespaceChar(l);
            if (c != '#') {
               if (accumulator == null)
                  accumulator = l.substring(1);
               else
                  accumulator = l.substring(1) + "\n" + accumulator;
               li.remove();
            }
         } else if (accumulator != null) {
            li.set(l + "\n" + accumulator);
            accumulator = null;
         }
      }

      lines = new ArrayList<>(lines);
      int last = lines.size()-1;
      int S1 = 1; // Looking for section.
      int S2 = 2; // Found section, looking for start.
      int state = S1;

      List<ConfigSection> sections = new ArrayList<>();

      for (int i = last; i >= 0; i--) {
         String l = lines.get(i);
         char c = firstChar(l);

         if (state == S1) {
            if (c == '[') {
               state = S2;
            }
         } else {
            if (c != '#' && (c == '[' || l.indexOf('=') != -1)) {
               sections.add(new ConfigSection(lines.subList(i+1, last+1)));
               last = i + 1;// (c == '[' ? i+1 : i);
               state = (c == '[' ? S2 : S1);
            }
         }
      }

      sections.add(new ConfigSection(lines.subList(0, last+1)));

      for (int i = sections.size() - 1; i >= 0; i--) {
         ConfigSection cs = sections.get(i);
         if (entries.containsKey(cs.name))
            throw new ConfigException("Duplicate section found in configuration:  [{0}]", cs.name);
         entries.put(cs.name, cs);
       }

      oentries.putAll(entries);
      return this;
   }

   //-----------------------------------------------------------------------------------------------------------------
   // Getters
   //-----------------------------------------------------------------------------------------------------------------

   /**
    * Reads an entry from this map.
    *
    * @param section
    *    The section name.
    *    <br>Must not be <jk>null</jk>.
    *    <br>Use blank to refer to the default section.
    * @param key
    *    The entry key.
    *    <br>Must not be <jk>null</jk>.
    * @return The entry, or <jk>null</jk> if the entry doesn't exist.
    */
   public ConfigEntry getEntry(String section, String key) {
      checkSectionName(section);
      checkKeyName(key);
      readLock();
      try {
         ConfigSection cs = entries.get(section);
         ConfigEntry ce = cs == null ? null : cs.entries.get(key);

         if (ce == null) {
            for (Import i : imports) {
               ce = i.getConfigMap().getEntry(section, key);
               if (ce != null)
                  break;
            }
         }

         return ce;
      } finally {
         readUnlock();
      }
   }

   /**
    * Returns the pre-lines on the specified section.
    *
    * <p>
    * The pre-lines are all lines such as blank lines and comments that preceed a section.
    *
    * @param section
    *    The section name.
    *    <br>Must not be <jk>null</jk>.
    *    <br>Use blank to refer to the default section.
    * @return
    *    An unmodifiable list of lines, or <jk>null</jk> if the section doesn't exist.
    */
   public List<String> getPreLines(String section) {
      checkSectionName(section);
      readLock();
      try {
         ConfigSection cs = entries.get(section);
         return cs == null ? null : cs.preLines;
      } finally {
         readUnlock();
      }
   }

   /**
    * Returns the keys of the entries in the specified section.
    *
    * @return
    *    An unmodifiable set of keys.
    */
   public Set<String> getSections() {
      Set<String> s = null;
      if (imports.isEmpty()) {
         s = entries.keySet();
      } else {
         s = new LinkedHashSet<>();
         for (Import ir : imports)
            s.addAll(ir.getConfigMap().getSections());
         s.addAll(entries.keySet());
      }
      return Collections.unmodifiableSet(s);
   }

   /**
    * Returns the keys of the entries in the specified section.
    *
    * @param section
    *    The section name.
    *    <br>Must not be <jk>null</jk>.
    *    <br>Use blank to refer to the default section.
    * @return
    *    An unmodifiable set of keys, or an empty set if the section doesn't exist.
    */
   public Set<String> getKeys(String section) {
      checkSectionName(section);
      Set<String> s = null;
      ConfigSection cs = entries.get(section);
      if (imports.isEmpty()) {
         s = cs == null ? Collections.<String>emptySet() : cs.entries.keySet();
      } else {
         s = new LinkedHashSet<>();
         for (Import i : imports)
            s.addAll(i.getConfigMap().getKeys(section));
         if (cs != null)
            s.addAll(cs.entries.keySet());
      }
      return Collections.unmodifiableSet(s);
   }

