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diff --git a/js/src/irregexp/imported/special-case.h b/js/src/irregexp/imported/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 "irregexp/RegExpShim.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.
+//
+// For unicode ignoreCase ("iu" and "iv"),
+// UnicodeSet::closeOver(USET_CASE_INSENSITIVE) adds all characters that are in
+// the same equivalence class. This includes characaters that are in the same
+// equivalence class using full case folding. According to the spec, only
+// simple case folding shall be considered. We therefore create
+// UnicodeNonSimpleCloseOverSet containing all characters for which
+// UnicodeSet::closeOver adds characters that are not simple case folds. This
+// set should be used similar to IgnoreSet described above.
+
+class RegExpCaseFolding final : public AllStatic {
+ public:
+  static const icu::UnicodeSet& IgnoreSet();
+  static const icu::UnicodeSet& SpecialAddSet();
+  static const icu::UnicodeSet& UnicodeNonSimpleCloseOverSet();
+
+  // 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_