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 // © 2016 and later: Unicode, Inc. and others.
 // License & terms of use: http://www.unicode.org/copyright.html
 /*
 ******************************************************************************
 *   Copyright (C) 1996-2016, International Business Machines
 *   Corporation and others.  All Rights Reserved.
 ******************************************************************************
 */
 
 /**
  * \file
  * \brief C++ API: Collation Service.
  */
 
 /**
 * File coll.h
 *
 * Created by: Helena Shih
 *
 * Modification History:
 *
 *  Date        Name        Description
 * 02/5/97      aliu        Modified createDefault to load collation data from
 *                          binary files when possible.  Added related methods
 *                          createCollationFromFile, chopLocale, createPathName.
 * 02/11/97     aliu        Added members addToCache, findInCache, and fgCache.
 * 02/12/97     aliu        Modified to create objects from RuleBasedCollator cache.
 *                          Moved cache out of Collation class.
 * 02/13/97     aliu        Moved several methods out of this class and into
 *                          RuleBasedCollator, with modifications.  Modified
 *                          createDefault() to call new RuleBasedCollator(Locale&)
 *                          constructor.  General clean up and documentation.
 * 02/20/97     helena      Added clone, operator==, operator!=, operator=, copy
 *                          constructor and getDynamicClassID.
 * 03/25/97     helena      Updated with platform independent data types.
 * 05/06/97     helena      Added memory allocation error detection.
 * 06/20/97     helena      Java class name change.
 * 09/03/97     helena      Added createCollationKeyValues().
 * 02/10/98     damiba      Added compare() with length as parameter.
 * 04/23/99     stephen     Removed EDecompositionMode, merged with
 *                          Normalizer::EMode.
 * 11/02/99     helena      Collator performance enhancements.  Eliminates the
 *                          UnicodeString construction and special case for NO_OP.
 * 11/23/99     srl         More performance enhancements. Inlining of
 *                          critical accessors.
 * 05/15/00     helena      Added version information API.
 * 01/29/01     synwee      Modified into a C++ wrapper which calls C apis
 *                          (ucol.h).
 * 2012-2014    markus      Rewritten in C++ again.
 */
 
 #ifndef COLL_H
 #define COLL_H
 
 #include "unicode/utypes.h"
 
 #if U_SHOW_CPLUSPLUS_API
 
 #if !UCONFIG_NO_COLLATION
 
 #include <functional>
 #include <string_view>
 #include <type_traits>
 
 #include "unicode/char16ptr.h"
 #include "unicode/uobject.h"
 #include "unicode/ucol.h"
 #include "unicode/unorm.h"
 #include "unicode/locid.h"
 #include "unicode/uniset.h"
 #include "unicode/umisc.h"
 #include "unicode/unistr.h"
 #include "unicode/uiter.h"
 #include "unicode/stringpiece.h"
 
 U_NAMESPACE_BEGIN
 
 class StringEnumeration;
 
 #if !UCONFIG_NO_SERVICE
 /**
  * @stable ICU 2.6
  */
 class CollatorFactory;
 #endif
 
 /**
 * @stable ICU 2.0
 */
 class CollationKey;
 
 /**
 * The <code>Collator</code> class performs locale-sensitive string
 * comparison.<br>
 * You use this class to build searching and sorting routines for natural
 * language text.
 * <p>
 * <code>Collator</code> is an abstract base class. Subclasses implement
 * specific collation strategies. One subclass,
 * <code>RuleBasedCollator</code>, is currently provided and is applicable
 * to a wide set of languages. Other subclasses may be created to handle more
 * specialized needs.
 * <p>
 * Like other locale-sensitive classes, you can use the static factory method,
 * <code>createInstance</code>, to obtain the appropriate
 * <code>Collator</code> object for a given locale. You will only need to
 * look at the subclasses of <code>Collator</code> if you need to
 * understand the details of a particular collation strategy or if you need to
 * modify that strategy.
 * <p>
 * The following example shows how to compare two strings using the
 * <code>Collator</code> for the default locale.
 * \htmlonly<blockquote>\endhtmlonly
 * <pre>
 * \code
 * // Compare two strings in the default locale
 * UErrorCode success = U_ZERO_ERROR;
 * Collator* myCollator = Collator::createInstance(success);
 * if (myCollator->compare("abc", "ABC") < 0)
 *   cout << "abc is less than ABC" << endl;
 * else
 *   cout << "abc is greater than or equal to ABC" << endl;
 * \endcode
 * </pre>
 * \htmlonly</blockquote>\endhtmlonly
 * <p>
 * You can set a <code>Collator</code>'s <em>strength</em> attribute to
 * determine the level of difference considered significant in comparisons.
 * Five strengths are provided: <code>PRIMARY</code>, <code>SECONDARY</code>,
 * <code>TERTIARY</code>, <code>QUATERNARY</code> and <code>IDENTICAL</code>.
 * The exact assignment of strengths to language features is locale dependent.
 * For example, in Czech, "e" and "f" are considered primary differences,
 * while "e" and "\u00EA" are secondary differences, "e" and "E" are tertiary
 * differences and "e" and "e" are identical. The following shows how both case
 * and accents could be ignored for US English.
 * \htmlonly<blockquote>\endhtmlonly
 * <pre>
 * \code
 * //Get the Collator for US English and set its strength to PRIMARY
 * UErrorCode success = U_ZERO_ERROR;
 * Collator* usCollator = Collator::createInstance(Locale::getUS(), success);
 * usCollator->setStrength(Collator::PRIMARY);
 * if (usCollator->compare("abc", "ABC") == 0)
 *     cout << "'abc' and 'ABC' strings are equivalent with strength PRIMARY" << endl;
 * \endcode
 * </pre>
 * \htmlonly</blockquote>\endhtmlonly
 *
 * The <code>getSortKey</code> methods
 * convert a string to a series of bytes that can be compared bitwise against
 * other sort keys using <code>strcmp()</code>. Sort keys are written as
 * zero-terminated byte strings.
 *
 * Another set of APIs returns a <code>CollationKey</code> object that wraps
 * the sort key bytes instead of returning the bytes themselves.
 * </p>
 * <p>
 * <strong>Note:</strong> <code>Collator</code>s with different Locale,
 * and CollationStrength settings will return different sort
 * orders for the same set of strings. Locales have specific collation rules,
 * and the way in which secondary and tertiary differences are taken into
 * account, for example, will result in a different sorting order for same
 * strings.
 * </p>
 * @see         RuleBasedCollator
 * @see         CollationKey
 * @see         CollationElementIterator
 * @see         Locale
 * @see         Normalizer2
 * @version     2.0 11/15/01
 */
 
 class U_I18N_API Collator : public UObject {
 public:
 
