/*
 * 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 net.hasor.utils.io;
import java.io.File;
import java.util.Collection;

/**
 * General filename and filepath manipulation utilities.
 * <p>
 * When dealing with filenames you can hit problems when moving from a Windows
 * based development machine to a Unix based production machine.
 * This class aims to help avoid those problems.
 * <p>
 * <b>NOTE</b>: You may be able to avoid using this class entirely simply by
 * using JDK {@link File File} objects and the two argument constructor
 * {@link File#File(File, String) File(File,String)}.
 * <p>
 * Most methods on this class are designed to work the same on both Unix and Windows.
 * Those that don't include 'System', 'Unix' or 'Windows' in their name.
 * <p>
 * Most methods recognise both separators (forward and back), and both
 * sets of prefixes. See the javadoc of each method for details.
 * <p>
 * This class defines six components within a filename
 * (example C:\dev\project\file.txt):
 * <ul>
 * <li>the prefix - C:\</li>
 * <li>the path - dev\project\</li>
 * <li>the full path - C:\dev\project\</li>
 * <li>the name - file.txt</li>
 * <li>the base name - file</li>
 * <li>the extension - txt</li>
 * </ul>
 * Note that this class works best if directory filenames end with a separator.
 * If you omit the last separator, it is impossible to determine if the filename
 * corresponds to a file or a directory. As a result, we have chosen to say
 * it corresponds to a file.
 * <p>
 * This class only supports Unix and Windows style names.
 * Prefixes are matched as follows:
 * <pre>
 * Windows:
 * a\b\c.txt           --> ""          --> relative
 * \a\b\c.txt          --> "\"         --> current drive absolute
 * C:a\b\c.txt         --> "C:"        --> drive relative
 * C:\a\b\c.txt        --> "C:\"       --> absolute
 * \\server\a\b\c.txt  --> "\\server\" --> UNC
 *
 * Unix:
 * a/b/c.txt           --> ""          --> relative
 * /a/b/c.txt          --> "/"         --> absolute
 * ~/a/b/c.txt         --> "~/"        --> current user
 * ~                   --> "~/"        --> current user (slash added)
 * ~user/a/b/c.txt     --> "~user/"    --> named user
 * ~user               --> "~user/"    --> named user (slash added)
 * </pre>
 * Both prefix styles are matched always, irrespective of the machine that you are
 * currently running on.
 * <p>
 * Origin of code: Excalibur, Alexandria, Tomcat, Commons-Utils.
 *
 * @version $Id: FilenameUtils.java 1307462 2012-03-30 15:13:11Z ggregory $
 * @since 1.1
 */
public class FilenameUtils {
    /**
     * The extension separator character.
     * @since 1.4
     */
    public static final  char   EXTENSION_SEPARATOR     = '.';
    /**
     * The extension separator String.
     * @since 1.4
     */
    public static final  String EXTENSION_SEPARATOR_STR = Character.toString(EXTENSION_SEPARATOR);
    /**
     * The Unix separator character.
     */
    private static final char   UNIX_SEPARATOR          = '/';
    /**
     * The Windows separator character.
     */
    private static final char   WINDOWS_SEPARATOR       = '\\';
    /**
     * The system separator character.
     */
    private static final char   SYSTEM_SEPARATOR        = File.separatorChar;
    /**
     * The separator character that is the opposite of the system separator.
     */
    private static final char   OTHER_SEPARATOR;

    static {
        if (isSystemWindows()) {
            OTHER_SEPARATOR = UNIX_SEPARATOR;
        } else {
            OTHER_SEPARATOR = WINDOWS_SEPARATOR;
        }
    }

    /**
     * Instances should NOT be constructed in standard programming.
     */
    public FilenameUtils() {
        super();
    }
    //-----------------------------------------------------------------------

    /**
     * Determines if Windows file system is in use.
     *
     * @return true if the system is Windows
     */
    static boolean isSystemWindows() {
        return SYSTEM_SEPARATOR == WINDOWS_SEPARATOR;
    }
    //-----------------------------------------------------------------------

    /**
     * Checks if the character is a separator.
     *
     * @param ch  the character to check
     * @return true if it is a separator character
     */
    private static boolean isSeparator(char ch) {
        return ch == UNIX_SEPARATOR || ch == WINDOWS_SEPARATOR;
    }
    //-----------------------------------------------------------------------

