tesseract/ccutil/unicharcompress.h

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///////////////////////////////////////////////////////////////////////
// File: unicharcompress.h
// Description: Unicode re-encoding using a sequence of smaller numbers in
// place of a single large code for CJK, similarly for Indic,
// and dissection of ligatures for other scripts.
// Author: Ray Smith
// Created: Wed Mar 04 14:45:01 PST 2015
//
// (C) Copyright 2015, Google Inc.
// Licensed 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.
//
///////////////////////////////////////////////////////////////////////
#ifndef TESSERACT_CCUTIL_UNICHARCOMPRESS_H_
#define TESSERACT_CCUTIL_UNICHARCOMPRESS_H_
#include <unordered_map>
#include "serialis.h"
#include "strngs.h"
#include "unicharset.h"
namespace tesseract {
// Trivial class to hold the code for a recoded unichar-id.
class RecodedCharID {
public:
// The maximum length of a code.
static const int kMaxCodeLen = 9;
RecodedCharID() : self_normalized_(1), length_(0) {
memset(code_, 0, sizeof(code_));
}
void Truncate(int length) { length_ = length; }
// Sets the code value at the given index in the code.
void Set(int index, int value) {
code_[index] = value;
if (length_ <= index) length_ = index + 1;
}
// Shorthand for setting codes of length 3, as all Hangul and Han codes are
// length 3.
void Set3(int code0, int code1, int code2) {
length_ = 3;
code_[0] = code0;
code_[1] = code1;
code_[2] = code2;
}
// Accessors
int length() const { return length_; }
int operator()(int index) const { return code_[index]; }
// Writes to the given file. Returns false in case of error.
bool Serialize(TFile* fp) const {
if (fp->FWrite(&self_normalized_, sizeof(self_normalized_), 1) != 1)
return false;
if (fp->FWrite(&length_, sizeof(length_), 1) != 1) return false;
if (fp->FWrite(code_, sizeof(code_[0]), length_) != length_) return false;
return true;
}
// Reads from the given file. Returns false in case of error.
// If swap is true, assumes a big/little-endian swap is needed.
bool DeSerialize(TFile* fp) {
if (fp->FRead(&self_normalized_, sizeof(self_normalized_), 1) != 1)
return false;
if (fp->FReadEndian(&length_, sizeof(length_), 1) != 1) return false;
if (fp->FReadEndian(code_, sizeof(code_[0]), length_) != length_)
return false;
return true;
}
bool operator==(const RecodedCharID& other) const {
if (length_ != other.length_) return false;
for (int i = 0; i < length_; ++i) {
if (code_[i] != other.code_[i]) return false;
}
return true;
}
// Hash functor for RecodedCharID.
struct RecodedCharIDHash {
size_t operator()(const RecodedCharID& code) const {
size_t result = 0;
for (int i = 0; i < code.length_; ++i) {
result ^= code(i) << (7 * i);
}
return result;
}
};
private:
// True if this code is self-normalizing, ie is the master entry for indices
// that map to the same code. Has boolean value, but inT8 for serialization.
inT8 self_normalized_;
// The number of elements in use in code_;
inT32 length_;
// The re-encoded form of the unichar-id to which this RecodedCharID relates.
inT32 code_[kMaxCodeLen];
};
// Class holds a "compression" of a unicharset to simplify the learning problem
// for a neural-network-based classifier.
// Objectives:
// 1 (CJK): Ids of a unicharset with a large number of classes are expressed as
// a sequence of 3 codes with much fewer values.
// This is achieved using the Jamo coding for Hangul and the Unicode
// Radical-Stroke-index for Han.
// 2 (Indic): Instead of thousands of codes with one for each grapheme, re-code
// as the unicode sequence (but coded in a more compact space).
// 3 (the rest): Eliminate multi-path problems with ligatures and fold confusing
// and not significantly distinct shapes (quotes) togther, ie
// represent the fi ligature as the f-i pair, and fold u+2019 and
// friends all onto ascii single '
// 4 The null character and mapping to target activations:
// To save horizontal coding space, the compressed codes are generally mapped
// to target network activations without intervening null characters, BUT
// in the case of ligatures, such as ff, null characters have to be included
// so existence of repeated codes is detected at codebook-building time, and
// null characters are embedded directly into the codes, so the rest of the
// system doesn't need to worry about the problem (much). There is still an
// effect on the range of ways in which the target activations can be
// generated.
