/*
 * 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.commons.configuration2;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.PrintWriter;
import java.io.Reader;
import java.io.Writer;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;

import org.apache.commons.configuration2.convert.ListDelimiterHandler;
import org.apache.commons.configuration2.ex.ConfigurationException;
import org.apache.commons.configuration2.ex.ConfigurationRuntimeException;
import org.apache.commons.configuration2.tree.ImmutableNode;
import org.apache.commons.configuration2.tree.InMemoryNodeModel;
import org.apache.commons.configuration2.tree.InMemoryNodeModelSupport;
import org.apache.commons.configuration2.tree.NodeHandler;
import org.apache.commons.configuration2.tree.NodeHandlerDecorator;
import org.apache.commons.configuration2.tree.NodeSelector;
import org.apache.commons.configuration2.tree.TrackedNodeModel;

/**
 * <p>
 * A specialized hierarchical configuration implementation for parsing ini files.
 * </p>
 * <p>
 * An initialization or ini file is a configuration file typically found on Microsoft's Windows operating system and
 * contains data for Windows based applications.
 * </p>
 * <p>
 * Although popularized by Windows, ini files can be used on any system or platform due to the fact that they are merely
 * text files that can easily be parsed and modified by both humans and computers.
 * </p>
 * <p>
 * A typical ini file could look something like:
 * </p>
 *
 * <pre>
 * [section1]
 * ; this is a comment!
 * var1 = foo
 * var2 = bar
 *
 * [section2]
 * var1 = doo
 * </pre>
 * <p>
 * The format of ini files is fairly straight forward and is composed of three components:
 * </p>
 * <ul>
 * <li><b>Sections:</b> Ini files are split into sections, each section starting with a section declaration. A section
 * declaration starts with a '[' and ends with a ']'. Sections occur on one line only.</li>
 * <li><b>Parameters:</b> Items in a section are known as parameters. Parameters have a typical {@code key = value}
 * format.</li>
 * <li><b>Comments:</b> Lines starting with a ';' are assumed to be comments.</li>
 * </ul>
 * <p>
 * There are various implementations of the ini file format by various vendors which has caused a number of differences
 * to appear. As far as possible this configuration tries to be lenient and support most of the differences.
 * </p>
 * <p>
 * Some of the differences supported are as follows:
 * </p>
 * <ul>
 * <li><b>Comments:</b> The '#' character is also accepted as a comment signifier.</li>
 * <li><b>Key value separator:</b> The ':' character is also accepted in place of '=' to separate keys and values in
 * parameters, for example {@code var1 : foo}.</li>
 * <li><b>Duplicate sections:</b> Typically duplicate sections are not allowed, this configuration does however support
 * this feature. In the event of a duplicate section, the two section's values are merged so that there is only a single
 * section. <strong>Note</strong>: This also affects the internal data of the configuration. If it is saved, only a
 * single section is written!</li>
 * <li><b>Duplicate parameters:</b> Typically duplicate parameters are only allowed if they are in two different
 * sections, thus they are local to sections; this configuration simply merges duplicates; if a section has a duplicate
 * parameter the values are then added to the key as a list.</li>
 * </ul>
 * <p>
 * Global parameters are also allowed; any parameters declared before a section is declared are added to a global
 * section. It is important to note that this global section does not have a name.
 * </p>
 * <p>
 * In all instances, a parameter's key is prepended with its section name and a '.' (period). Thus a parameter named
 * "var1" in "section1" will have the key {@code section1.var1} in this configuration. (This is the default behavior.
 * Because this is a hierarchical configuration you can change this by setting a different
 * {@link org.apache.commons.configuration2.tree.ExpressionEngine}.)
 * </p>
 * <p>
 * <strong>Implementation Details:</strong> Consider the following ini file:
 * </p>
 * <pre>
 *  default = ok
 *
 *  [section1]
 *  var1 = foo
 *  var2 = doodle
 *
 *  [section2]
 *  ; a comment
 *  var1 = baz
 *  var2 = shoodle
 *  bad =
 *  = worse
 *
 *  [section3]
