special-case.h

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// Copyright 2019 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
 
#ifndef V8_REGEXP_SPECIAL_CASE_H_
#define V8_REGEXP_SPECIAL_CASE_H_
 
#ifdef V8_INTL_SUPPORT
#include "src/base/logging.h"
#include "src/common/globals.h"
 
#include "unicode/uchar.h"
#include "unicode/uniset.h"
#include "unicode/unistr.h"
 
namespace v8 {
namespace internal {
 
// Sets of Unicode characters that need special handling under "i" mode
 
// For non-unicode ignoreCase matches (aka "i", not "iu"), ECMA 262
// defines slightly different case-folding rules than Unicode. An
// input character should match a pattern character if the result of
// the Canonicalize algorithm is the same for both characters.
//
// Roughly speaking, for "i" regexps, Canonicalize(c) is the same as
// c.toUpperCase(), unless a) c.toUpperCase() is a multi-character
// string, or b) c is non-ASCII, and c.toUpperCase() is ASCII. See
// https://tc39.es/ecma262/#sec-runtime-semantics-canonicalize-ch for
// the precise definition.
//
// While compiling such regular expressions, we need to compute the
// set of characters that should match a given input character. (See
// GetCaseIndependentLetters and CharacterRange::AddCaseEquivalents.)
// For almost all characters, this can be efficiently computed using
// UnicodeSet::closeOver(USET_CASE_INSENSITIVE). These sets represent
// the remaining special cases.
//
// For a character c, the rules are as follows:
//
// 1. If c is in neither IgnoreSet nor SpecialAddSet, then calling
//    UnicodeSet::closeOver(USET_CASE_INSENSITIVE) on a UnicodeSet
//    containing c will produce the set of characters that should
//    match /c/i (or /[c]/i), and only those characters.
//
// 2. If c is in IgnoreSet, then the only character it should match is
//    itself. However, closeOver will add additional incorrect
//    matches. For example, consider SHARP S: 'ß' (U+00DF) and 'ẞ'
//    (U+1E9E). Although closeOver('ß') = "ßẞ", uppercase('ß') is
//    "SS".  Step 3.e therefore requires that 'ß' canonicalizes to
//    itself, and should not match 'ẞ'. In these cases, we can skip
//    the closeOver entirely, because it will never add an equivalent
//    character.
//
// 3. If c is in SpecialAddSet, then it should match at least one
//    character other than itself. However, closeOver will add at
//    least one additional incorrect match. For example, consider the
//    letter 'k'. Closing over 'k' gives "kKK" (lowercase k, uppercase
//    K, U+212A KELVIN SIGN). However, because of step 3.g, KELVIN
//    SIGN should not match either of the other two characters. As a
//    result, "k" and "K" are in SpecialAddSet (and KELVIN SIGN is in
//    IgnoreSet). To find the correct matches for characters in
//    SpecialAddSet, we closeOver the original character, but filter
//    out the results that do not have the same canonical value.
//
// The contents of these sets are calculated at build time by
// src/regexp/gen-regexp-special-case.cc, which generates
// gen/src/regexp/special-case.cc. This is done by iterating over the
// result of closeOver for each BMP character, and finding sets for
// which at least one character has a different canonical value than
// another character. Characters that match no other characters in
// their equivalence class are added to IgnoreSet. Characters that
// match at least one other character are added to SpecialAddSet.
 
class RegExpCaseFolding final : public AllStatic {
 public:
  static const icu::UnicodeSet& IgnoreSet();
  static const icu::UnicodeSet& SpecialAddSet();
 
  // This implements ECMAScript 2020 21.2.2.8.2 (Runtime Semantics:
  // Canonicalize) step 3, which is used to determine whether
  // characters match when ignoreCase is true and unicode is false.
  static UChar32 Canonicalize(UChar32 ch) {
    // a. Assert: ch is a UTF-16 code unit.
    CHECK_LE(ch, 0xffff);
 
    // b. Let s be the String value consisting of the single code unit ch.
    icu::UnicodeString s(ch);
 
    // c. Let u be the same result produced as if by performing the algorithm
    // for String.prototype.toUpperCase using s as the this value.
    // d. Assert: Type(u) is String.
    icu::UnicodeString& u = s.toUpper();
 
    // e. If u does not consist of a single code unit, return ch.
    if (u.length() != 1) {
      return ch;
    }
 
    // f. Let cu be u's single code unit element.
    UChar32 cu = u.char32At(0);
 
    // g. If the value of ch >= 128 and the value of cu < 128, return ch.
    if (ch >= 128 && cu < 128) {
      return ch;
    }
 
    // h. Return cu.
    return cu;
  }
};
 
}  // namespace internal
}  // namespace v8
 
#endif  // V8_INTL_SUPPORT
 
#endif  // V8_REGEXP_SPECIAL_CASE_H_