    /**
     * Normalizes a path, removing double and single dot path steps.
     * <p>
     * This method normalizes a path to a standard format.
     * The input may contain separators in either Unix or Windows format.
     * The output will contain separators in the format of the system.
     * <p>
     * A trailing slash will be retained.
     * A double slash will be merged to a single slash (but UNC names are handled).
     * A single dot path segment will be removed.
     * A double dot will cause that path segment and the one before to be removed.
     * If the double dot has no parent path segment to work with, {@code null}
     * is returned.
     * <p>
     * The output will be the same on both Unix and Windows except
     * for the separator character.
     * <pre>
     * /foo//               -->   /foo/
     * /foo/./              -->   /foo/
     * /foo/../bar          -->   /bar
     * /foo/../bar/         -->   /bar/
     * /foo/../bar/../baz   -->   /baz
     * //foo//./bar         -->   /foo/bar
     * /../                 -->   null
     * ../foo               -->   null
     * foo/bar/..           -->   foo/
     * foo/../../bar        -->   null
     * foo/../bar           -->   bar
     * //server/foo/../bar  -->   //server/bar
     * //server/../bar      -->   null
     * C:\foo\..\bar        -->   C:\bar
     * C:\..\bar            -->   null
     * ~/foo/../bar/        -->   ~/bar/
     * ~/../bar             -->   null
     * </pre>
     * (Note the file separator returned will be correct for Windows/Unix)
     *
     * @param filename  the filename to normalize, null returns null
     * @return the normalized filename, or null if invalid
     */
    public static String normalize(String filename) {
        return doNormalize(filename, SYSTEM_SEPARATOR, true);
    }

    /**
     * Normalizes a path, removing double and single dot path steps.
     * <p>
     * This method normalizes a path to a standard format.
     * The input may contain separators in either Unix or Windows format.
     * The output will contain separators in the format specified.
     * <p>
     * A trailing slash will be retained.
     * A double slash will be merged to a single slash (but UNC names are handled).
     * A single dot path segment will be removed.
     * A double dot will cause that path segment and the one before to be removed.
     * If the double dot has no parent path segment to work with, {@code null}
     * is returned.
     * <p>
     * The output will be the same on both Unix and Windows except
     * for the separator character.
     * <pre>
     * /foo//               -->   /foo/
     * /foo/./              -->   /foo/
     * /foo/../bar          -->   /bar
     * /foo/../bar/         -->   /bar/
     * /foo/../bar/../baz   -->   /baz
     * //foo//./bar         -->   /foo/bar
     * /../                 -->   null
     * ../foo               -->   null
     * foo/bar/..           -->   foo/
     * foo/../../bar        -->   null
     * foo/../bar           -->   bar
     * //server/foo/../bar  -->   //server/bar
     * //server/../bar      -->   null
     * C:\foo\..\bar        -->   C:\bar
     * C:\..\bar            -->   null
     * ~/foo/../bar/        -->   ~/bar/
     * ~/../bar             -->   null
     * </pre>
     * The output will be the same on both Unix and Windows including
     * the separator character.
     *
     * @param filename  the filename to normalize, null returns null
     * @param unixSeparator {@code true} if a unix separator should
     * be used or {@code false} if a windows separator should be used.
     * @return the normalized filename, or null if invalid
     * @since 2.0
     */
    public static String normalize(String filename, boolean unixSeparator) {
        char separator = unixSeparator ? UNIX_SEPARATOR : WINDOWS_SEPARATOR;
        return doNormalize(filename, separator, true);
    }
    //-----------------------------------------------------------------------