     // Collator public enums -----------------------------------------------
 
     /**
      * Base letter represents a primary difference. Set comparison level to
      * PRIMARY to ignore secondary and tertiary differences.<br>
      * Use this to set the strength of a Collator object.<br>
      * Example of primary difference, "abc" &lt; "abd"
      *
      * Diacritical differences on the same base letter represent a secondary
      * difference. Set comparison level to SECONDARY to ignore tertiary
      * differences. Use this to set the strength of a Collator object.<br>
      * Example of secondary difference, "&auml;" >> "a".
      *
      * Uppercase and lowercase versions of the same character represents a
      * tertiary difference.  Set comparison level to TERTIARY to include all
      * comparison differences. Use this to set the strength of a Collator
      * object.<br>
      * Example of tertiary difference, "abc" &lt;&lt;&lt; "ABC".
      *
      * Two characters are considered "identical" when they have the same unicode
      * spellings.<br>
      * For example, "&auml;" == "&auml;".
      *
      * UCollationStrength is also used to determine the strength of sort keys
      * generated from Collator objects.
      * @stable ICU 2.0
      */
     enum ECollationStrength
     {
         PRIMARY    = UCOL_PRIMARY,  // 0
         SECONDARY  = UCOL_SECONDARY,  // 1
         TERTIARY   = UCOL_TERTIARY,  // 2
         QUATERNARY = UCOL_QUATERNARY,  // 3
         IDENTICAL  = UCOL_IDENTICAL  // 15
     };
 
 
     // Cannot use #ifndef U_HIDE_DEPRECATED_API for the following, it is
     // used by virtual methods that cannot have that conditional.
 #ifndef U_FORCE_HIDE_DEPRECATED_API
     /**
      * LESS is returned if source string is compared to be less than target
      * string in the compare() method.
      * EQUAL is returned if source string is compared to be equal to target
      * string in the compare() method.
      * GREATER is returned if source string is compared to be greater than
      * target string in the compare() method.
      * @see Collator#compare
      * @deprecated ICU 2.6. Use C enum UCollationResult defined in ucol.h
      */
     enum EComparisonResult
     {
         LESS = UCOL_LESS,  // -1
         EQUAL = UCOL_EQUAL,  // 0
         GREATER = UCOL_GREATER  // 1
     };
 #endif  // U_FORCE_HIDE_DEPRECATED_API
 
     // Collator public destructor -----------------------------------------
 
     /**
      * Destructor
      * @stable ICU 2.0
      */
     virtual ~Collator();
 
     // Collator public methods --------------------------------------------
 
     /**
      * Returns true if "other" is the same as "this".
      *
      * The base class implementation returns true if "other" has the same type/class as "this":
      * `typeid(*this) == typeid(other)`.
      *
      * Subclass implementations should do something like the following:
      *
      *     if (this == &other) { return true; }
      *     if (!Collator::operator==(other)) { return false; }  // not the same class
      *
      *     const MyCollator &o = (const MyCollator&)other;
      *     (compare this vs. o's subclass fields)
      *
      * @param other Collator object to be compared
      * @return true if other is the same as this.
      * @stable ICU 2.0
      */
     virtual bool operator==(const Collator& other) const;
 
     /**
      * Returns true if "other" is not the same as "this".
      * Calls ! operator==(const Collator&) const which works for all subclasses.
      * @param other Collator object to be compared
      * @return true if other is not the same as this.
      * @stable ICU 2.0
      */
     virtual bool operator!=(const Collator& other) const;
 
     /**
      * Makes a copy of this object.
      * @return a copy of this object, owned by the caller
      * @stable ICU 2.0
      */
     virtual Collator* clone() const = 0;
 
     /**
      * Creates the Collator object for the current default locale.
      * The default locale is determined by Locale::getDefault.
      * The UErrorCode& err parameter is used to return status information to the user.
      * To check whether the construction succeeded or not, you should check the
      * value of U_SUCCESS(err).  If you wish more detailed information, you can
      * check for informational error results which still indicate success.
      * U_USING_FALLBACK_ERROR indicates that a fall back locale was used. For
      * example, 'de_CH' was requested, but nothing was found there, so 'de' was
      * used. U_USING_DEFAULT_ERROR indicates that the default locale data was
      * used; neither the requested locale nor any of its fall back locales
      * could be found.
      * The caller owns the returned object and is responsible for deleting it.
      *
      * @param err    the error code status.
      * @return       the collation object of the default locale.(for example, en_US)
      * @see Locale#getDefault
      * @stable ICU 2.0
      */
     static Collator* U_EXPORT2 createInstance(UErrorCode&  err);
 
     /**
      * Gets the collation object for the desired locale. The
      * resource of the desired locale will be loaded.
      *
      * Locale::getRoot() is the base collation table and all other languages are
      * built on top of it with additional language-specific modifications.
      *
      * For some languages, multiple collation types are available;
      * for example, "de@collation=phonebook".
      * Starting with ICU 54, collation attributes can be specified via locale keywords as well,
      * in the old locale extension syntax ("el@colCaseFirst=upper")
      * or in language tag syntax ("el-u-kf-upper").
      * See <a href="https://unicode-org.github.io/icu/userguide/collation/api">User Guide: Collation API</a>.
      *
      * The UErrorCode& err parameter is used to return status information to the user.
      * To check whether the construction succeeded or not, you should check
      * the value of U_SUCCESS(err).  If you wish more detailed information, you
      * can check for informational error results which still indicate success.
      * U_USING_FALLBACK_ERROR indicates that a fall back locale was used.  For
      * example, 'de_CH' was requested, but nothing was found there, so 'de' was
      * used.  U_USING_DEFAULT_ERROR indicates that the default locale data was
      * used; neither the requested locale nor any of its fall back locales
      * could be found.
      *
      * The caller owns the returned object and is responsible for deleting it.
      * @param loc    The locale ID for which to open a collator.
      * @param err    the error code status.
      * @return       the created table-based collation object based on the desired
      *               locale.
      * @see Locale
      * @see ResourceLoader
      * @stable ICU 2.2
      */
     static Collator* U_EXPORT2 createInstance(const Locale& loc, UErrorCode& err);
 