//
// The computed code values are compact (no unused values), and, for CJK,
// unique (each code position uses a disjoint set of values from each other code
// position). For non-CJK, the same code value CAN be used in multiple
// positions, eg the ff ligature is converted to <f> <nullchar> <f>, where <f>
// is the same code as is used for the single f.
class UnicharCompress {
public:
UnicharCompress();
UnicharCompress(const UnicharCompress& src);
~UnicharCompress();
UnicharCompress& operator=(const UnicharCompress& src);
// The 1st Hangul unicode.
static const int kFirstHangul = 0xac00;
// The number of Hangul unicodes.
static const int kNumHangul = 11172;
// The number of Jamos for each of the 3 parts of a Hangul character, being
// the Leading consonant, Vowel and Trailing consonant.
static const int kLCount = 19;
static const int kVCount = 21;
static const int kTCount = 28;
// Computes the encoding for the given unicharset. It is a requirement that
// the file training/langdata/radical-stroke.txt have been read into the
// input string radical_stroke_table.
// Returns false if the encoding cannot be constructed.
bool ComputeEncoding(const UNICHARSET& unicharset, int null_id,
STRING* radical_stroke_table);
// Sets up an encoder that doesn't change the unichars at all, so it just
// passes them through unchanged.
void SetupPassThrough(const UNICHARSET& unicharset);
// Sets up an encoder directly using the given encoding vector, which maps
// unichar_ids to the given codes.
void SetupDirect(const GenericVector<RecodedCharID>& codes);
// Returns the number of different values that can be used in a code, ie
// 1 + the maximum value that will ever be used by an RecodedCharID code in
// any position in its array.
int code_range() const { return code_range_; }
// Encodes a single unichar_id. Returns the length of the code, (or zero if
// invalid input), and the encoding itself in code.
int EncodeUnichar(int unichar_id, RecodedCharID* code) const;
// Decodes code, returning the original unichar-id, or
// INVALID_UNICHAR_ID if the input is invalid.
int DecodeUnichar(const RecodedCharID& code) const;
// Returns true if the given code is a valid start or single code.
bool IsValidFirstCode(int code) const { return is_valid_start_[code]; }
// Returns a list of valid non-final next codes for a given prefix code,
// which may be empty.
const GenericVector<int>* GetNextCodes(const RecodedCharID& code) const {
auto it = next_codes_.find(code);
return it == next_codes_.end() ? NULL : it->second;
}
// Returns a list of valid final codes for a given prefix code, which may
// be empty.
const GenericVector<int>* GetFinalCodes(const RecodedCharID& code) const {
auto it = final_codes_.find(code);
return it == final_codes_.end() ? NULL : it->second;
}
// Writes to the given file. Returns false in case of error.
bool Serialize(TFile* fp) const;
// Reads from the given file. Returns false in case of error.
bool DeSerialize(TFile* fp);
// Returns a STRING containing a text file that describes the encoding thus:
// <index>[,<index>]*<tab><UTF8-str><newline>
// In words, a comma-separated list of one or more indices, followed by a tab
// and the UTF-8 string that the code represents per line. Most simple scripts
// will encode a single index to a UTF8-string, but Chinese, Japanese, Korean
// and the Indic scripts will contain a many-to-many mapping.
// See the class comment above for details.
STRING GetEncodingAsString(const UNICHARSET& unicharset) const;
// Helper decomposes a Hangul unicode to 3 parts, leading, vowel, trailing.
// Note that the returned values are 0-based indices, NOT unicode Jamo.
// Returns false if the input is not in the Hangul unicode range.
static bool DecomposeHangul(int unicode, int* leading, int* vowel,
int* trailing);
private:
// Renumbers codes to eliminate unused values.
void DefragmentCodeValues(int encoded_null);
// Computes the value of code_range_ from the encoder_.
void ComputeCodeRange();
// Initializes the decoding hash_map from the encoder_ array.
void SetupDecoder();
// Frees allocated memory.
void Cleanup();
// The encoder that maps a unichar-id to a sequence of small codes.
// encoder_ is the only part that is serialized. The rest is computed on load.
GenericVector<RecodedCharID> encoder_;
// Decoder converts the output of encoder back to a unichar-id.
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std::unordered_map<RecodedCharID, int, RecodedCharID::RecodedCharIDHash>
decoder_;
// True if the index is a valid single or start code.
GenericVector<bool> is_valid_start_;
// Maps a prefix code to a list of valid next codes.
// The map owns the vectors.
std::unordered_map<RecodedCharID, GenericVectorEqEq<int>*,
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RecodedCharID::RecodedCharIDHash>
next_codes_;
// Maps a prefix code to a list of valid final codes.
// The map owns the vectors.
std::unordered_map<RecodedCharID, GenericVectorEqEq<int>*,
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RecodedCharID::RecodedCharIDHash>
final_codes_;
// Max of any value in encoder_ + 1.
int code_range_;
};
} // namespace tesseract.
#endif // TESSERACT_CCUTIL_UNICHARCOMPRESS_H_