    /**
     * Normalizes a path, removing double and single dot path steps,
     * and removing any final directory separator.
     * <p>
     * This method normalizes a path to a standard format.
     * The input may contain separators in either Unix or Windows format.
     * The output will contain separators in the format of the system.
     * <p>
     * A trailing slash will be removed.
     * A double slash will be merged to a single slash (but UNC names are handled).
     * A single dot path segment will be removed.
     * A double dot will cause that path segment and the one before to be removed.
     * If the double dot has no parent path segment to work with, {@code null}
     * is returned.
     * <p>
     * The output will be the same on both Unix and Windows except
     * for the separator character.
     * <pre>
     * /foo//               -->   /foo
     * /foo/./              -->   /foo
     * /foo/../bar          -->   /bar
     * /foo/../bar/         -->   /bar
     * /foo/../bar/../baz   -->   /baz
     * //foo//./bar         -->   /foo/bar
     * /../                 -->   null
     * ../foo               -->   null
     * foo/bar/..           -->   foo
     * foo/../../bar        -->   null
     * foo/../bar           -->   bar
     * //server/foo/../bar  -->   //server/bar
     * //server/../bar      -->   null
     * C:\foo\..\bar        -->   C:\bar
     * C:\..\bar            -->   null
     * ~/foo/../bar/        -->   ~/bar
     * ~/../bar             -->   null
     * </pre>
     * (Note the file separator returned will be correct for Windows/Unix)
     *
     * @param filename  the filename to normalize, null returns null
     * @return the normalized filename, or null if invalid
     */
    public static String normalizeNoEndSeparator(String filename) {
        return doNormalize(filename, SYSTEM_SEPARATOR, false);
    }

    /**
     * Normalizes a path, removing double and single dot path steps,
     * and removing any final directory separator.
     * <p>
     * This method normalizes a path to a standard format.
     * The input may contain separators in either Unix or Windows format.
     * The output will contain separators in the format specified.
     * <p>
     * A trailing slash will be removed.
     * A double slash will be merged to a single slash (but UNC names are handled).
     * A single dot path segment will be removed.
     * A double dot will cause that path segment and the one before to be removed.
     * If the double dot has no parent path segment to work with, {@code null}
     * is returned.
     * <p>
     * The output will be the same on both Unix and Windows including
     * the separator character.
     * <pre>
     * /foo//               -->   /foo
     * /foo/./              -->   /foo
     * /foo/../bar          -->   /bar
     * /foo/../bar/         -->   /bar
     * /foo/../bar/../baz   -->   /baz
     * //foo//./bar         -->   /foo/bar
     * /../                 -->   null
     * ../foo               -->   null
     * foo/bar/..           -->   foo
     * foo/../../bar        -->   null
     * foo/../bar           -->   bar
     * //server/foo/../bar  -->   //server/bar
     * //server/../bar      -->   null
     * C:\foo\..\bar        -->   C:\bar
     * C:\..\bar            -->   null
     * ~/foo/../bar/        -->   ~/bar
     * ~/../bar             -->   null
     * </pre>
     *
     * @param filename  the filename to normalize, null returns null
     * @param unixSeparator {@code true} if a unix separator should
     * be used or {@code false} if a windows separtor should be used.
     * @return the normalized filename, or null if invalid
     * @since 2.0
     */
    public static String normalizeNoEndSeparator(String filename, boolean unixSeparator) {
        char separator = unixSeparator ? UNIX_SEPARATOR : WINDOWS_SEPARATOR;
        return doNormalize(filename, separator, false);
    }

    /**
     * Internal method to perform the normalization.
     *
     * @param filename  the filename
     * @param separator The separator character to use
     * @param keepSeparator  true to keep the final separator
     * @return the normalized filename
     */
    private static String doNormalize(String filename, char separator, boolean keepSeparator) {
        if (filename == null) {
            return null;
        }
        int size = filename.length();
        if (size == 0) {
            return filename;
        }
        int prefix = getPrefixLength(filename);
        if (prefix < 0) {
            return null;
        }
        char[] array = new char[size + 2];  // +1 for possible extra slash, +2 for arraycopy
        filename.getChars(0, filename.length(), array, 0);
        // fix separators throughout
        char otherSeparator = separator == SYSTEM_SEPARATOR ? OTHER_SEPARATOR : SYSTEM_SEPARATOR;
        for (int i = 0; i < array.length; i++) {
            if (array[i] == otherSeparator) {
                array[i] = separator;
            }
        }
        // add extra separator on the end to simplify code below
        boolean lastIsDirectory = true;
        if (array[size - 1] != separator) {
            array[size++] = separator;
            lastIsDirectory = false;
        }
        // adjoining slashes
        for (int i = prefix + 1; i < size; i++) {
            if (array[i] == separator && array[i - 1] == separator) {
                System.arraycopy(array, i, array, i - 1, size - i);
                size--;
                i--;
            }
        }
        // dot slash
        for (int i = prefix + 1; i < size; i++) {
            if (array[i] == separator && array[i - 1] == '.' && (i == prefix + 1 || array[i - 2] == separator)) {
                if (i == size - 1) {
                    lastIsDirectory = true;
                }
                System.arraycopy(array, i + 1, array, i - 1, size - i);
                size -= 2;
                i--;
            }
        }
        // double dot slash
        outer:
        for (int i = prefix + 2; i < size; i++) {
            if (array[i] == separator && array[i - 1] == '.' && array[i - 2] == '.' && (i == prefix + 2 || array[i - 3] == separator)) {
                if (i == prefix + 2) {
                    return null;
                }
                if (i == size - 1) {
                    lastIsDirectory = true;
                }
                int j;
                for (j = i - 4; j >= prefix; j--) {
                    if (array[j] == separator) {
                        // remove b/../ from a/b/../c
                        System.arraycopy(array, i + 1, array, j + 1, size - i);
                        size -= i - j;
                        i = j + 1;
                        continue outer;
                    }
                }
                // remove a/../ from a/../c
                System.arraycopy(array, i + 1, array, prefix, size - i);
                size -= i + 1 - prefix;
                i = prefix + 1;
            }
        }
        if (size <= 0) {  // should never be less than 0
            return "";
        }
        if (size <= prefix) {  // should never be less than prefix
            return new String(array, 0, size);
        }
        if (lastIsDirectory && keepSeparator) {
            return new String(array, 0, size);  // keep trailing separator
        }
        return new String(array, 0, size - 1);  // lose trailing separator
    }
    //-----------------------------------------------------------------------