 #ifndef U_FORCE_HIDE_DEPRECATED_API
     /**
      * The comparison function compares the character data stored in two
      * different strings. Returns information about whether a string is less
      * than, greater than or equal to another string.
      * @param source the source string to be compared with.
      * @param target the string that is to be compared with the source string.
      * @return Returns a byte value. GREATER if source is greater
      * than target; EQUAL if source is equal to target; LESS if source is less
      * than target
      * @deprecated ICU 2.6 use the overload with UErrorCode &
      */
     virtual EComparisonResult compare(const UnicodeString& source,
                                       const UnicodeString& target) const;
 #endif  // U_FORCE_HIDE_DEPRECATED_API
 
     /**
      * The comparison function compares the character data stored in two
      * different strings. Returns information about whether a string is less
      * than, greater than or equal to another string.
      * @param source the source string to be compared with.
      * @param target the string that is to be compared with the source string.
      * @param status possible error code
      * @return Returns an enum value. UCOL_GREATER if source is greater
      * than target; UCOL_EQUAL if source is equal to target; UCOL_LESS if source is less
      * than target
      * @stable ICU 2.6
      */
     virtual UCollationResult compare(const UnicodeString& source,
                                       const UnicodeString& target,
                                       UErrorCode &status) const = 0;
 
 #ifndef U_FORCE_HIDE_DEPRECATED_API
     /**
      * Does the same thing as compare but limits the comparison to a specified
      * length
      * @param source the source string to be compared with.
      * @param target the string that is to be compared with the source string.
      * @param length the length the comparison is limited to
      * @return Returns a byte value. GREATER if source (up to the specified
      *         length) is greater than target; EQUAL if source (up to specified
      *         length) is equal to target; LESS if source (up to the specified
      *         length) is less  than target.
      * @deprecated ICU 2.6 use the overload with UErrorCode &
      */
     virtual EComparisonResult compare(const UnicodeString& source,
                                       const UnicodeString& target,
                                       int32_t length) const;
 #endif  // U_FORCE_HIDE_DEPRECATED_API
 
     /**
      * Does the same thing as compare but limits the comparison to a specified
      * length
      * @param source the source string to be compared with.
      * @param target the string that is to be compared with the source string.
      * @param length the length the comparison is limited to
      * @param status possible error code
      * @return Returns an enum value. UCOL_GREATER if source (up to the specified
      *         length) is greater than target; UCOL_EQUAL if source (up to specified
      *         length) is equal to target; UCOL_LESS if source (up to the specified
      *         length) is less  than target.
      * @stable ICU 2.6
      */
     virtual UCollationResult compare(const UnicodeString& source,
                                       const UnicodeString& target,
                                       int32_t length,
                                       UErrorCode &status) const = 0;
 
 #ifndef U_FORCE_HIDE_DEPRECATED_API
     /**
      * The comparison function compares the character data stored in two
      * different string arrays. Returns information about whether a string array
      * is less than, greater than or equal to another string array.
      * <p>Example of use:
      * <pre>
      * .       char16_t ABC[] = {0x41, 0x42, 0x43, 0};  // = "ABC"
      * .       char16_t abc[] = {0x61, 0x62, 0x63, 0};  // = "abc"
      * .       UErrorCode status = U_ZERO_ERROR;
      * .       Collator *myCollation =
      * .                         Collator::createInstance(Locale::getUS(), status);
      * .       if (U_FAILURE(status)) return;
      * .       myCollation->setStrength(Collator::PRIMARY);
      * .       // result would be Collator::EQUAL ("abc" == "ABC")
      * .       // (no primary difference between "abc" and "ABC")
      * .       Collator::EComparisonResult result =
      * .                             myCollation->compare(abc, 3, ABC, 3);
      * .       myCollation->setStrength(Collator::TERTIARY);
      * .       // result would be Collator::LESS ("abc" &lt;&lt;&lt; "ABC")
      * .       // (with tertiary difference between "abc" and "ABC")
      * .       result = myCollation->compare(abc, 3, ABC, 3);
      * </pre>
      * @param source the source string array to be compared with.
      * @param sourceLength the length of the source string array.  If this value
      *        is equal to -1, the string array is null-terminated.
      * @param target the string that is to be compared with the source string.
      * @param targetLength the length of the target string array.  If this value
      *        is equal to -1, the string array is null-terminated.
      * @return Returns a byte value. GREATER if source is greater than target;
      *         EQUAL if source is equal to target; LESS if source is less than
      *         target
      * @deprecated ICU 2.6 use the overload with UErrorCode &
      */
     virtual EComparisonResult compare(const char16_t* source, int32_t sourceLength,
                                       const char16_t* target, int32_t targetLength)
                                       const;
 #endif  // U_FORCE_HIDE_DEPRECATED_API
 
     /**
      * The comparison function compares the character data stored in two
      * different string arrays. Returns information about whether a string array
      * is less than, greater than or equal to another string array.
      * @param source the source string array to be compared with.
      * @param sourceLength the length of the source string array.  If this value
      *        is equal to -1, the string array is null-terminated.
      * @param target the string that is to be compared with the source string.
      * @param targetLength the length of the target string array.  If this value
      *        is equal to -1, the string array is null-terminated.
      * @param status possible error code
      * @return Returns an enum value. UCOL_GREATER if source is greater
      * than target; UCOL_EQUAL if source is equal to target; UCOL_LESS if source is less
      * than target
      * @stable ICU 2.6
      */
     virtual UCollationResult compare(const char16_t* source, int32_t sourceLength,
                                       const char16_t* target, int32_t targetLength,
                                       UErrorCode &status) const = 0;
 