 *  # another comment
 *  var1 : foo
 *  var2 : bar
 *  var5 : test1
 *
 *  [section3]
 *  var3 = foo
 *  var4 = bar
 *  var5 = test2
 *
 *  [sectionSeparators]
 *  passwd : abc=def
 *  a:b = "value"
 * </pre>
 * <p>
 * This ini file will be parsed without error. Note:
 * </p>
 * <ul>
 * <li>The parameter named "default" is added to the global section, it's value is accessed simply using
 * {@code getProperty("default")}.</li>
 * <li>Section 1's parameters can be accessed using {@code getProperty("section1.var1")}.</li>
 * <li>The parameter named "bad" simply adds the parameter with an empty value.</li>
 * <li>The empty key with value "= worse" is added using a key consisting of a single space character. This key is still
 * added to section 2 and the value can be accessed using {@code getProperty("section2. ")}, notice the period '.' and
 * the space following the section name.</li>
 * <li>Section three uses both '=' and ':' to separate keys and values.</li>
 * <li>Section 3 has a duplicate key named "var5". The value for this key is [test1, test2], and is represented as a
 * List.</li>
 * <li>The section called <em>sectionSeparators</em> demonstrates how the configuration deals with multiple occurrences
 * of separator characters. Per default the first separator character in a line is detected and used to split the key
 * from the value. Therefore the first property definition in this section has the key {@code passwd} and the value
 * {@code abc=def}. This default behavior can be changed by using quotes. If there is a separator character before the
 * first quote character (ignoring whitespace), this character is used as separator. Thus the second property definition
 * in the section has the key {@code a:b} and the value {@code value}.</li>
 * </ul>
 * <p>
 * Internally, this configuration maps the content of the represented ini file to its node structure in the following
 * way:
 * </p>
 * <ul>
 * <li>Sections are represented by direct child nodes of the root node.</li>
 * <li>For the content of a section, corresponding nodes are created as children of the section node.</li>
 * </ul>
 * <p>
 * This explains how the keys for the properties can be constructed. You can also use other methods of
 * {@link HierarchicalConfiguration} for querying or manipulating the hierarchy of configuration nodes, for instance the
 * {@code configurationAt()} method for obtaining the data of a specific section. However, be careful that the storage
 * scheme described above is not violated (e.g. by adding multiple levels of nodes or inserting duplicate section
 * nodes). Otherwise, the special methods for ini configurations may not work correctly!
 * </p>
 * <p>
 * The set of sections in this configuration can be retrieved using the {@code getSections()} method. For obtaining a
 * {@code SubnodeConfiguration} with the content of a specific section the {@code getSection()} method can be used.
 * </p>
 * <p>
 * Like other {@code Configuration} implementations, this class uses a {@code Synchronizer} object to control concurrent
 * access. By choosing a suitable implementation of the {@code Synchronizer} interface, an instance can be made
 * thread-safe or not. Note that access to most of the properties typically set through a builder is not protected by
 * the {@code Synchronizer}. The intended usage is that these properties are set once at construction time through the
 * builder and after that remain constant. If you wish to change such properties during life time of an instance, you
 * have to use the {@code lock()} and {@code unlock()} methods manually to ensure that other threads see your changes.
 * </p>
 * <p>
 * As this class extends {@link AbstractConfiguration}, all basic features like variable interpolation, list handling,
 * or data type conversions are available as well. This is described in the chapter
 * <a href="https://commons.apache.org/proper/commons-configuration/userguide/howto_basicfeatures.html"> Basic features
 * and AbstractConfiguration</a> of the user's guide.
 * </p>
 * <p>
 * Note that this configuration does not support properties with null values. Such properties are considered to be
 * section nodes.
 * </p>
 *
 * @since 1.6
 */
public class INIConfiguration extends BaseHierarchicalConfiguration implements FileBasedConfiguration {