    /**
     * Concatenates a filename to a base path using normal command line style rules.
     * <p>
     * The effect is equivalent to resultant directory after changing
     * directory to the first argument, followed by changing directory to
     * the second argument.
     * <p>
     * The first argument is the base path, the second is the path to concatenate.
     * The returned path is always normalized via {@link #normalize(String)},
     * thus <code>..</code> is handled.
     * <p>
     * If <code>pathToAdd</code> is absolute (has an absolute prefix), then
     * it will be normalized and returned.
     * Otherwise, the paths will be joined, normalized and returned.
     * <p>
     * The output will be the same on both Unix and Windows except
     * for the separator character.
     * <pre>
     * /foo/ + bar          -->   /foo/bar
     * /foo + bar           -->   /foo/bar
     * /foo + /bar          -->   /bar
     * /foo + C:/bar        -->   C:/bar
     * /foo + C:bar         -->   C:bar (*)
     * /foo/a/ + ../bar     -->   foo/bar
     * /foo/ + ../../bar    -->   null
     * /foo/ + /bar         -->   /bar
     * /foo/.. + /bar       -->   /bar
     * /foo + bar/c.txt     -->   /foo/bar/c.txt
     * /foo/c.txt + bar     -->   /foo/c.txt/bar (!)
     * </pre>
     * (*) Note that the Windows relative drive prefix is unreliable when
     * used with this method.
     * (!) Note that the first parameter must be a path. If it ends with a name, then
     * the name will be built into the concatenated path. If this might be a problem,
     * use {@link #getFullPath(String)} on the base path argument.
     *
     * @param basePath  the base path to attach to, always treated as a path
     * @param fullFilenameToAdd  the filename (or path) to attach to the base
     * @return the concatenated path, or null if invalid
     */
    public static String concat(String basePath, String fullFilenameToAdd) {
        int prefix = getPrefixLength(fullFilenameToAdd);
        if (prefix < 0) {
            return null;
        }
        if (prefix > 0) {
            return normalize(fullFilenameToAdd);
        }
        if (basePath == null) {
            return null;
        }
        int len = basePath.length();
        if (len == 0) {
            return normalize(fullFilenameToAdd);
        }
        char ch = basePath.charAt(len - 1);
        if (isSeparator(ch)) {
            return normalize(basePath + fullFilenameToAdd);
        } else {
            return normalize(basePath + '/' + fullFilenameToAdd);
        }
    }
    //-----------------------------------------------------------------------

    /**
     * Converts all separators to the Unix separator of forward slash.
     *
     * @param path  the path to be changed, null ignored
     * @return the updated path
     */
    public static String separatorsToUnix(String path) {
        if (path == null || path.indexOf(WINDOWS_SEPARATOR) == -1) {
            return path;
        }
        return path.replace(WINDOWS_SEPARATOR, UNIX_SEPARATOR);
    }