     /**
      * Compares two strings using the Collator.
      * Returns whether the first one compares less than/equal to/greater than
      * the second one.
      * This version takes UCharIterator input.
      * @param sIter the first ("source") string iterator
      * @param tIter the second ("target") string iterator
      * @param status ICU status
      * @return UCOL_LESS, UCOL_EQUAL or UCOL_GREATER
      * @stable ICU 4.2
      */
     virtual UCollationResult compare(UCharIterator &sIter,
                                      UCharIterator &tIter,
                                      UErrorCode &status) const;
 
     /**
      * Compares two UTF-8 strings using the Collator.
      * Returns whether the first one compares less than/equal to/greater than
      * the second one.
      * This version takes UTF-8 input.
      * Note that a StringPiece can be implicitly constructed
      * from a std::string or a NUL-terminated const char * string.
      * @param source the first UTF-8 string
      * @param target the second UTF-8 string
      * @param status ICU status
      * @return UCOL_LESS, UCOL_EQUAL or UCOL_GREATER
      * @stable ICU 4.2
      */
     virtual UCollationResult compareUTF8(const StringPiece &source,
                                          const StringPiece &target,
                                          UErrorCode &status) const;
 
     /**
      * Transforms the string into a series of characters that can be compared
      * with CollationKey::compareTo. It is not possible to restore the original
      * string from the chars in the sort key.
      * <p>Use CollationKey::equals or CollationKey::compare to compare the
      * generated sort keys.
      * If the source string is null, a null collation key will be returned.
      *
      * Note that sort keys are often less efficient than simply doing comparison.
      * For more details, see the ICU User Guide.
      *
      * @param source the source string to be transformed into a sort key.
      * @param key the collation key to be filled in
      * @param status the error code status.
      * @return the collation key of the string based on the collation rules.
      * @see CollationKey#compare
      * @stable ICU 2.0
      */
     virtual CollationKey& getCollationKey(const UnicodeString&  source,
                                           CollationKey& key,
                                           UErrorCode& status) const = 0;
 
     /**
      * Transforms the string into a series of characters that can be compared
      * with CollationKey::compareTo. It is not possible to restore the original
      * string from the chars in the sort key.
      * <p>Use CollationKey::equals or CollationKey::compare to compare the
      * generated sort keys.
      * <p>If the source string is null, a null collation key will be returned.
      *
      * Note that sort keys are often less efficient than simply doing comparison.
      * For more details, see the ICU User Guide.
      *
      * @param source the source string to be transformed into a sort key.
      * @param sourceLength length of the collation key
      * @param key the collation key to be filled in
      * @param status the error code status.
      * @return the collation key of the string based on the collation rules.
      * @see CollationKey#compare
      * @stable ICU 2.0
      */
     virtual CollationKey& getCollationKey(const char16_t*source,
                                           int32_t sourceLength,
                                           CollationKey& key,
                                           UErrorCode& status) const = 0;
     /**
      * Generates the hash code for the collation object
      * @stable ICU 2.0
      */
     virtual int32_t hashCode() const = 0;
 
 #ifndef U_FORCE_HIDE_DEPRECATED_API
     /**
      * Gets the locale of the Collator
      *
      * @param type can be either requested, valid or actual locale. For more
      *             information see the definition of ULocDataLocaleType in
      *             uloc.h
      * @param status the error code status.
      * @return locale where the collation data lives. If the collator
      *         was instantiated from rules, locale is empty.
      * @deprecated ICU 2.8 This API is under consideration for revision
      * in ICU 3.0.
      */
     virtual Locale getLocale(ULocDataLocaleType type, UErrorCode& status) const = 0;
 #endif  // U_FORCE_HIDE_DEPRECATED_API
 
     /**
      * Convenience method for comparing two strings based on the collation rules.
      * @param source the source string to be compared with.
      * @param target the target string to be compared with.
      * @return true if the first string is greater than the second one,
      *         according to the collation rules. false, otherwise.
      * @see Collator#compare
      * @stable ICU 2.0
      */
     UBool greater(const UnicodeString& source, const UnicodeString& target)
                   const;
 
     /**
      * Convenience method for comparing two strings based on the collation rules.
      * @param source the source string to be compared with.
      * @param target the target string to be compared with.
      * @return true if the first string is greater than or equal to the second
      *         one, according to the collation rules. false, otherwise.
      * @see Collator#compare
      * @stable ICU 2.0
      */
     UBool greaterOrEqual(const UnicodeString& source,
                          const UnicodeString& target) const;
 
     /**
      * Convenience method for comparing two strings based on the collation rules.
      * @param source the source string to be compared with.
      * @param target the target string to be compared with.
      * @return true if the strings are equal according to the collation rules.
      *         false, otherwise.
      * @see Collator#compare
      * @stable ICU 2.0
      */
     UBool equals(const UnicodeString& source, const UnicodeString& target) const;
 
 #ifndef U_HIDE_DRAFT_API
 
     /**
      * Creates a comparison function object that uses this collator.
      * Like <code>std::equal_to</code> but uses the collator instead of <code>operator==</code>.
      * @draft ICU 76
      */
     inline auto equal_to() const { return Predicate<std::equal_to, UCOL_EQUAL>(*this); }
 
     /**
      * Creates a comparison function object that uses this collator.
      * Like <code>std::greater</code> but uses the collator instead of <code>operator&gt;</code>.
      * @draft ICU 76
      */
     inline auto greater() const { return Predicate<std::equal_to, UCOL_GREATER>(*this); }
 
     /**
      * Creates a comparison function object that uses this collator.
      * Like <code>std::less</code> but uses the collator instead of <code>operator&lt;</code>.
      * @draft ICU 76
      */
     inline auto less() const { return Predicate<std::equal_to, UCOL_LESS>(*this); }
 