    /**
     * The default characters that signal the start of a comment line.
     */
    protected static final String COMMENT_CHARS = "#;";

    /**
     * The default characters used to separate keys from values.
     */
    protected static final String SEPARATOR_CHARS = "=:";

    /**
     * Constant for the line separator.
     */
    private static final String LINE_SEPARATOR = System.lineSeparator();

    /**
     * The characters used for quoting values.
     */
    private static final String QUOTE_CHARACTERS = "\"'";

    /**
     * The line continuation character.
     */
    private static final String LINE_CONT = "\\";

    /**
     * The separator used when writing an INI file.
     */
    private String separatorUsedInOutput = " = ";

    /**
     * The separator used when reading an INI file.
     */
    private String separatorUsedInInput = SEPARATOR_CHARS;

    /**
     * The characters used to separate keys from values when reading an INI file.
     */
    private String commentCharsUsedInInput = COMMENT_CHARS;

    /**
     * The flag for decision, whether inline comments on the section line are allowed.
     */
    private boolean sectionInLineCommentsAllowed;

    /**
     * Create a new empty INI Configuration.
     */
    public INIConfiguration() {
    }

    /**
     * Creates a new instance of {@code INIConfiguration} with the content of the specified
     * {@code HierarchicalConfiguration}.
     *
     * @param c the configuration to be copied
     * @since 2.0
     */
    public INIConfiguration(final HierarchicalConfiguration<ImmutableNode> c) {
        super(c);
    }

    /**
     * Create a new empty INI Configuration with option to allow inline comments on the section line.
     *
     * @param sectionInLineCommentsAllowed when true inline comments on the section line are allowed
     */
    private INIConfiguration(final boolean sectionInLineCommentsAllowed) {
        this.sectionInLineCommentsAllowed = sectionInLineCommentsAllowed;
    }

    /**
     * Creates a new builder.
     *
     * @return a new builder.
     * @since 2.9.0
     */
    public static Builder builder() {
        return new Builder();
    }

    /**
     * Builds instances of INIConfiguration.
     *
     * @since 2.9.0
     */
    public static class Builder {

        /**
         * The flag for decision, whether inline comments on the section line are allowed.
         */
        private boolean sectionInLineCommentsAllowed;

        public Builder setSectionInLineCommentsAllowed(final boolean sectionInLineCommentsAllowed) {
            this.sectionInLineCommentsAllowed = sectionInLineCommentsAllowed;
            return this;
        }

        public INIConfiguration build() {
            return new INIConfiguration(sectionInLineCommentsAllowed);
        }

    }

    /**
     * Gets separator used in INI output. see {@code setSeparatorUsedInOutput} for further explanation
     *
     * @return the current separator for writing the INI output
     * @since 2.2
     */
    public String getSeparatorUsedInOutput() {
        beginRead(false);
        try {
            return separatorUsedInOutput;
        } finally {
            endRead();
        }
    }

    /**
     * Allows setting the key and value separator which is used for the creation of the resulting INI output
     *
     * @param separator String of the new separator for INI output
     * @since 2.2
     */
    public void setSeparatorUsedInOutput(final String separator) {
        beginWrite(false);
        try {
            this.separatorUsedInOutput = separator;
        } finally {
            endWrite();
        }
    }

    /**
     * Gets separator used in INI reading. see {@code setSeparatorUsedInInput} for further explanation
     *
     * @return the current separator for reading the INI input
     * @since 2.5
     */
    public String getSeparatorUsedInInput() {
        beginRead(false);
        try {
            return separatorUsedInInput;
        } finally {
            endRead();
        }
    }

    /**
     * Allows setting the key and value separator which is used in reading an INI file
     *
     * @param separator String of the new separator for INI reading
     * @since 2.5
     */
    public void setSeparatorUsedInInput(final String separator) {
        beginRead(false);
        try {
            this.separatorUsedInInput = separator;
        } finally {
            endRead();
        }
    }