    /**
     * Converts all separators to the Windows separator of backslash.
     *
     * @param path  the path to be changed, null ignored
     * @return the updated path
     */
    public static String separatorsToWindows(String path) {
        if (path == null || path.indexOf(UNIX_SEPARATOR) == -1) {
            return path;
        }
        return path.replace(UNIX_SEPARATOR, WINDOWS_SEPARATOR);
    }

    /**
     * Converts all separators to the system separator.
     *
     * @param path  the path to be changed, null ignored
     * @return the updated path
     */
    public static String separatorsToSystem(String path) {
        if (path == null) {
            return null;
        }
        if (isSystemWindows()) {
            return separatorsToWindows(path);
        } else {
            return separatorsToUnix(path);
        }
    }
    //-----------------------------------------------------------------------

    /**
     * Returns the length of the filename prefix, such as <code>C:/</code> or <code>~/</code>.
     * <p>
     * This method will handle a file in either Unix or Windows format.
     * <p>
     * The prefix length includes the first slash in the full filename
     * if applicable. Thus, it is possible that the length returned is greater
     * than the length of the input string.
     * <pre>
     * Windows:
     * a\b\c.txt           --> ""          --> relative
     * \a\b\c.txt          --> "\"         --> current drive absolute
     * C:a\b\c.txt         --> "C:"        --> drive relative
     * C:\a\b\c.txt        --> "C:\"       --> absolute
     * \\server\a\b\c.txt  --> "\\server\" --> UNC
     *
     * Unix:
     * a/b/c.txt           --> ""          --> relative
     * /a/b/c.txt          --> "/"         --> absolute
     * ~/a/b/c.txt         --> "~/"        --> current user
     * ~                   --> "~/"        --> current user (slash added)
     * ~user/a/b/c.txt     --> "~user/"    --> named user
     * ~user               --> "~user/"    --> named user (slash added)
     * </pre>
     * <p>
     * The output will be the same irrespective of the machine that the code is running on.
     * ie. both Unix and Windows prefixes are matched regardless.
     *
     * @param filename  the filename to find the prefix in, null returns -1
     * @return the length of the prefix, -1 if invalid or null
     */
    public static int getPrefixLength(String filename) {
        if (filename == null) {
            return -1;
        }
        int len = filename.length();
        if (len == 0) {
            return 0;
        }
        char ch0 = filename.charAt(0);
        if (ch0 == ':') {
            return -1;
        }
        if (len == 1) {
            if (ch0 == '~') {
                return 2;  // return a length greater than the input
            }
            return isSeparator(ch0) ? 1 : 0;
        } else {
            if (ch0 == '~') {
                int posUnix = filename.indexOf(UNIX_SEPARATOR, 1);
                int posWin = filename.indexOf(WINDOWS_SEPARATOR, 1);
                if (posUnix == -1 && posWin == -1) {
                    return len + 1;  // return a length greater than the input
                }
                posUnix = posUnix == -1 ? posWin : posUnix;
                posWin = posWin == -1 ? posUnix : posWin;
                return Math.min(posUnix, posWin) + 1;
            }
            char ch1 = filename.charAt(1);
            if (ch1 == ':') {
                ch0 = Character.toUpperCase(ch0);
                if (ch0 >= 'A' && ch0 <= 'Z') {
                    if (len == 2 || isSeparator(filename.charAt(2)) == false) {
                        return 2;
                    }
                    return 3;
                }
                return -1;
            } else if (isSeparator(ch0) && isSeparator(ch1)) {
                int posUnix = filename.indexOf(UNIX_SEPARATOR, 2);
                int posWin = filename.indexOf(WINDOWS_SEPARATOR, 2);
                if (posUnix == -1 && posWin == -1 || posUnix == 2 || posWin == 2) {
                    return -1;
                }
                posUnix = posUnix == -1 ? posWin : posUnix;
                posWin = posWin == -1 ? posUnix : posWin;
                return Math.min(posUnix, posWin) + 1;
            } else {
                return isSeparator(ch0) ? 1 : 0;
            }
        }
    }

    /**
     * Returns the index of the last directory separator character.
     * <p>
     * This method will handle a file in either Unix or Windows format.
     * The position of the last forward or backslash is returned.
     * <p>
     * The output will be the same irrespective of the machine that the code is running on.
     *
     * @param filename  the filename to find the last path separator in, null returns -1
     * @return the index of the last separator character, or -1 if there
     * is no such character
     */
    public static int indexOfLastSeparator(String filename) {
        if (filename == null) {
            return -1;
        }
        int lastUnixPos = filename.lastIndexOf(UNIX_SEPARATOR);
        int lastWindowsPos = filename.lastIndexOf(WINDOWS_SEPARATOR);
        return Math.max(lastUnixPos, lastWindowsPos);
    }