     /**
      * Creates a comparison function object that uses this collator.
      * Like <code>std::not_equal_to</code> but uses the collator instead of <code>operator!=</code>.
      * @draft ICU 76
      */
     inline auto not_equal_to() const { return Predicate<std::not_equal_to, UCOL_EQUAL>(*this); }
 
     /**
      * Creates a comparison function object that uses this collator.
      * Like <code>std::greater_equal</code> but uses the collator instead of <code>operator&gt;=</code>.
      * @draft ICU 76
      */
     inline auto greater_equal() const { return Predicate<std::not_equal_to, UCOL_LESS>(*this); }
 
     /**
      * Creates a comparison function object that uses this collator.
      * Like <code>std::less_equal</code> but uses the collator instead of <code>operator&lt;=</code>.
      * @draft ICU 76
      */
     inline auto less_equal() const { return Predicate<std::not_equal_to, UCOL_GREATER>(*this); }
 
 #endif  // U_HIDE_DRAFT_API
 
 #ifndef U_FORCE_HIDE_DEPRECATED_API
     /**
      * Determines the minimum strength that will be used in comparison or
      * transformation.
      * <p>E.g. with strength == SECONDARY, the tertiary difference is ignored
      * <p>E.g. with strength == PRIMARY, the secondary and tertiary difference
      * are ignored.
      * @return the current comparison level.
      * @see Collator#setStrength
      * @deprecated ICU 2.6 Use getAttribute(UCOL_STRENGTH...) instead
      */
     virtual ECollationStrength getStrength() const;
 
     /**
      * Sets the minimum strength to be used in comparison or transformation.
      * <p>Example of use:
      * <pre>
      *  \code
      *  UErrorCode status = U_ZERO_ERROR;
      *  Collator*myCollation = Collator::createInstance(Locale::getUS(), status);
      *  if (U_FAILURE(status)) return;
      *  myCollation->setStrength(Collator::PRIMARY);
      *  // result will be "abc" == "ABC"
      *  // tertiary differences will be ignored
      *  Collator::ComparisonResult result = myCollation->compare("abc", "ABC");
      * \endcode
      * </pre>
      * @see Collator#getStrength
      * @param newStrength the new comparison level.
      * @deprecated ICU 2.6 Use setAttribute(UCOL_STRENGTH...) instead
      */
     virtual void setStrength(ECollationStrength newStrength);
 #endif  // U_FORCE_HIDE_DEPRECATED_API
 
     /**
      * Retrieves the reordering codes for this collator.
      * @param dest The array to fill with the script ordering.
      * @param destCapacity The length of dest. If it is 0, then dest may be nullptr and the function
      *  will only return the length of the result without writing any codes (pre-flighting).
      * @param status A reference to an error code value, which must not indicate
      * a failure before the function call.
      * @return The length of the script ordering array.
      * @see ucol_setReorderCodes
      * @see Collator#getEquivalentReorderCodes
      * @see Collator#setReorderCodes
      * @see UScriptCode
      * @see UColReorderCode
      * @stable ICU 4.8
      */
      virtual int32_t getReorderCodes(int32_t *dest,
                                      int32_t destCapacity,
                                      UErrorCode& status) const;
 
     /**
      * Sets the ordering of scripts for this collator.
      *
      * <p>The reordering codes are a combination of script codes and reorder codes.
      * @param reorderCodes An array of script codes in the new order. This can be nullptr if the
      * length is also set to 0. An empty array will clear any reordering codes on the collator.
      * @param reorderCodesLength The length of reorderCodes.
      * @param status error code
      * @see ucol_setReorderCodes
      * @see Collator#getReorderCodes
      * @see Collator#getEquivalentReorderCodes
      * @see UScriptCode
      * @see UColReorderCode
      * @stable ICU 4.8
      */
      virtual void setReorderCodes(const int32_t* reorderCodes,
                                   int32_t reorderCodesLength,
                                   UErrorCode& status) ;
 
     /**
      * Retrieves the reorder codes that are grouped with the given reorder code. Some reorder
      * codes will be grouped and must reorder together.
      * Beginning with ICU 55, scripts only reorder together if they are primary-equal,
      * for example Hiragana and Katakana.
      *
      * @param reorderCode The reorder code to determine equivalence for.
      * @param dest The array to fill with the script equivalence reordering codes.
      * @param destCapacity The length of dest. If it is 0, then dest may be nullptr and the
      * function will only return the length of the result without writing any codes (pre-flighting).
      * @param status A reference to an error code value, which must not indicate
      * a failure before the function call.
      * @return The length of the of the reordering code equivalence array.
      * @see ucol_setReorderCodes
      * @see Collator#getReorderCodes
      * @see Collator#setReorderCodes
      * @see UScriptCode
      * @see UColReorderCode
      * @stable ICU 4.8
      */
     static int32_t U_EXPORT2 getEquivalentReorderCodes(int32_t reorderCode,
                                 int32_t* dest,
                                 int32_t destCapacity,
                                 UErrorCode& status);
 
     /**
      * Get name of the object for the desired Locale, in the desired language
      * @param objectLocale must be from getAvailableLocales
      * @param displayLocale specifies the desired locale for output
      * @param name the fill-in parameter of the return value
      * @return display-able name of the object for the object locale in the
      *         desired language
      * @stable ICU 2.0
      */
     static UnicodeString& U_EXPORT2 getDisplayName(const Locale& objectLocale,
                                          const Locale& displayLocale,
                                          UnicodeString& name);
 
     /**
     * Get name of the object for the desired Locale, in the language of the
     * default locale.
     * @param objectLocale must be from getAvailableLocales
     * @param name the fill-in parameter of the return value
     * @return name of the object for the desired locale in the default language
     * @stable ICU 2.0
     */
     static UnicodeString& U_EXPORT2 getDisplayName(const Locale& objectLocale,
                                          UnicodeString& name);
 