    /**
     * Gets comment leading separator used in INI reading. see {@code setCommentLeadingCharsUsedInInput} for further
     * explanation
     *
     * @return the current separator for reading the INI input
     * @since 2.5
     */
    public String getCommentLeadingCharsUsedInInput() {
        beginRead(false);
        try {
            return commentCharsUsedInInput;
        } finally {
            endRead();
        }
    }

    /**
     * Allows setting the leading comment separator which is used in reading an INI file
     *
     * @param separator String of the new separator for INI reading
     * @since 2.5
     */
    public void setCommentLeadingCharsUsedInInput(final String separator) {
        beginRead(false);
        try {
            this.commentCharsUsedInInput = separator;
        } finally {
            endRead();
        }
    }

    /**
     * Save the configuration to the specified writer.
     *
     * @param writer - The writer to save the configuration to.
     * @throws ConfigurationException If an error occurs while writing the configuration
     * @throws IOException if an I/O error occurs.
     */
    @Override
    public void write(final Writer writer) throws ConfigurationException, IOException {
        final PrintWriter out = new PrintWriter(writer);
        boolean first = true;
        final String separator = getSeparatorUsedInOutput();

        beginRead(false);
        try {
            for (final ImmutableNode node : getModel().getNodeHandler().getRootNode().getChildren()) {
                if (isSectionNode(node)) {
                    if (!first) {
                        out.println();
                    }
                    out.print("[");
                    out.print(node.getNodeName());
                    out.print("]");
                    out.println();

                    node.forEach(child -> writeProperty(out, child.getNodeName(), child.getValue(), separator));
                } else {
                    writeProperty(out, node.getNodeName(), node.getValue(), separator);
                }
                first = false;
            }
            out.println();
            out.flush();
        } finally {
            endRead();
        }
    }

    /**
     * Load the configuration from the given reader. Note that the {@code clear()} method is not called so the configuration
     * read in will be merged with the current configuration.
     *
     * @param in the reader to read the configuration from.
     * @throws ConfigurationException If an error occurs while reading the configuration
     * @throws IOException if an I/O error occurs.
     */
    @Override
    public void read(final Reader in) throws ConfigurationException, IOException {
        final BufferedReader bufferedReader = new BufferedReader(in);
        final Map<String, ImmutableNode.Builder> sectionBuilders = new LinkedHashMap<>();
        final ImmutableNode.Builder rootBuilder = new ImmutableNode.Builder();

        createNodeBuilders(bufferedReader, rootBuilder, sectionBuilders);
        final ImmutableNode rootNode = createNewRootNode(rootBuilder, sectionBuilders);
        addNodes(null, rootNode.getChildren());
    }

    /**
     * Creates a new root node from the builders constructed while reading the configuration file.
     *
     * @param rootBuilder the builder for the top-level section
     * @param sectionBuilders a map storing the section builders
     * @return the root node of the newly created hierarchy
     */
    private static ImmutableNode createNewRootNode(final ImmutableNode.Builder rootBuilder, final Map<String, ImmutableNode.Builder> sectionBuilders) {
        sectionBuilders.forEach((k, v) -> rootBuilder.addChild(v.name(k).create()));
        return rootBuilder.create();
    }