   /**
    * Returns <jk>true</jk> if this config has the specified section.
    *
    * @param section
    *    The section name.
    *    <br>Must not be <jk>null</jk>.
    *    <br>Use blank to refer to the default section.
    * @return <jk>true</jk> if this config has the specified section.
    */
   public boolean hasSection(String section) {
      checkSectionName(section);
      for (Import i : imports)
         if (i.getConfigMap().hasSection(section))
            return true;
      return entries.get(section) != null;
   }

   //-----------------------------------------------------------------------------------------------------------------
   // Setters
   //-----------------------------------------------------------------------------------------------------------------

   /**
    * Adds a new section or replaces the pre-lines on an existing section.
    *
    * @param section
    *    The section name.
    *    <br>Must not be <jk>null</jk>.
    *    <br>Use blank to refer to the default section.
    * @param preLines
    *    The pre-lines on the section.
    *    <br>If <jk>null</jk>, the previous value will not be overwritten.
    * @return This object (for method chaining).
    */
   public ConfigMap setSection(String section, List<String> preLines) {
      checkSectionName(section);
      return applyChange(true, ConfigEvent.setSection(name, section, preLines));
   }

   /**
    * Adds or overwrites an existing entry.
    *
    * @param section
    *    The section name.
    *    <br>Must not be <jk>null</jk>.
    *    <br>Use blank to refer to the default section.
    * @param key
    *    The entry key.
    *    <br>Must not be <jk>null</jk>.
    * @param value
    *    The entry value.
    *    <br>If <jk>null</jk>, the previous value will not be overwritten.
    * @param modifiers
    *    Optional modifiers.
    *    <br>If <jk>null</jk>, the previous value will not be overwritten.
    * @param comment
    *    Optional comment.
    *    <br>If <jk>null</jk>, the previous value will not be overwritten.
    * @param preLines
    *    Optional pre-lines.
    *    <br>If <jk>null</jk>, the previous value will not be overwritten.
    * @return This object (for method chaining).
    */
   public ConfigMap setEntry(String section, String key, String value, String modifiers, String comment, List<String> preLines) {
      checkSectionName(section);
      checkKeyName(key);
      if (modifiers != null && ! MOD_CHARS.containsOnly(modifiers))
         throw new ConfigException("Invalid modifiers: {0}", modifiers);
      return applyChange(true, ConfigEvent.setEntry(name, section, key, value, modifiers, comment, preLines));
   }


   /**
    * Not implemented.
    *
    * @param section
    *    The section name where to place the import statement.
    *    <br>Must not be <jk>null</jk>.
    *    <br>Use blank for the default section.
    * @param importName
    *    The import name.
    *    <br>Must not be <jk>null</jk>.
    * @param preLines
    *    Optional comment and blank lines to add immediately before the import statement.
    *    <br>If <jk>null</jk>, previous pre-lines will not be replaced.
    * @return This object (for method chaining).
    */
   public ConfigMap setImport(String section, String importName, List<String> preLines) {
      throw new UnsupportedOperationException();
   }

   /**
    * Removes a section.
    *
    * <p>
    * This eliminates all entries in the section as well.
    *
    * @param section
    *    The section name.
    *    <br>Must not be <jk>null</jk>.
    *    <br>Use blank to refer to the default section.
    * @return This object (for method chaining).
    */
   public ConfigMap removeSection(String section) {
      checkSectionName(section);
      return applyChange(true, ConfigEvent.removeSection(name, section));
   }

   /**
    * Removes an entry.
    *
    * @param section
    *    The section name.
    *    <br>Must not be <jk>null</jk>.
    *    <br>Use blank to refer to the default section.
    * @param key
    *    The entry key.
    *    <br>Must not be <jk>null</jk>.
    * @return This object (for method chaining).
    */
   public ConfigMap removeEntry(String section, String key) {
      checkSectionName(section);
      checkKeyName(key);
      return applyChange(true, ConfigEvent.removeEntry(name, section, key));
   }