     /**
      * Get the set of Locales for which Collations are installed.
      *
      * <p>Note this does not include locales supported by registered collators.
      * If collators might have been registered, use the overload of getAvailableLocales
      * that returns a StringEnumeration.</p>
      *
      * @param count the output parameter of number of elements in the locale list
      * @return the list of available locales for which collations are installed
      * @stable ICU 2.0
      */
     static const Locale* U_EXPORT2 getAvailableLocales(int32_t& count);
 
     /**
      * Return a StringEnumeration over the locales available at the time of the call,
      * including registered locales.  If a severe error occurs (such as out of memory
      * condition) this will return null. If there is no locale data, an empty enumeration
      * will be returned.
      * @return a StringEnumeration over the locales available at the time of the call
      * @stable ICU 2.6
      */
     static StringEnumeration* U_EXPORT2 getAvailableLocales();
 
     /**
      * Create a string enumerator of all possible keywords that are relevant to
      * collation. At this point, the only recognized keyword for this
      * service is "collation".
      * @param status input-output error code
      * @return a string enumeration over locale strings. The caller is
      * responsible for closing the result.
      * @stable ICU 3.0
      */
     static StringEnumeration* U_EXPORT2 getKeywords(UErrorCode& status);
 
     /**
      * Given a keyword, create a string enumeration of all values
      * for that keyword that are currently in use.
      * @param keyword a particular keyword as enumerated by
      * ucol_getKeywords. If any other keyword is passed in, status is set
      * to U_ILLEGAL_ARGUMENT_ERROR.
      * @param status input-output error code
      * @return a string enumeration over collation keyword values, or nullptr
      * upon error. The caller is responsible for deleting the result.
      * @stable ICU 3.0
      */
     static StringEnumeration* U_EXPORT2 getKeywordValues(const char *keyword, UErrorCode& status);
 
     /**
      * Given a key and a locale, returns an array of string values in a preferred
      * order that would make a difference. These are all and only those values where
      * the open (creation) of the service with the locale formed from the input locale
      * plus input keyword and that value has different behavior than creation with the
      * input locale alone.
      * @param keyword        one of the keys supported by this service.  For now, only
      *                      "collation" is supported.
      * @param locale        the locale
      * @param commonlyUsed  if set to true it will return only commonly used values
      *                      with the given locale in preferred order.  Otherwise,
      *                      it will return all the available values for the locale.
      * @param status ICU status
      * @return a string enumeration over keyword values for the given key and the locale.
      * @stable ICU 4.2
      */
     static StringEnumeration* U_EXPORT2 getKeywordValuesForLocale(const char* keyword, const Locale& locale,
                                                                     UBool commonlyUsed, UErrorCode& status);
 
     /**
      * Return the functionally equivalent locale for the given
      * requested locale, with respect to given keyword, for the
      * collation service.  If two locales return the same result, then
      * collators instantiated for these locales will behave
      * equivalently.  The converse is not always true; two collators
      * may in fact be equivalent, but return different results, due to
      * internal details.  The return result has no other meaning than
      * that stated above, and implies nothing as to the relationship
      * between the two locales.  This is intended for use by
      * applications who wish to cache collators, or otherwise reuse
      * collators when possible.  The functional equivalent may change
      * over time.  For more information, please see the <a
      * href="https://unicode-org.github.io/icu/userguide/locale#locales-and-services">
      * Locales and Services</a> section of the ICU User Guide.
      * @param keyword a particular keyword as enumerated by
      * ucol_getKeywords.
      * @param locale the requested locale
      * @param isAvailable reference to a fillin parameter that
      * indicates whether the requested locale was 'available' to the
      * collation service. A locale is defined as 'available' if it
      * physically exists within the collation locale data.
      * @param status reference to input-output error code
      * @return the functionally equivalent collation locale, or the root
      * locale upon error.
      * @stable ICU 3.0
      */
     static Locale U_EXPORT2 getFunctionalEquivalent(const char* keyword, const Locale& locale,
                                           UBool& isAvailable, UErrorCode& status);
 
 #if !UCONFIG_NO_SERVICE
     /**
      * Register a new Collator.  The collator will be adopted.
      * Because ICU may choose to cache collators internally, this must be
      * called at application startup, prior to any calls to
      * Collator::createInstance to avoid undefined behavior.
      * @param toAdopt the Collator instance to be adopted
      * @param locale the locale with which the collator will be associated
      * @param status the in/out status code, no special meanings are assigned
      * @return a registry key that can be used to unregister this collator
      * @stable ICU 2.6
      */
     static URegistryKey U_EXPORT2 registerInstance(Collator* toAdopt, const Locale& locale, UErrorCode& status);
 
     /**
      * Register a new CollatorFactory.  The factory will be adopted.
      * Because ICU may choose to cache collators internally, this must be
      * called at application startup, prior to any calls to
      * Collator::createInstance to avoid undefined behavior.
      * @param toAdopt the CollatorFactory instance to be adopted
      * @param status the in/out status code, no special meanings are assigned
      * @return a registry key that can be used to unregister this collator
      * @stable ICU 2.6
      */
     static URegistryKey U_EXPORT2 registerFactory(CollatorFactory* toAdopt, UErrorCode& status);
 
     /**
      * Unregister a previously-registered Collator or CollatorFactory
      * using the key returned from the register call.  Key becomes
      * invalid after a successful call and should not be used again.
      * The object corresponding to the key will be deleted.
      * Because ICU may choose to cache collators internally, this should
      * be called during application shutdown, after all calls to
      * Collator::createInstance to avoid undefined behavior.
      * @param key the registry key returned by a previous call to registerInstance
      * @param status the in/out status code, no special meanings are assigned
      * @return true if the collator for the key was successfully unregistered
      * @stable ICU 2.6
      */
     static UBool U_EXPORT2 unregister(URegistryKey key, UErrorCode& status);
 #endif /* UCONFIG_NO_SERVICE */
 
     /**
      * Gets the version information for a Collator.
      * @param info the version # information, the result will be filled in
      * @stable ICU 2.0
      */
     virtual void getVersion(UVersionInfo info) const = 0;
 