    /**
     * Reads the content of an INI file from the passed in reader and creates a structure of builders for constructing the
     * {@code ImmutableNode} objects representing the data.
     *
     * @param in the reader
     * @param rootBuilder the builder for the top-level section
     * @param sectionBuilders a map storing the section builders
     * @throws IOException if an I/O error occurs.
     */
    private void createNodeBuilders(final BufferedReader in, final ImmutableNode.Builder rootBuilder, final Map<String, ImmutableNode.Builder> sectionBuilders)
        throws IOException {
        ImmutableNode.Builder sectionBuilder = rootBuilder;
        String line = in.readLine();
        while (line != null) {
            line = line.trim();
            if (!isCommentLine(line)) {
                if (isSectionLine(line)) {
                    final int length = sectionInLineCommentsAllowed ? line.indexOf("]") : line.length() - 1;
                    final String section = line.substring(1, length);
                    sectionBuilder = sectionBuilders.computeIfAbsent(section, k -> new ImmutableNode.Builder());
                } else {
                    String key;
                    String value = "";
                    final int index = findSeparator(line);
                    if (index >= 0) {
                        key = line.substring(0, index);
                        value = parseValue(line.substring(index + 1), in);
                    } else {
                        key = line;
                    }
                    key = key.trim();
                    if (key.isEmpty()) {
                        // use space for properties with no key
                        key = " ";
                    }
                    createValueNodes(sectionBuilder, key, value);
                }
            }

            line = in.readLine();
        }
    }

    /**
     * Creates the node(s) for the given key value-pair. If delimiter parsing is enabled, the value string is split if
     * possible, and for each single value a node is created. Otherwise only a single node is added to the section.
     *
     * @param sectionBuilder the section builder for adding new nodes
     * @param key the key
     * @param value the value string
     */
    private void createValueNodes(final ImmutableNode.Builder sectionBuilder, final String key, final String value) {
        getListDelimiterHandler().split(value, false).forEach(v -> sectionBuilder.addChild(new ImmutableNode.Builder().name(key).value(v).create()));
    }

    /**
     * Writes data about a property into the given stream.
     *
     * @param out the output stream
     * @param key the key
     * @param value the value
     */
    private void writeProperty(final PrintWriter out, final String key, final Object value, final String separator) {
        out.print(key);
        out.print(separator);
        out.print(escapeValue(value.toString()));
        out.println();
    }

    /**
     * Parse the value to remove the quotes and ignoring the comment. Example:
     *
     * <pre>
     * &quot;value&quot; ; comment -&gt; value
     * </pre>
     *
     * <pre>
     * 'value' ; comment -&gt; value
     * </pre>
     *
     * Note that a comment character is only recognized if there is at least one whitespace character before it. So it can
     * appear in the property value, e.g.:
     *
     * <pre>
     * C:\\Windows;C:\\Windows\\system32
     * </pre>
     *
     * @param val the value to be parsed
     * @param reader the reader (needed if multiple lines have to be read)
     * @throws IOException if an IO error occurs
     */
    private String parseValue(final String val, final BufferedReader reader) throws IOException {
        final StringBuilder propertyValue = new StringBuilder();
        boolean lineContinues;
        String value = val.trim();

        do {
            final boolean quoted = value.startsWith("\"") || value.startsWith("'");
            boolean stop = false;
            boolean escape = false;

            final char quote = quoted ? value.charAt(0) : 0;

            int i = quoted ? 1 : 0;

            final StringBuilder result = new StringBuilder();
            char lastChar = 0;
            while (i < value.length() && !stop) {
                final char c = value.charAt(i);

                if (quoted) {
                    if ('\\' == c && !escape) {
                        escape = true;
                    } else if (!escape && quote == c) {
                        stop = true;
                    } else {
                        if (escape && quote == c) {
                            escape = false;
                        } else if (escape) {
                            escape = false;
                            result.append('\\');
                        }
                        result.append(c);
                    }
                } else if (isCommentChar(c) && Character.isWhitespace(lastChar)) {
                    stop = true;
                } else {
                    result.append(c);
                }

                i++;
                lastChar = c;
            }

            String v = result.toString();
            if (!quoted) {
                v = v.trim();
                lineContinues = lineContinues(v);
                if (lineContinues) {
                    // remove trailing "\"
                    v = v.substring(0, v.length() - 1).trim();
                }
            } else {
                lineContinues = lineContinues(value, i);
            }
            propertyValue.append(v);

            if (lineContinues) {
                propertyValue.append(LINE_SEPARATOR);
                value = reader.readLine();
            }
        } while (lineContinues && value != null);

        return propertyValue.toString();
    }

    /**
     * Tests whether the specified string contains a line continuation marker.
     *
     * @param line the string to check
     * @return a flag whether this line continues
     */
    private static boolean lineContinues(final String line) {
        final String s = line.trim();
        return s.equals(LINE_CONT) || s.length() > 2 && s.endsWith(LINE_CONT) && Character.isWhitespace(s.charAt(s.length() - 2));
    }