   /**
    * Not implemented.
    *
    * @param section
    *    The section name where to place the import statement.
    *    <br>Must not be <jk>null</jk>.
    *    <br>Use blank for the default section.
    * @param importName
    *    The import name.
    *    <br>Must not be <jk>null</jk>.
    * @return This object (for method chaining).
    */
   public ConfigMap removeImport(String section, String importName) {
      throw new UnsupportedOperationException();
   }

   private ConfigMap applyChange(boolean addToChangeList, ConfigEvent ce) {
      if (ce == null)
         return this;
      writeLock();
      try {
         String section = ce.getSection();
         ConfigSection cs = entries.get(section);
         if (ce.getType() == SET_ENTRY) {
            if (cs == null) {
               cs = new ConfigSection(section);
               entries.put(section, cs);
            }
            ConfigEntry oe = cs.entries.get(ce.getKey());
            if (oe == null)
               oe = ConfigEntry.NULL;
            cs.addEntry(
               ce.getKey(),
               ce.getValue() == null ? oe.value : ce.getValue(),
               ce.getModifiers() == null ? oe.modifiers : ce.getModifiers(),
               ce.getComment() == null ? oe.comment : ce.getComment(),
               ce.getPreLines() == null ? oe.preLines : ce.getPreLines()
            );
         } else if (ce.getType() == SET_SECTION) {
            if (cs == null) {
               cs = new ConfigSection(section);
               entries.put(section, cs);
            }
            if (ce.getPreLines() != null)
               cs.setPreLines(ce.getPreLines());
         } else if (ce.getType() == REMOVE_ENTRY) {
            if (cs != null)
               cs.entries.remove(ce.getKey());
         } else if (ce.getType() == REMOVE_SECTION) {
            if (cs != null)
               entries.remove(section);
         }
         if (addToChangeList)
            changes.add(ce);
      } finally {
         writeUnlock();
      }
      return this;
   }

   /**
    * Overwrites the contents of the config file.
    *
    * @param contents The new contents of the config file.
    * @param synchronous Wait until the change has been persisted before returning this map.
    * @return This object (for method chaining).
    * @throws IOException Thrown by underlying stream.
    * @throws InterruptedException Thread was interrupted.
    */
   public ConfigMap load(String contents, boolean synchronous) throws IOException, InterruptedException {

      if (synchronous) {
         final CountDownLatch latch = new CountDownLatch(1);
         ConfigStoreListener l = new ConfigStoreListener() {
            @Override
            public void onChange(String contents) {
               latch.countDown();
            }
         };
         store.register(name, l);
         store.write(name, null, contents);
         latch.await(30, TimeUnit.SECONDS);
         store.unregister(name, l);
      } else {
         store.write(name, null, contents);
      }
      return this;
   }

   //-----------------------------------------------------------------------------------------------------------------
   // Lifecycle events
   //-----------------------------------------------------------------------------------------------------------------

   /**
    * Persist any changes made to this map and signal all listeners.
    *
    * <p>
    * If the underlying contents of the file have changed, this will reload it and apply the changes
    * on top of the modified file.
    *
    * <p>
    * Subsequent changes made to the underlying file will also be signaled to all listeners.
    *
    * <p>
    * We try saving the file up to 10 times.
    * <br>If the file keeps changing on the file system, we throw an exception.
    *
    * @return This object (for method chaining).
    * @throws IOException Thrown by underlying stream.
    */
   public ConfigMap commit() throws IOException {
      writeLock();
      try {
         String newContents = asString();
         for (int i = 0; i <= 10; i++) {
            if (i == 10)
               throw new ConfigException("Unable to store contents of config to store.");
            String currentContents = store.write(name, contents, newContents);
            if (currentContents == null)
               break;
            onChange(currentContents);
         }
         this.changes.clear();
      } finally {
         writeUnlock();
      }
      return this;
   }


   //-----------------------------------------------------------------------------------------------------------------
   // Listeners
   //-----------------------------------------------------------------------------------------------------------------

