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 (SYSTEM_SEPARATOR == WINDOWS_SEPARATOR) {
            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.