    /**
     * Tests whether the specified string contains a line continuation marker after the specified position. This method
     * parses the string to remove a comment that might be present. Then it checks whether a line continuation marker can be
     * found at the end.
     *
     * @param line the line to check
     * @param pos the start position
     * @return a flag whether this line continues
     */
    private boolean lineContinues(final String line, final int pos) {
        final String s;

        if (pos >= line.length()) {
            s = line;
        } else {
            int end = pos;
            while (end < line.length() && !isCommentChar(line.charAt(end))) {
                end++;
            }
            s = line.substring(pos, end);
        }

        return lineContinues(s);
    }

    /**
     * Tests whether the specified character is a comment character.
     *
     * @param c the character
     * @return a flag whether this character starts a comment
     */
    private boolean isCommentChar(final char c) {
        return getCommentLeadingCharsUsedInInput().indexOf(c) >= 0;
    }

    /**
     * Tries to find the index of the separator character in the given string. This method checks for the presence of
     * separator characters in the given string. If multiple characters are found, the first one is assumed to be the
     * correct separator. If there are quoting characters, they are taken into account, too.
     *
     * @param line the line to be checked
     * @return the index of the separator character or -1 if none is found
     */
    private int findSeparator(final String line) {
        int index = findSeparatorBeforeQuote(line, findFirstOccurrence(line, QUOTE_CHARACTERS));
        if (index < 0) {
            index = findFirstOccurrence(line, getSeparatorUsedInInput());
        }
        return index;
    }

    /**
     * Checks for the occurrence of the specified separators in the given line. The index of the first separator is
     * returned.
     *
     * @param line the line to be investigated
     * @param separators a string with the separator characters to look for
     * @return the lowest index of a separator character or -1 if no separator is found
     */
    private static int findFirstOccurrence(final String line, final String separators) {
        int index = -1;

        for (int i = 0; i < separators.length(); i++) {
            final char sep = separators.charAt(i);
            final int pos = line.indexOf(sep);
            if (pos >= 0 && (index < 0 || pos < index)) {
                index = pos;
            }
        }

        return index;
    }

    /**
     * Searches for a separator character directly before a quoting character. If the first non-whitespace character before
     * a quote character is a separator, it is considered the "real" separator in this line - even if there are other
     * separators before.
     *
     * @param line the line to be investigated
     * @param quoteIndex the index of the quote character
     * @return the index of the separator before the quote or &lt; 0 if there is none
     */
    private static int findSeparatorBeforeQuote(final String line, final int quoteIndex) {
        int index = quoteIndex - 1;
        while (index >= 0 && Character.isWhitespace(line.charAt(index))) {
            index--;
        }

        if (index >= 0 && SEPARATOR_CHARS.indexOf(line.charAt(index)) < 0) {
            index = -1;
        }

        return index;
    }

    /**
     * Escapes the given property value before it is written. This method add quotes around the specified value if it
     * contains a comment character and handles list delimiter characters.
     *
     * @param value the string to be escaped
     */
    private String escapeValue(final String value) {
        return String.valueOf(getListDelimiterHandler().escape(escapeComments(value), ListDelimiterHandler.NOOP_TRANSFORMER));
    }