   /**
    * Registers an event listener on this map.
    *
    * @param listener The new listener.
    * @return This object (for method chaining).
    */
   public ConfigMap register(ConfigEventListener listener) {
      listeners.add(listener);
      for (Import ir : imports)
         ir.register(listener);
      return this;
   }

   boolean hasEntry(String section, String key) {
      ConfigSection cs = entries.get(section);
      ConfigEntry ce = cs == null ? null : cs.entries.get(key);
      return ce != null;
   }

   /**
    * Unregisters an event listener from this map.
    *
    * @param listener The listener to remove.
    * @return This object (for method chaining).
    */
   public ConfigMap unregister(ConfigEventListener listener) {
      listeners.remove(listener);
      for (Import ir : imports)
         ir.register(listener);
      return this;
   }

   /**
    * Returns the listeners currently associated with this config map.
    *
    * @return The listeners currently associated with this config map.
    */
   public Set<ConfigEventListener> getListeners() {
      return Collections.unmodifiableSet(listeners);
   }

   @Override /* ConfigStoreListener */
   public void onChange(String newContents) {
      ConfigEvents changes = null;
      writeLock();
      try {
         if (! StringUtils.isEquals(contents, newContents)) {
            changes = findDiffs(newContents);
            load(newContents);

            // Reapply our changes on top of the modifications.
            for (ConfigEvent ce : this.changes)
               applyChange(false, ce);
         }
      } catch (IOException e) {
         throw new RuntimeException(e);
      } finally {
         writeUnlock();
      }
      if (changes != null && ! changes.isEmpty())
         signal(changes);
   }

   @Override /* Object */
   public String toString() {
      readLock();
      try {
         return asString();
      } finally {
         readUnlock();
      }
   }

   /**
    * Returns the values in this config map as a map of maps.
    *
    * <p>
    * This is considered a snapshot copy of the config map.
    *
    * <p>
    * The returned map is modifiable, but modifications to the returned map are not reflected in the config map.
    *
    * @return A copy of this config as a map of maps.
    */
   public OMap asMap() {
      OMap m = new OMap();
      readLock();
      try {
         for (Import i : imports)
            m.putAll(i.getConfigMap().asMap());
         for (ConfigSection cs : entries.values()) {
            Map<String,String> m2 = new LinkedHashMap<>();
            for (ConfigEntry ce : cs.entries.values())
               m2.put(ce.key, ce.value);
            m.put(cs.name, m2);
         }
      } finally {
         readUnlock();
      }
      return m;
   }

   /**
    * Serializes this map to the specified writer.
    *
    * @param w The writer to serialize to.
    * @return The same writer passed in.
    * @throws IOException Thrown by underlying stream.
    */
   public Writer writeTo(Writer w) throws IOException {
      readLock();
      try {
         for (ConfigSection cs : entries.values())
            cs.writeTo(w);
      } finally {
         readUnlock();
      }
      return w;
   }

   /**
    * Does a rollback of any changes on this map currently in memory.
    *
    * @return This object (for method chaining).
    */
   public ConfigMap rollback() {
      if (changes.size() > 0) {
         writeLock();
         try {
            changes.clear();
            load(contents);
         } catch (IOException e) {
            throw new RuntimeException(e);
         } finally {
            writeUnlock();
         }
      }
      return this;
   }


   //-----------------------------------------------------------------------------------------------------------------
   // Private methods
   //-----------------------------------------------------------------------------------------------------------------

   private void readLock() {
      lock.readLock().lock();
   }

   private void readUnlock() {
      lock.readLock().unlock();
   }

   private void writeLock() {
      lock.writeLock().lock();
   }

   private void writeUnlock() {
      lock.writeLock().unlock();
   }

   private void checkSectionName(String s) {
      if (! ("".equals(s) || isValidNewSectionName(s)))
         throw new IllegalArgumentException("Invalid section name: '" + s + "'");
   }

   private void checkKeyName(String s) {
      if (! isValidKeyName(s))
         throw new IllegalArgumentException("Invalid key name: '" + s + "'");
   }

   private boolean isValidKeyName(String s) {
      if (s == null)
         return false;
      s = s.trim();
      if (s.isEmpty())
         return false;
      for (int i = 0; i < s.length(); i++) {
         char c = s.charAt(i);
         if (c == '/' || c == '\\' || c == '[' || c == ']' || c == '=' || c == '#')
            return false;
      }
      return true;
   }