    /**
     * Returns the index of the last extension separator character, which is a dot.
     * <p>
     * This method also checks that there is no directory separator after the last dot.
     * To do this it uses {@link #indexOfLastSeparator(String)} which will
     * handle a file in either Unix or Windows format.
     * <p>
     * The output will be the same irrespective of the machine that the code is running on.
     *
     * @param filename  the filename to find the last path separator in, null returns -1
     * @return the index of the last separator character, or -1 if there
     * is no such character
     */
    public static int indexOfExtension(String filename) {
        if (filename == null) {
            return -1;
        }
        int extensionPos = filename.lastIndexOf(EXTENSION_SEPARATOR);
        int lastSeparator = indexOfLastSeparator(filename);
        return lastSeparator > extensionPos ? -1 : extensionPos;
    }
    //-----------------------------------------------------------------------

    /**
     * Gets the prefix from a full filename, such as <code>C:/</code>
     * or <code>~/</code>.
     * <p>
     * This method will handle a file in either Unix or Windows format.
     * The prefix includes the first slash in the full filename where applicable.
     * <pre>
     * Windows:
     * a\b\c.txt           --> ""          --> relative
     * \a\b\c.txt          --> "\"         --> current drive absolute
     * C:a\b\c.txt         --> "C:"        --> drive relative
     * C:\a\b\c.txt        --> "C:\"       --> absolute
     * \\server\a\b\c.txt  --> "\\server\" --> UNC
     *
     * Unix:
     * a/b/c.txt           --> ""          --> relative
     * /a/b/c.txt          --> "/"         --> absolute
     * ~/a/b/c.txt         --> "~/"        --> current user
     * ~                   --> "~/"        --> current user (slash added)
     * ~user/a/b/c.txt     --> "~user/"    --> named user
     * ~user               --> "~user/"    --> named user (slash added)
     * </pre>
     * <p>
     * The output will be the same irrespective of the machine that the code is running on.
     * ie. both Unix and Windows prefixes are matched regardless.
     *
     * @param filename  the filename to query, null returns null
     * @return the prefix of the file, null if invalid
     */
    public static String getPrefix(String filename) {
        if (filename == null) {
            return null;
        }
        int len = getPrefixLength(filename);
        if (len < 0) {
            return null;
        }
        if (len > filename.length()) {
            return filename + UNIX_SEPARATOR;  // we know this only happens for unix
        }
        return filename.substring(0, len);
    }

    /**
     * Gets the path from a full filename, which excludes the prefix.
     * <p>
     * This method will handle a file in either Unix or Windows format.
     * The method is entirely text based, and returns the text before and
     * including the last forward or backslash.
     * <pre>
     * C:\a\b\c.txt --> a\b\
     * ~/a/b/c.txt  --> a/b/
     * a.txt        --> ""
     * a/b/c        --> a/b/
     * a/b/c/       --> a/b/c/
     * </pre>
     * <p>
     * The output will be the same irrespective of the machine that the code is running on.
     * <p>
     * This method drops the prefix from the result.
     * See {@link #getFullPath(String)} for the method that retains the prefix.
     *
     * @param filename  the filename to query, null returns null
     * @return the path of the file, an empty string if none exists, null if invalid
     */
    public static String getPath(String filename) {
        return doGetPath(filename, 1);
    }

    /**
     * Gets the path from a full filename, which excludes the prefix, and
     * also excluding the final directory separator.
     * <p>
     * This method will handle a file in either Unix or Windows format.
     * The method is entirely text based, and returns the text before the
     * last forward or backslash.
     * <pre>
     * C:\a\b\c.txt --> a\b
     * ~/a/b/c.txt  --> a/b
     * a.txt        --> ""
     * a/b/c        --> a/b
     * a/b/c/       --> a/b/c
     * </pre>
     * <p>
     * The output will be the same irrespective of the machine that the code is running on.
     * <p>
     * This method drops the prefix from the result.
     * See {@link #getFullPathNoEndSeparator(String)} for the method that retains the prefix.
     *
     * @param filename  the filename to query, null returns null
     * @return the path of the file, an empty string if none exists, null if invalid
     */
    public static String getPathNoEndSeparator(String filename) {
        return doGetPath(filename, 0);
    }

