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RFC 3987 (Internationlaized Resource Identifier) libraries.
There is a newer version: 1.1.3
/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  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.  For additional information regarding
 * copyright in this work, please see the NOTICE file in the top level
 * directory of this distribution.
 */
package org.apache.abdera.i18n.text;

import java.text.AttributedString;
import java.util.Arrays;

import org.apache.abdera.i18n.rfc4646.Lang;

/**
 * Bidi guessing algorithms
 */
public class Bidi {

    public enum Direction {
        UNSPECIFIED, LTR, RTL
    };

    private static final String[] RTL_LANGS = {"ar", "dv", "fa", "he", "ps", "syr", "ur", "yi"};

    private static final String[] RTL_SCRIPTS =
        {"arab", "avst", "hebr", "hung", "lydi", "mand", "mani", "mero", "mong", "nkoo", "orkh", "phlv", "phnx",
         "samr", "syrc", "syre", "syrj", "syrn", "tfng", "thaa"};
    // charset encodings that one may typically expect to be RTL
    private static final String[] RTL_ENCODINGS =
        {"iso-8859-6", "iso-8859-6-bidi", "iso-8859-6-i", "iso-ir-127", "ecma-114", "asmo-708", "arabic",
         "csisolatinarabic", "windows-1256", "ibm-864", "macarabic", "macfarsi", "iso-8859-8-i", "iso-8859-8-bidi",
         "windows-1255", "iso-8859-8", "ibm-862", "machebrew", "asmo-449", "iso-9036", "arabic7", "iso-ir-89",
         "csiso89asmo449", "iso-unicode-ibm-1264", "csunicodeibm1264", "iso_8859-8:1988", "iso-ir-138", "hebrew",
         "csisolatinhebrew", "iso-unicode-ibm-1265", "csunicodeibm1265", "cp862", "862", "cspc862latinhebrew"};

    /**
     * Algorithm that will determine text direction by looking at the characteristics of the language tag. If the tag
     * uses a language or script that is known to be RTL, then Direction.RTL will be returned
     */
    public static Direction guessDirectionFromLanguage(Lang lang) {
        if (lang.getScript() != null) {
            String script = lang.getScript().getName();
            if (Arrays.binarySearch(RTL_SCRIPTS, script.toLowerCase()) > -1)
                return Direction.RTL;
        }
        String primary = lang.getLanguage().getName();
        if (Arrays.binarySearch(RTL_LANGS, primary.toLowerCase()) > -1)
            return Direction.RTL;
        return Direction.UNSPECIFIED;
    }

    /**
     * Algorithm that will determine text direction by looking at the character set encoding. If the charset is
     * typically used for RTL languages, Direction.RTL will be returned
     */
    public static Direction guessDirectionFromEncoding(String charset) {
        if (charset == null)
            return Direction.UNSPECIFIED;
        charset = charset.replace('_', '-');
        Arrays.sort(RTL_ENCODINGS);
        if (Arrays.binarySearch(RTL_ENCODINGS, charset.toLowerCase()) > -1)
            return Direction.RTL;
        return Direction.UNSPECIFIED;
    }

    /**
     * Algorithm that analyzes properties of the text to determine text direction. If the majority of characters in the
     * text are RTL characters, then Direction.RTL will be returned.
     */
    public static Direction guessDirectionFromTextProperties(String text) {
        if (text != null && text.length() > 0) {
            if (text.charAt(0) == 0x200F)
                return Direction.RTL; // if using the unicode right-to-left mark
            if (text.charAt(0) == 0x200E)
                return Direction.LTR; // if using the unicode left-to-right mark
            int c = 0;
            for (int n = 0; n < text.length(); n++) {
                char ch = text.charAt(n);
                if (java.text.Bidi.requiresBidi(new char[] {ch}, 0, 1))
                    c++;
                else
                    c--;
            }
            return c > 0 ? Direction.RTL : Direction.LTR;
        }
        return Direction.UNSPECIFIED;
    }

    /**
     * Algorithm that defers to the Java Bidi implementation to determine text direction.
     */
    public static Direction guessDirectionFromJavaBidi(String text) {
        if (text != null) {
            AttributedString s = new AttributedString(text);
            java.text.Bidi bidi = new java.text.Bidi(s.getIterator());
            return bidi.baseIsLeftToRight() ? Direction.LTR : Direction.RTL;
        }
        return Direction.UNSPECIFIED;
    }
}