    /**
     * Escapes comment characters in the given value.
     *
     * @param value the value to be escaped
     * @return the value with comment characters escaped
     */
    private String escapeComments(final String value) {
        final String commentChars = getCommentLeadingCharsUsedInInput();
        boolean quoted = false;

        for (int i = 0; i < commentChars.length(); i++) {
            final char c = commentChars.charAt(i);
            if (value.indexOf(c) != -1) {
                quoted = true;
                break;
            }
        }

        if (quoted) {
            return '"' + value.replace("\"", "\\\"") + '"';
        }
        return value;
    }

    /**
     * Determine if the given line is a comment line.
     *
     * @param line The line to check.
     * @return true if the line is empty or starts with one of the comment characters
     */
    protected boolean isCommentLine(final String line) {
        if (line == null) {
            return false;
        }
        // blank lines are also treated as comment lines
        return line.isEmpty() || getCommentLeadingCharsUsedInInput().indexOf(line.charAt(0)) >= 0;
    }

    /**
     * Determine if the given line is a section.
     *
     * @param line The line to check.
     * @return true if the line contains a section
     */
    protected boolean isSectionLine(final String line) {
        if (line == null) {
            return false;
        }
        return sectionInLineCommentsAllowed ? isNonStrictSection(line) : isStrictSection(line);
    }

    /**
     * Determine if the entire given line is a section - inline comments are not allowed.
     *
     * @param line The line to check.
     * @return true if the entire line is a section
     */
    private static boolean isStrictSection(final String line) {
        return line.startsWith("[") && line.endsWith("]");
    }

    /**
     * Determine if the given line contains a section - inline comments are allowed.
     *
     * @param line The line to check.
     * @return true if the line contains a section
     */
    private static boolean isNonStrictSection(final String line) {
        return line.startsWith("[") && line.contains("]");
    }

    /**
     * Gets a set containing the sections in this ini configuration. Note that changes to this set do not affect the
     * configuration.
     *
     * @return a set containing the sections.
     */
    public Set<String> getSections() {
        final Set<String> sections = new LinkedHashSet<>();
        boolean globalSection = false;
        boolean inSection = false;

        beginRead(false);
        try {
            for (final ImmutableNode node : getModel().getNodeHandler().getRootNode().getChildren()) {
                if (isSectionNode(node)) {
                    inSection = true;
                    sections.add(node.getNodeName());
                } else if (!inSection && !globalSection) {
                    globalSection = true;
                    sections.add(null);
                }
            }
        } finally {
            endRead();
        }

        return sections;
    }

    /**
     * Gets a configuration with the content of the specified section. This provides an easy way of working with a single
     * section only. The way this configuration is structured internally, this method is very similar to calling
     * {@link HierarchicalConfiguration#configurationAt(String)} with the name of the section in question. There are the
     * following differences however:
     * <ul>
     * <li>This method never throws an exception. If the section does not exist, it is created now. The configuration
     * returned in this case is empty.</li>
     * <li>If section is contained multiple times in the configuration, the configuration returned by this method is
     * initialized with the first occurrence of the section. (This can only happen if {@code addProperty()} has been used in
     * a way that does not conform to the storage scheme used by {@code INIConfiguration}. If used correctly, there will not
     * be duplicate sections.)</li>
     * <li>There is special support for the global section: Passing in <b>null</b> as section name returns a configuration
     * with the content of the global section (which may also be empty).</li>
     * </ul>
     *
     * @param name the name of the section in question; <b>null</b> represents the global section
     * @return a configuration containing only the properties of the specified section
     */
    public SubnodeConfiguration getSection(final String name) {
        if (name == null) {
            return getGlobalSection();
        }
        try {
            return (SubnodeConfiguration) configurationAt(name, true);
        } catch (final ConfigurationRuntimeException iex) {
            // the passed in key does not map to exactly one node
            // obtain the node for the section, create it on demand
            final InMemoryNodeModel parentModel = getSubConfigurationParentModel();
            final NodeSelector selector = parentModel.trackChildNodeWithCreation(null, name, this);
            return createSubConfigurationForTrackedNode(selector, this);
        }
    }