    /**
     * Does the work of getting the path.
     *
     * @param filename  the filename
     * @param separatorAdd  0 to omit the end separator, 1 to return it
     * @return the path
     */
    private static String doGetPath(String filename, int separatorAdd) {
        if (filename == null) {
            return null;
        }
        int prefix = getPrefixLength(filename);
        if (prefix < 0) {
            return null;
        }
        int index = indexOfLastSeparator(filename);
        int endIndex = index + separatorAdd;
        if (prefix >= filename.length() || index < 0 || prefix >= endIndex) {
            return "";
        }
        return filename.substring(prefix, endIndex);
    }

    /**
     * Gets the full path from a full filename, which is the prefix + path.
     * <p>
     * This method will handle a file in either Unix or Windows format.
     * The method is entirely text based, and returns the text before and
     * including the last forward or backslash.
     * <pre>
     * C:\a\b\c.txt --> C:\a\b\
     * ~/a/b/c.txt  --> ~/a/b/
     * a.txt        --> ""
     * a/b/c        --> a/b/
     * a/b/c/       --> a/b/c/
     * C:           --> C:
     * C:\          --> C:\
     * ~            --> ~/
     * ~/           --> ~/
     * ~user        --> ~user/
     * ~user/       --> ~user/
     * </pre>
     * <p>
     * The output will be the same irrespective of the machine that the code is running on.
     *
     * @param filename  the filename to query, null returns null
     * @return the path of the file, an empty string if none exists, null if invalid
     */
    public static String getFullPath(String filename) {
        return doGetFullPath(filename, true);
    }

    /**
     * Gets the full path from a full filename, which is the prefix + path,
     * and also excluding the final directory separator.
     * <p>
     * This method will handle a file in either Unix or Windows format.
     * The method is entirely text based, and returns the text before the
     * last forward or backslash.
     * <pre>
     * C:\a\b\c.txt --> C:\a\b
     * ~/a/b/c.txt  --> ~/a/b
     * a.txt        --> ""
     * a/b/c        --> a/b
     * a/b/c/       --> a/b/c
     * C:           --> C:
     * C:\          --> C:\
     * ~            --> ~
     * ~/           --> ~
     * ~user        --> ~user
     * ~user/       --> ~user
     * </pre>
     * <p>
     * The output will be the same irrespective of the machine that the code is running on.
     *
     * @param filename  the filename to query, null returns null
     * @return the path of the file, an empty string if none exists, null if invalid
     */
    public static String getFullPathNoEndSeparator(String filename) {
        return doGetFullPath(filename, false);
    }

    /**
     * Does the work of getting the path.
     *
     * @param filename  the filename
     * @param includeSeparator  true to include the end separator
     * @return the path
     */
    private static String doGetFullPath(String filename, boolean includeSeparator) {
        if (filename == null) {
            return null;
        }
        int prefix = getPrefixLength(filename);
        if (prefix < 0) {
            return null;
        }
        if (prefix >= filename.length()) {
            if (includeSeparator) {
                return getPrefix(filename);  // add end slash if necessary
            } else {
                return filename;
            }
        }
        int index = indexOfLastSeparator(filename);
        if (index < 0) {
            return filename.substring(0, prefix);
        }
        int end = index + (includeSeparator ? 1 : 0);
        if (end == 0) {
            end++;
        }
        return filename.substring(0, end);
    }

    /**
     * Gets the name minus the path from a full filename.
     * <p>
     * This method will handle a file in either Unix or Windows format.
     * The text after the last forward or backslash is returned.
     * <pre>
     * a/b/c.txt --> c.txt
     * a.txt     --> a.txt
     * a/b/c     --> c
     * a/b/c/    --> ""
     * </pre>
     * <p>
     * The output will be the same irrespective of the machine that the code is running on.
     *
     * @param filename  the filename to query, null returns null
     * @return the name of the file without the path, or an empty string if none exists
     */
    public static String getName(String filename) {
        if (filename == null) {
            return null;
        }
        int index = indexOfLastSeparator(filename);
        return filename.substring(index + 1);
    }