   private boolean isValidNewSectionName(String s) {
      if (s == null)
         return false;
      s = s.trim();
      if (s.isEmpty())
         return false;
      for (int i = 0; i < s.length(); i++) {
         char c = s.charAt(i);
         if (c == '/' || c == '\\' || c == '[' || c == ']')
            return false;
      }
      return true;
   }

   private boolean isValidConfigName(String s) {
      if (s == null)
         return false;
      s = s.trim();
      if (s.isEmpty())
         return false;
      for (int i = 0; i < s.length(); i++) {
         char c = s.charAt(i);
         if (i == 0) {
            if (! Character.isJavaIdentifierStart(c))
               return false;
         } else {
            if (! Character.isJavaIdentifierPart(c))
               return false;
         }
      }
      return true;
   }

   private void signal(ConfigEvents changes) {
      if (changes.size() > 0)
         for (ConfigEventListener l : listeners)
            l.onConfigChange(changes);
   }

   private ConfigEvents findDiffs(String updatedContents) throws IOException {
      ConfigEvents changes = new ConfigEvents();
      ConfigMap newMap = new ConfigMap(store, name, updatedContents);

      // Imports added.
      for (Import i : newMap.imports) {
         if (! imports.contains(i)) {
            for (ConfigSection s : i.getConfigMap().entries.values()) {
               for (ConfigEntry e : s.oentries.values()) {
                  if (! newMap.hasEntry(s.name, e.key)) {
                     changes.add(ConfigEvent.setEntry(name, s.name, e.key, e.value, e.modifiers, e.comment, e.preLines));
                  }
               }
            }
         }
      }

      // Imports removed.
      for (Import i : imports) {
         if (! newMap.imports.contains(i)) {
            for (ConfigSection s : i.getConfigMap().entries.values()) {
               for (ConfigEntry e : s.oentries.values()) {
                  if (! newMap.hasEntry(s.name, e.key)) {
                     changes.add(ConfigEvent.removeEntry(name, s.name, e.key));
                  }
               }
            }
         }
      }

      for (ConfigSection ns : newMap.oentries.values()) {
         ConfigSection s = oentries.get(ns.name);
         if (s == null) {
            //changes.add(ConfigEvent.setSection(ns.name, ns.preLines));
            for (ConfigEntry ne : ns.entries.values()) {
               changes.add(ConfigEvent.setEntry(name, ns.name, ne.key, ne.value, ne.modifiers, ne.comment, ne.preLines));
            }
         } else {
            for (ConfigEntry ne : ns.oentries.values()) {
               ConfigEntry e = s.oentries.get(ne.key);
               if (e == null || ! isEquals(e.value, ne.value)) {
                  changes.add(ConfigEvent.setEntry(name, s.name, ne.key, ne.value, ne.modifiers, ne.comment, ne.preLines));
               }
            }
            for (ConfigEntry e : s.oentries.values()) {
               ConfigEntry ne = ns.oentries.get(e.key);
               if (ne == null) {
                  changes.add(ConfigEvent.removeEntry(name, s.name, e.key));
               }
            }
         }
      }

      for (ConfigSection s : oentries.values()) {
         ConfigSection ns = newMap.oentries.get(s.name);
         if (ns == null) {
            //changes.add(ConfigEvent.removeSection(s.name));
            for (ConfigEntry e : s.oentries.values())
               changes.add(ConfigEvent.removeEntry(name, s.name, e.key));
         }
      }

      return changes;
   }

   // This method should only be called from behind a lock.
   private String asString() {
      try {
         StringWriter sw = new StringWriter();
         for (ConfigSection cs : entries.values())
            cs.writeTo(sw);
         return sw.toString();
      } catch (IOException e) {
         throw new RuntimeException(e);  // Not possible.
      }
   }


   //---------------------------------------------------------------------------------------------
   // ConfigSection
   //---------------------------------------------------------------------------------------------

   class ConfigSection {

      final String name;   // The config section name, or blank if the default section.  Never null.

