/* ***** BEGIN LICENSE BLOCK ***** * Version: MPL 1.1/GPL 2.0/LGPL 2.1 * * The contents of this file are subject to the Mozilla Public License Version * 1.1 (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.mozilla.org/MPL/ * * Software distributed under the License is distributed on an "AS IS" basis, * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License * for the specific language governing rights and limitations under the * License. * * The Original Code is Mozilla Universal charset detector code. * * The Initial Developer of the Original Code is * Netscape Communications Corporation. * Portions created by the Initial Developer are Copyright (C) 2001 * the Initial Developer. All Rights Reserved. * * Contributor(s): * Shy Shalom * Rudi Pettazzi (C# port) * * Alternatively, the contents of this file may be used under the terms of * either the GNU General Public License Version 2 or later (the "GPL"), or * the GNU Lesser General Public License Version 2.1 or later (the "LGPL"), * in which case the provisions of the GPL or the LGPL are applicable instead * of those above. If you wish to allow use of your version of this file only * under the terms of either the GPL or the LGPL, and not to allow others to * use your version of this file under the terms of the MPL, indicate your * decision by deleting the provisions above and replace them with the notice * and other provisions required by the GPL or the LGPL. If you do not delete * the provisions above, a recipient may use your version of this file under * the terms of any one of the MPL, the GPL or the LGPL. * * ***** END LICENSE BLOCK ***** */ using System; /** * General ideas of the Hebrew charset recognition * * Four main charsets exist in Hebrew: * "ISO-8859-8" - Visual Hebrew * "windows-1255" - Logical Hebrew * "ISO-8859-8-I" - Logical Hebrew * "x-mac-hebrew" - ?? Logical Hebrew ?? * * Both "ISO" charsets use a completely identical set of code points, whereas * "windows-1255" and "x-mac-hebrew" are two different proper supersets of * these code points. windows-1255 defines additional characters in the range * 0x80-0x9F as some misc punctuation marks as well as some Hebrew-specific * diacritics and additional 'Yiddish' ligature letters in the range 0xc0-0xd6. * x-mac-hebrew defines similar additional code points but with a different * mapping. * * As far as an average Hebrew text with no diacritics is concerned, all four * charsets are identical with respect to code points. Meaning that for the * main Hebrew alphabet, all four map the same values to all 27 Hebrew letters * (including final letters). * * The dominant difference between these charsets is their directionality. * "Visual" directionality means that the text is ordered as if the renderer is * not aware of a BIDI rendering algorithm. The renderer sees the text and * draws it from left to right. The text itself when ordered naturally is read * backwards. A buffer of Visual Hebrew generally looks like so: * "[last word of first line spelled backwards] [whole line ordered backwards * and spelled backwards] [first word of first line spelled backwards] * [end of line] [last word of second line] ... etc' " * adding punctuation marks, numbers and English text to visual text is * naturally also "visual" and from left to right. * * "Logical" directionality means the text is ordered "naturally" according to * the order it is read. It is the responsibility of the renderer to display * the text from right to left. A BIDI algorithm is used to place general * punctuation marks, numbers and English text in the text. * * Texts in x-mac-hebrew are almost impossible to find on the Internet. From * what little evidence I could find, it seems that its general directionality * is Logical. * * To sum up all of the above, the Hebrew probing mechanism knows about two * charsets: * Visual Hebrew - "ISO-8859-8" - backwards text - Words and sentences are * backwards while line order is natural. For charset recognition purposes * the line order is unimportant (In fact, for this implementation, even * word order is unimportant). * Logical Hebrew - "windows-1255" - normal, naturally ordered text. * * "ISO-8859-8-I" is a subset of windows-1255 and doesn't need to be * specifically identified. * "x-mac-hebrew" is also identified as windows-1255. A text in x-mac-hebrew * that contain