    /**
     * Gets the base name, minus the full path and extension, from a full filename.
     * <p>
     * This method will handle a file in either Unix or Windows format.
     * The text after the last forward or backslash and before the last dot is returned.
     * <pre>
     * a/b/c.txt --> c
     * a.txt     --> a
     * a/b/c     --> c
     * a/b/c/    --> ""
     * </pre>
     * <p>
     * The output will be the same irrespective of the machine that the code is running on.
     *
     * @param filename  the filename to query, null returns null
     * @return the name of the file without the path, or an empty string if none exists
     */
    public static String getBaseName(String filename) {
        return removeExtension(getName(filename));
    }

    /**
     * Gets the extension of a filename.
     * <p>
     * This method returns the textual part of the filename after the last dot.
     * There must be no directory separator after the dot.
     * <pre>
     * foo.txt      --> "txt"
     * a/b/c.jpg    --> "jpg"
     * a/b.txt/c    --> ""
     * a/b/c        --> ""
     * </pre>
     * <p>
     * The output will be the same irrespective of the machine that the code is running on.
     *
     * @param filename the filename to retrieve the extension of.
     * @return the extension of the file or an empty string if none exists or {@code null}
     * if the filename is {@code null}.
     */
    public static String getExtension(String filename) {
        if (filename == null) {
            return null;
        }
        int index = indexOfExtension(filename);
        if (index == -1) {
            return "";
        } else {
            return filename.substring(index + 1);
        }
    }
    //-----------------------------------------------------------------------

    /**
     * Removes the extension from a filename.
     * <p>
     * This method returns the textual part of the filename before the last dot.
     * There must be no directory separator after the dot.
     * <pre>
     * foo.txt    --> foo
     * a\b\c.jpg  --> a\b\c
     * a\b\c      --> a\b\c
     * a.b\c      --> a.b\c
     * </pre>
     * <p>
     * The output will be the same irrespective of the machine that the code is running on.
     *
     * @param filename  the filename to query, null returns null
     * @return the filename minus the extension
     */
    public static String removeExtension(String filename) {
        if (filename == null) {
            return null;
        }
        int index = indexOfExtension(filename);
        if (index == -1) {
            return filename;
        } else {
            return filename.substring(0, index);
        }
    }
    //-----------------------------------------------------------------------

    /**
     * Checks whether the extension of the filename is that specified.
     * <p>
     * This method obtains the extension as the textual part of the filename
     * after the last dot. There must be no directory separator after the dot.
     * The extension check is case-sensitive on all platforms.
     *
     * @param filename  the filename to query, null returns false
     * @param extension  the extension to check for, null or empty checks for no extension
     * @return true if the filename has the specified extension
     */
    public static boolean isExtension(String filename, String extension) {
        if (filename == null) {
            return false;
        }
        if (extension == null || extension.length() == 0) {
            return indexOfExtension(filename) == -1;
        }
        String fileExt = getExtension(filename);
        return fileExt.equals(extension);
    }

    /**
     * Checks whether the extension of the filename is one of those specified.
     * <p>
     * This method obtains the extension as the textual part of the filename
     * after the last dot. There must be no directory separator after the dot.
     * The extension check is case-sensitive on all platforms.
     *
     * @param filename  the filename to query, null returns false
     * @param extensions  the extensions to check for, null checks for no extension
     * @return true if the filename is one of the extensions
     */
    public static boolean isExtension(String filename, String[] extensions) {
        if (filename == null) {
            return false;
        }
        if (extensions == null || extensions.length == 0) {
            return indexOfExtension(filename) == -1;
        }
        String fileExt = getExtension(filename);
        for (String extension : extensions) {
            if (fileExt.equals(extension)) {
                return true;
            }
        }
        return false;
    }

    /**
     * Checks whether the extension of the filename is one of those specified.
     * <p>
     * This method obtains the extension as the textual part of the filename
     * after the last dot. There must be no directory separator after the dot.
     * The extension check is case-sensitive on all platforms.
     *
     * @param filename  the filename to query, null returns false
     * @param extensions  the extensions to check for, null checks for no extension
     * @return true if the filename is one of the extensions
     */
    public static boolean isExtension(String filename, Collection<String> extensions) {
        if (filename == null) {
            return false;
        }
        if (extensions == null || extensions.isEmpty()) {
            return indexOfExtension(filename) == -1;
        }
        String fileExt = getExtension(filename);
        for (String extension : extensions) {
            if (fileExt.equals(extension)) {
                return true;
            }
        }
        return false;
    }
}