    /**
     * Creates a sub configuration for the global section of the represented INI configuration.
     *
     * @return the sub configuration for the global section
     */
    private SubnodeConfiguration getGlobalSection() {
        final InMemoryNodeModel parentModel = getSubConfigurationParentModel();
        final NodeSelector selector = new NodeSelector(null); // selects parent
        parentModel.trackNode(selector, this);
        final GlobalSectionNodeModel model = new GlobalSectionNodeModel(this, selector);
        final SubnodeConfiguration sub = new SubnodeConfiguration(this, model);
        initSubConfigurationForThisParent(sub);
        return sub;
    }

    /**
     * Checks whether the specified configuration node represents a section.
     *
     * @param node the node in question
     * @return a flag whether this node represents a section
     */
    private static boolean isSectionNode(final ImmutableNode node) {
        return node.getValue() == null;
    }

    /**
     * A specialized node model implementation for the sub configuration representing the global section of the INI file.
     * This is a regular {@code TrackedNodeModel} with one exception: The {@code NodeHandler} used by this model applies a
     * filter on the children of the root node so that only nodes are visible that are no sub sections.
     */
    private static final class GlobalSectionNodeModel extends TrackedNodeModel {
        /**
         * Creates a new instance of {@code GlobalSectionNodeModel} and initializes it with the given underlying model.
         *
         * @param modelSupport the underlying {@code InMemoryNodeModel}
         * @param selector the {@code NodeSelector}
         */
        public GlobalSectionNodeModel(final InMemoryNodeModelSupport modelSupport, final NodeSelector selector) {
            super(modelSupport, selector, true);
        }

        @Override
        public NodeHandler<ImmutableNode> getNodeHandler() {
            return new NodeHandlerDecorator<ImmutableNode>() {
                @Override
                public List<ImmutableNode> getChildren(final ImmutableNode node) {
                    final List<ImmutableNode> children = super.getChildren(node);
                    return filterChildrenOfGlobalSection(node, children);
                }

                @Override
                public List<ImmutableNode> getChildren(final ImmutableNode node, final String name) {
                    final List<ImmutableNode> children = super.getChildren(node, name);
                    return filterChildrenOfGlobalSection(node, children);
                }

                @Override
                public int getChildrenCount(final ImmutableNode node, final String name) {
                    final List<ImmutableNode> children = name != null ? super.getChildren(node, name) : super.getChildren(node);
                    return filterChildrenOfGlobalSection(node, children).size();
                }

                @Override
                public ImmutableNode getChild(final ImmutableNode node, final int index) {
                    final List<ImmutableNode> children = super.getChildren(node);
                    return filterChildrenOfGlobalSection(node, children).get(index);
                }

                @Override
                public int indexOfChild(final ImmutableNode parent, final ImmutableNode child) {
                    final List<ImmutableNode> children = super.getChildren(parent);
                    return filterChildrenOfGlobalSection(parent, children).indexOf(child);
                }

                @Override
                protected NodeHandler<ImmutableNode> getDecoratedNodeHandler() {
                    return GlobalSectionNodeModel.super.getNodeHandler();
                }

                /**
                 * Filters the child nodes of the global section. This method checks whether the passed in node is the root node of the
                 * configuration. If so, from the list of children all nodes are filtered which are section nodes.
                 *
                 * @param node the node in question
                 * @param children the children of this node
                 * @return a list with the filtered children
                 */
                private List<ImmutableNode> filterChildrenOfGlobalSection(final ImmutableNode node, final List<ImmutableNode> children) {
                    final List<ImmutableNode> filteredList;
                    if (node == getRootNode()) {
                        filteredList = children.stream().filter(child -> !isSectionNode(child)).collect(Collectors.toList());
                    } else {
                        filteredList = children;
                    }

                    return filteredList;
                }
            };
        }
    }
}