special punctuation marks or diacritics is displayed with * some unconverted characters showing as question marks. This problem might * be corrected using another model prober for x-mac-hebrew. Due to the fact * that x-mac-hebrew texts are so rare, writing another model prober isn't * worth the effort and performance hit. * * *** The Prober *** * * The prober is divided between two nsSBCharSetProbers and an nsHebrewProber, * all of which are managed, created, fed data, inquired and deleted by the * nsSBCSGroupProber. The two nsSBCharSetProbers identify that the text is in * fact some kind of Hebrew, Logical or Visual. The final decision about which * one is it is made by the nsHebrewProber by combining final-letter scores * with the scores of the two nsSBCharSetProbers to produce a final answer. * * The nsSBCSGroupProber is responsible for stripping the original text of HTML * tags, English characters, numbers, low-ASCII punctuation characters, spaces * and new lines. It reduces any sequence of such characters to a single space. * The buffer fed to each prober in the SBCS group prober is pure text in * high-ASCII. * The two nsSBCharSetProbers (model probers) share the same language model: * Win1255Model. * The first nsSBCharSetProber uses the model normally as any other * nsSBCharSetProber does, to recognize windows-1255, upon which this model was * built. The second nsSBCharSetProber is told to make the pair-of-letter * lookup in the language model backwards. This in practice exactly simulates * a visual Hebrew model using the windows-1255 logical Hebrew model. * * The nsHebrewProber is not using any language model. All it does is look for * final-letter evidence suggesting the text is either logical Hebrew or visual * Hebrew. Disjointed from the model probers, the results of the nsHebrewProber * alone are meaningless. nsHebrewProber always returns 0.00 as confidence * since it never identifies a charset by itself. Instead, the pointer to the * nsHebrewProber is passed to the model probers as a helper "Name Prober". * When the Group prober receives a positive identification from any prober, * it asks for the name of the charset identified. If the prober queried is a * Hebrew model prober, the model prober forwards the call to the * nsHebrewProber to make the final decision. In the nsHebrewProber, the * decision is made according to the final-letters scores maintained and Both * model probers scores. The answer is returned in the form of the name of the * charset identified, either "windows-1255" or "ISO-8859-8". * */ namespace UniversalDetector.Core { /// /// This prober doesn't actually recognize a language or a charset. /// It is a helper prober for the use of the Hebrew model probers /// public class HebrewProber : CharsetProber { // windows-1255 / ISO-8859-8 code points of interest private const byte FINAL_KAF = 0xEA; private const byte NORMAL_KAF = 0xEB; private const byte FINAL_MEM = 0xED; private const byte NORMAL_MEM = 0xEE; private const byte FINAL_NUN = 0xEF; private const byte NORMAL_NUN = 0xF0; private const byte FINAL_PE = 0xF3; private const byte NORMAL_PE = 0xF4; private const byte FINAL_TSADI = 0xF5; private const byte NORMAL_TSADI = 0xF6; // Minimum Visual vs Logical final letter score difference. // If the difference is below this, don't rely solely on the final letter score distance. private const int MIN_FINAL_CHAR_DISTANCE = 5; // Minimum Visual vs Logical model score difference. // If the difference is below this, don't rely at all on the model score distance. private const float MIN_MODEL_DISTANCE = 0.01f; protected const string VISUAL_HEBREW_NAME = "ISO-8859-8"; protected const string LOGICAL_HEBREW_NAME = "windows-1255"; // owned by the group prober. protected CharsetProber logicalProber, visualProber; protected int finalCharLogicalScore, finalCharVisualScore; // The two last bytes seen in the previous buffer. protected byte prev, beforePrev; public HebrewProber() { Reset(); } public void SetModelProbers(CharsetProber logical, CharsetProber visual) { logicalProber = logical; visualProber = visual; } /** * Final letter analysis for logical-visual decision. * Look for evidence that the received buffer is either logical Hebrew or * visual Hebrew. * The following cases are checked: * 1) A word longer than 1 letter, ending with a final letter. This is