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