     /**
      * Returns a unique class ID POLYMORPHICALLY. Pure virtual method.
      * This method is to implement a simple version of RTTI, since not all C++
      * compilers support genuine RTTI. Polymorphic operator==() and clone()
      * methods call this method.
      * @return The class ID for this object. All objects of a given class have
      *         the same class ID.  Objects of other classes have different class
      *         IDs.
      * @stable ICU 2.0
      */
     virtual UClassID getDynamicClassID() const override = 0;
 
     /**
      * Universal attribute setter
      * @param attr attribute type
      * @param value attribute value
      * @param status to indicate whether the operation went on smoothly or
      *        there were errors
      * @stable ICU 2.2
      */
     virtual void setAttribute(UColAttribute attr, UColAttributeValue value,
                               UErrorCode &status) = 0;
 
     /**
      * Universal attribute getter
      * @param attr attribute type
      * @param status to indicate whether the operation went on smoothly or
      *        there were errors
      * @return attribute value
      * @stable ICU 2.2
      */
     virtual UColAttributeValue getAttribute(UColAttribute attr,
                                             UErrorCode &status) const = 0;
 
     /**
      * Sets the variable top to the top of the specified reordering group.
      * The variable top determines the highest-sorting character
      * which is affected by UCOL_ALTERNATE_HANDLING.
      * If that attribute is set to UCOL_NON_IGNORABLE, then the variable top has no effect.
      *
      * The base class implementation sets U_UNSUPPORTED_ERROR.
      * @param group one of UCOL_REORDER_CODE_SPACE, UCOL_REORDER_CODE_PUNCTUATION,
      *              UCOL_REORDER_CODE_SYMBOL, UCOL_REORDER_CODE_CURRENCY;
      *              or UCOL_REORDER_CODE_DEFAULT to restore the default max variable group
      * @param errorCode Standard ICU error code. Its input value must
      *                  pass the U_SUCCESS() test, or else the function returns
      *                  immediately. Check for U_FAILURE() on output or use with
      *                  function chaining. (See User Guide for details.)
      * @return *this
      * @see getMaxVariable
      * @stable ICU 53
      */
     virtual Collator &setMaxVariable(UColReorderCode group, UErrorCode &errorCode);
 
     /**
      * Returns the maximum reordering group whose characters are affected by UCOL_ALTERNATE_HANDLING.
      *
      * The base class implementation returns UCOL_REORDER_CODE_PUNCTUATION.
      * @return the maximum variable reordering group.
      * @see setMaxVariable
      * @stable ICU 53
      */
     virtual UColReorderCode getMaxVariable() const;
 
 #ifndef U_FORCE_HIDE_DEPRECATED_API
     /**
      * Sets the variable top to the primary weight of the specified string.
      *
      * Beginning with ICU 53, the variable top is pinned to
      * the top of one of the supported reordering groups,
      * and it must not be beyond the last of those groups.
      * See setMaxVariable().
      * @param varTop one or more (if contraction) char16_ts to which the variable top should be set
      * @param len length of variable top string. If -1 it is considered to be zero terminated.
      * @param status error code. If error code is set, the return value is undefined. Errors set by this function are: <br>
      *    U_CE_NOT_FOUND_ERROR if more than one character was passed and there is no such contraction<br>
      *    U_ILLEGAL_ARGUMENT_ERROR if the variable top is beyond
      *    the last reordering group supported by setMaxVariable()
      * @return variable top primary weight
      * @deprecated ICU 53 Call setMaxVariable() instead.
      */
     virtual uint32_t setVariableTop(const char16_t *varTop, int32_t len, UErrorCode &status) = 0;
 
     /**
      * Sets the variable top to the primary weight of the specified string.
      *
      * Beginning with ICU 53, the variable top is pinned to
      * the top of one of the supported reordering groups,
      * and it must not be beyond the last of those groups.
      * See setMaxVariable().
      * @param varTop a UnicodeString size 1 or more (if contraction) of char16_ts to which the variable top should be set
      * @param status error code. If error code is set, the return value is undefined. Errors set by this function are: <br>
      *    U_CE_NOT_FOUND_ERROR if more than one character was passed and there is no such contraction<br>
      *    U_ILLEGAL_ARGUMENT_ERROR if the variable top is beyond
      *    the last reordering group supported by setMaxVariable()
      * @return variable top primary weight
      * @deprecated ICU 53 Call setMaxVariable() instead.
      */
     virtual uint32_t setVariableTop(const UnicodeString &varTop, UErrorCode &status) = 0;
 
     /**
      * Sets the variable top to the specified primary weight.
      *
      * Beginning with ICU 53, the variable top is pinned to
      * the top of one of the supported reordering groups,
      * and it must not be beyond the last of those groups.
      * See setMaxVariable().
      * @param varTop primary weight, as returned by setVariableTop or ucol_getVariableTop
      * @param status error code
      * @deprecated ICU 53 Call setMaxVariable() instead.
      */
     virtual void setVariableTop(uint32_t varTop, UErrorCode &status) = 0;
 #endif  // U_FORCE_HIDE_DEPRECATED_API
 
     /**
      * Gets the variable top value of a Collator.
      * @param status error code (not changed by function). If error code is set, the return value is undefined.
      * @return the variable top primary weight
      * @see getMaxVariable
      * @stable ICU 2.0
      */
     virtual uint32_t getVariableTop(UErrorCode &status) const = 0;
 
     /**
      * Get a UnicodeSet that contains all the characters and sequences
      * tailored in this collator.
      * @param status      error code of the operation
      * @return a pointer to a UnicodeSet object containing all the
      *         code points and sequences that may sort differently than
      *         in the root collator. The object must be disposed of by using delete
      * @stable ICU 2.4
      */
     virtual UnicodeSet *getTailoredSet(UErrorCode &status) const;
 
 #ifndef U_FORCE_HIDE_DEPRECATED_API
     /**
      * Same as clone().
      * The base class implementation simply calls clone().
      * @return a copy of this object, owned by the caller
      * @see clone()
      * @deprecated ICU 50 no need to have two methods for cloning
      */
     virtual Collator* safeClone() const;
 #endif  // U_FORCE_HIDE_DEPRECATED_API
 
