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124 lines
4.9 KiB
C++
124 lines
4.9 KiB
C++
// Copyright 2018 Mozilla Foundation. See the COPYRIGHT
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// file at the top-level directory of this distribution.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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// Mostly copied and pasted from
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// third_party/rust/shift_or_euc/src/lib.rs , so
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// "top-level directory of this distribution" above refers to
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// third_party/rust/shift_or_euc/
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#ifndef mozilla_JapaneseDetector_h
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#define mozilla_JapaneseDetector_h
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#include "mozilla/Encoding.h"
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namespace mozilla {
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class JapaneseDetector;
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}; // namespace mozilla
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#define SHIFT_OR_EUC_DETECTOR mozilla::JapaneseDetector
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#include "shift_or_euc.h"
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namespace mozilla {
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/**
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* A Japanese legacy encoding detector for detecting between Shift_JIS,
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* EUC-JP, and, optionally, ISO-2022-JP _given_ the assumption that the
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* encoding is one of those.
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*
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* # Principle of Operation
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*
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* The detector is based on two observations:
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*
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* 1. The ISO-2022-JP escape sequences don't normally occur in Shift_JIS or
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* EUC-JP, so encountering such an escape sequence (before non-ASCII has been
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* encountered) can be taken as indication of ISO-2022-JP.
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* 2. When normal (full-with) kana or common kanji encoded as Shift_JIS is
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* decoded as EUC-JP, or vice versa, the result is either an error or
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* half-width katakana, and it's very uncommon for Japanese HTML to have
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* half-width katakana character before a normal kana or common kanji
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* character. Therefore, if decoding as Shift_JIS results in error or
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* have-width katakana, the detector decides that the content is EUC-JP, and
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* vice versa.
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*
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* # Failure Modes
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*
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* The detector gives the wrong answer if the text has a half-width katakana
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* character before normal kana or common kanji. Some uncommon kanji are
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* undecidable. (All JIS X 0208 Level 1 kanji are decidable.)
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*
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* The half-width katakana issue is mainly relevant for old 8-bit JIS X
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* 0201-only text files that would decode correctly as Shift_JIS but that the
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* detector detects as EUC-JP.
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*
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* The undecidable kanji issue does not realistically show up when a full
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* document is fed to the detector, because, realistically, in a full
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* document, there is at least one kana or common kanji. It can occur,
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* though, if the detector is only run on a prefix of a document and the
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* prefix only contains the title of the document. It is possible for
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* document title to consist entirely of undecidable kanji. (Indeed,
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* Japanese Wikipedia has articles with such titles.) If the detector is
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* undecided, a fallback to Shift_JIS should be used.
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*/
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class JapaneseDetector final {
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public:
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~JapaneseDetector() {}
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static void operator delete(void* aDetector) {
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shift_or_euc_detector_free(reinterpret_cast<JapaneseDetector*>(aDetector));
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}
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/**
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* Instantiates the detector. If `aAllow2022` is `true` the possible
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* guesses are Shift_JIS, EUC-JP, ISO-2022-JP, and undecided. If
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* `aAllow2022` is `false`, the possible guesses are Shift_JIS, EUC-JP,
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* and undecided.
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*/
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static inline UniquePtr<JapaneseDetector> Create(bool aAllow2022) {
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UniquePtr<JapaneseDetector> detector(shift_or_euc_detector_new(aAllow2022));
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return detector;
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}
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/**
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* Feeds bytes to the detector. If `aLast` is `true` the end of the stream
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* is considered to occur immediately after the end of `aBuffer`.
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* Otherwise, the stream is expected to continue. `aBuffer` may be empty.
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*
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* If you're running the detector only on a prefix of a complete
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* document, _do not_ pass `aLast` as `true` after the prefix if the
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* stream as a whole still contains more content.
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*
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* Returns `SHIFT_JIS_ENCODING` if the detector guessed
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* Shift_JIS. Returns `EUC_JP_ENCODING` if the detector
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* guessed EUC-JP. Returns `ISO_2022_JP_ENCODING` if the
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* detector guessed ISO-2022-JP (only possible if `true` was passed as
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* `aAllow2022` when instantiating the detector). Returns `nullptr` if the
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* detector is undecided. If `nullptr` is returned even when passing `true`
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* as `aLast`, falling back to Shift_JIS is the best guess for Web
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* purposes.
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*
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* Do not call again after the method has returned non-`nullptr` or after
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* the method has been called with `true` as `aLast`. (Asserts if the
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* previous sentence isn't adhered to.)
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*/
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inline const mozilla::Encoding* Feed(Span<const uint8_t> aBuffer,
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bool aLast) {
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return shift_or_euc_detector_feed(this, aBuffer.Elements(),
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aBuffer.Length(), aLast);
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}
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private:
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JapaneseDetector() = delete;
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JapaneseDetector(const JapaneseDetector&) = delete;
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JapaneseDetector& operator=(const JapaneseDetector&) = delete;
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};
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}; // namespace mozilla
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#endif // mozilla_JapaneseDetector_h
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