      final List<String> preLines = Collections.synchronizedList(new ArrayList<String>());
      private final String rawLine;

      final Map<String,ConfigEntry> oentries = Collections.synchronizedMap(new LinkedHashMap<String,ConfigEntry>());
      final Map<String,ConfigEntry> entries = Collections.synchronizedMap(new LinkedHashMap<String,ConfigEntry>());

      /**
       * Constructor.
       */
      ConfigSection(String name) {
         this.name = name;
         this.rawLine = "[" + name + "]";
      }

      /**
       * Constructor.
       */
      ConfigSection(List<String> lines) {

         String name = null, rawLine = null;

         int S1 = 1; // Looking for section.
         int S2 = 2; // Found section, looking for end.
         int state = S1;
         int start = 0;

         for (int i = 0; i < lines.size(); i++) {
            String l = lines.get(i);
            char c = StringUtils.firstNonWhitespaceChar(l);
            if (state == S1) {
               if (c == '[') {
                  int i1 = l.indexOf('['), i2 = l.indexOf(']');
                  name = l.substring(i1+1, i2).trim();
                  rawLine = l;
                  state = S2;
                  start = i+1;
               } else {
                  preLines.add(l);
               }
            } else {
               if (c != '#' && l.indexOf('=') != -1) {
                  ConfigEntry e = new ConfigEntry(l, lines.subList(start, i));
                  if (entries.containsKey(e.key))
                     throw new ConfigException("Duplicate entry found in section [{0}] of configuration:  {1}", name, e.key);
                  entries.put(e.key, e);
                  start = i+1;
               }
            }
         }

         this.name = name;
         this.rawLine = rawLine;
         this.oentries.putAll(entries);
      }

      ConfigSection addEntry(String key, String value, String modifiers, String comment, List<String> preLines) {
         ConfigEntry e = new ConfigEntry(key, value, modifiers, comment, preLines);
         this.entries.put(e.key, e);
         return this;
      }

      ConfigSection setPreLines(List<String> preLines) {
         this.preLines.clear();
         this.preLines.addAll(preLines);
         return this;
      }

      Writer writeTo(Writer w) throws IOException {
         for (String s : preLines)
            w.append(s).append('\n');

         if (! name.equals(""))
            w.append(rawLine).append('\n');
         else {
            // Need separation between default prelines and first-entry prelines.
            if (! preLines.isEmpty())
               w.append('\n');
         }

         for (ConfigEntry e : entries.values())
            e.writeTo(w);

         return w;
      }
   }


   //---------------------------------------------------------------------------------------------
   // Import
   //---------------------------------------------------------------------------------------------

   class Import {

      private final ConfigMap configMap;
      private final Map<ConfigEventListener,ConfigEventListener> listenerMap = Collections.synchronizedMap(new LinkedHashMap<>());

      Import(ConfigMap configMap) {
         this.configMap = configMap;
      }

      synchronized Import register(Collection<ConfigEventListener> listeners) {
         for (ConfigEventListener l : listeners)
            register(l);
         return this;
      }

      synchronized Import register(final ConfigEventListener listener) {
         ConfigEventListener l2 = new ConfigEventListener() {
            @Override
            public void onConfigChange(ConfigEvents events) {
               ConfigEvents events2 = new ConfigEvents();
               for (ConfigEvent cev : events) {
                  if (! hasEntry(cev.getSection(), cev.getKey()))
                     events2.add(cev);
               }
               if (events2.size() > 0)
                  listener.onConfigChange(events2);
            }
         };
         listenerMap.put(listener, l2);
         configMap.register(l2);
         return this;
      }

      synchronized Import unregister(final ConfigEventListener listener) {
         configMap.unregister(listenerMap.remove(listener));
         return this;
      }

      synchronized Import unregisterAll() {
         for (ConfigEventListener l : listenerMap.values())
            configMap.unregister(l);
         listenerMap.clear();
         return this;
      }

      String getConfigName() {
         return configMap.name;
      }

      ConfigMap getConfigMap() {
         return configMap;
      }

      @Override
      public boolean equals(Object o) {
         if (o instanceof Import) {
            Import ir = (Import)o;
            if (ir.getConfigName().equals(getConfigName()))
               return true;
         }
         return false;
      }

      @Override
      public int hashCode() {
         return getConfigName().hashCode();
      }
   }
}