an * indication that the text is laid out "naturally" since the final letter * really appears at the end. +1 for logical score. * 2) A word longer than 1 letter, ending with a Non-Final letter. In normal * Hebrew, words ending with Kaf, Mem, Nun, Pe or Tsadi, should not end with * the Non-Final form of that letter. Exceptions to this rule are mentioned * above in isNonFinal(). This is an indication that the text is laid out * backwards. +1 for visual score * 3) A word longer than 1 letter, starting with a final letter. Final letters * should not appear at the beginning of a word. This is an indication that * the text is laid out backwards. +1 for visual score. * * The visual score and logical score are accumulated throughout the text and * are finally checked against each other in GetCharSetName(). * No checking for final letters in the middle of words is done since that case * is not an indication for either Logical or Visual text. * * The input buffer should not contain any white spaces that are not (' ') * or any low-ascii punctuation marks. */ public override ProbingState HandleData(byte[] buf, int offset, int len) { // Both model probers say it's not them. No reason to continue. if (GetState() == ProbingState.NotMe) return ProbingState.NotMe; int max = offset + len; for (int i = offset; i < max; i++) { byte b = buf[i]; // a word just ended if (b == 0x20) { // *(curPtr-2) was not a space so prev is not a 1 letter word if (beforePrev != 0x20) { // case (1) [-2:not space][-1:final letter][cur:space] if (IsFinal(prev)) finalCharLogicalScore++; // case (2) [-2:not space][-1:Non-Final letter][cur:space] else if (IsNonFinal(prev)) finalCharVisualScore++; } } else { // case (3) [-2:space][-1:final letter][cur:not space] if ((beforePrev == 0x20) && (IsFinal(prev)) && (b != ' ')) ++finalCharVisualScore; } beforePrev = prev; prev = b; } // Forever detecting, till the end or until both model probers // return NotMe (handled above). return ProbingState.Detecting; } // Make the decision: is it Logical or Visual? public override string GetCharsetName() { // If the final letter score distance is dominant enough, rely on it. int finalsub = finalCharLogicalScore - finalCharVisualScore; if (finalsub >= MIN_FINAL_CHAR_DISTANCE) return LOGICAL_HEBREW_NAME; if (finalsub <= -(MIN_FINAL_CHAR_DISTANCE)) return VISUAL_HEBREW_NAME; // It's not dominant enough, try to rely on the model scores instead. float modelsub = logicalProber.GetConfidence() - visualProber.GetConfidence(); if (modelsub > MIN_MODEL_DISTANCE) return LOGICAL_HEBREW_NAME; if (modelsub < -(MIN_MODEL_DISTANCE)) return VISUAL_HEBREW_NAME; // Still no good, back to final letter distance, maybe it'll save the day. if (finalsub < 0) return VISUAL_HEBREW_NAME; // (finalsub > 0 - Logical) or (don't know what to do) default to Logical. return LOGICAL_HEBREW_NAME; } public override void Reset() { finalCharLogicalScore = 0; finalCharVisualScore = 0; prev = 0x20; beforePrev = 0x20; } public override ProbingState GetState() { // Remain active as long as any of the model probers are active. if (logicalProber.GetState() == ProbingState.NotMe && visualProber.GetState() == ProbingState.NotMe) return ProbingState.NotMe; return ProbingState.Detecting; } public override void DumpStatus() { //Console.WriteLine(" HEB: {0} - {1} [Logical-Visual score]", finalCharLogicalScore, finalCharVisualScore); } public override float GetConfidence() { return 0.0f; } protected static bool IsFinal(byte b) { return (b == FINAL_KAF || b == FINAL_MEM || b == FINAL_NUN || b == FINAL_PE || b == FINAL_TSADI); } protected static bool IsNonFinal(byte b) { // The normal Tsadi is not a good Non-Final letter due to words like // 'lechotet' (to chat) containing an apostrophe after the tsadi. This // apostrophe is converted to a space in FilterWithoutEnglishLetters causing // the Non-Final tsadi to appear at an end of a word even though this is not // the case in the original text. // The letters Pe and Kaf rarely display a related behavior of not being a // good Non-Final letter. Words like 'Pop', 'Winamp' and 'Mubarak' for // example legally end with a Non-Final Pe or Kaf. However, the benefit of // these letters as Non-Final letters outweighs the damage since these words // are quite rare. return (b == NORMAL_KAF || b == NORMAL_MEM || b == NORMAL_NUN || b == NORMAL_PE); } } }