     /**
      * Get the sort key as an array of bytes from a UnicodeString.
      * Sort key byte arrays are zero-terminated and can be compared using
      * strcmp().
      *
      * Note that sort keys are often less efficient than simply doing comparison.
      * For more details, see the ICU User Guide.
      *
      * @param source string to be processed.
      * @param result buffer to store result in. If nullptr, number of bytes needed
      *        will be returned.
      * @param resultLength length of the result buffer. If if not enough the
      *        buffer will be filled to capacity.
      * @return Number of bytes needed for storing the sort key
      * @stable ICU 2.2
      */
     virtual int32_t getSortKey(const UnicodeString& source,
                               uint8_t* result,
                               int32_t resultLength) const = 0;
 
     /**
      * Get the sort key as an array of bytes from a char16_t buffer.
      * Sort key byte arrays are zero-terminated and can be compared using
      * strcmp().
      *
      * Note that sort keys are often less efficient than simply doing comparison.
      * For more details, see the ICU User Guide.
      *
      * @param source string to be processed.
      * @param sourceLength length of string to be processed.
      *        If -1, the string is 0 terminated and length will be decided by the
      *        function.
      * @param result buffer to store result in. If nullptr, number of bytes needed
      *        will be returned.
      * @param resultLength length of the result buffer. If if not enough the
      *        buffer will be filled to capacity.
      * @return Number of bytes needed for storing the sort key
      * @stable ICU 2.2
      */
     virtual int32_t getSortKey(const char16_t*source, int32_t sourceLength,
                                uint8_t*result, int32_t resultLength) const = 0;
 
     /**
      * Produce a bound for a given sortkey and a number of levels.
      * Return value is always the number of bytes needed, regardless of
      * whether the result buffer was big enough or even valid.<br>
      * Resulting bounds can be used to produce a range of strings that are
      * between upper and lower bounds. For example, if bounds are produced
      * for a sortkey of string "smith", strings between upper and lower
      * bounds with one level would include "Smith", "SMITH", "sMiTh".<br>
      * There are two upper bounds that can be produced. If UCOL_BOUND_UPPER
      * is produced, strings matched would be as above. However, if bound
      * produced using UCOL_BOUND_UPPER_LONG is used, the above example will
      * also match "Smithsonian" and similar.<br>
      * For more on usage, see example in cintltst/capitst.c in procedure
      * TestBounds.
      * Sort keys may be compared using <TT>strcmp</TT>.
      * @param source The source sortkey.
      * @param sourceLength The length of source, or -1 if null-terminated.
      *                     (If an unmodified sortkey is passed, it is always null
      *                      terminated).
      * @param boundType Type of bound required. It can be UCOL_BOUND_LOWER, which
      *                  produces a lower inclusive bound, UCOL_BOUND_UPPER, that
      *                  produces upper bound that matches strings of the same length
      *                  or UCOL_BOUND_UPPER_LONG that matches strings that have the
      *                  same starting substring as the source string.
      * @param noOfLevels  Number of levels required in the resulting bound (for most
      *                    uses, the recommended value is 1). See users guide for
      *                    explanation on number of levels a sortkey can have.
      * @param result A pointer to a buffer to receive the resulting sortkey.
      * @param resultLength The maximum size of result.
      * @param status Used for returning error code if something went wrong. If the
      *               number of levels requested is higher than the number of levels
      *               in the source key, a warning (U_SORT_KEY_TOO_SHORT_WARNING) is
      *               issued.
      * @return The size needed to fully store the bound.
      * @see ucol_keyHashCode
      * @stable ICU 2.1
      */
     static int32_t U_EXPORT2 getBound(const uint8_t       *source,
             int32_t             sourceLength,
             UColBoundMode       boundType,
             uint32_t            noOfLevels,
             uint8_t             *result,
             int32_t             resultLength,
             UErrorCode          &status);
 
 
 protected:
 
     // Collator protected constructors -------------------------------------
 
     /**
     * Default constructor.
     * Constructor is different from the old default Collator constructor.
     * The task for determining the default collation strength and normalization
     * mode is left to the child class.
     * @stable ICU 2.0
     */
     Collator();
 
 #ifndef U_HIDE_DEPRECATED_API
     /**
     * Constructor.
     * Empty constructor, does not handle the arguments.
     * This constructor is done for backward compatibility with 1.7 and 1.8.
     * The task for handling the argument collation strength and normalization
     * mode is left to the child class.
     * @param collationStrength collation strength
     * @param decompositionMode
     * @deprecated ICU 2.4. Subclasses should use the default constructor
     * instead and handle the strength and normalization mode themselves.
     */
     Collator(UCollationStrength collationStrength,
              UNormalizationMode decompositionMode);
 #endif  /* U_HIDE_DEPRECATED_API */
 
     /**
     * Copy constructor.
     * @param other Collator object to be copied from
     * @stable ICU 2.0
     */
     Collator(const Collator& other);
 
 public:
    /**
     * Used internally by registration to define the requested and valid locales.
     * @param requestedLocale the requested locale
     * @param validLocale the valid locale
     * @param actualLocale the actual locale
     * @internal
     */
     virtual void setLocales(const Locale& requestedLocale, const Locale& validLocale, const Locale& actualLocale);
 
     /** Get the short definition string for a collator. This internal API harvests the collator's
      *  locale and the attribute set and produces a string that can be used for opening
      *  a collator with the same attributes using the ucol_openFromShortString API.
      *  This string will be normalized.
      *  The structure and the syntax of the string is defined in the "Naming collators"
      *  section of the users guide:
      *  https://unicode-org.github.io/icu/userguide/collation/concepts#collator-naming-scheme
      *  This function supports preflighting.
      *
      *  This is internal, and intended to be used with delegate converters.
      *
      *  @param locale a locale that will appear as a collators locale in the resulting
      *                short string definition. If nullptr, the locale will be harvested
      *                from the collator.
      *  @param buffer space to hold the resulting string
      *  @param capacity capacity of the buffer
      *  @param status for returning errors. All the preflighting errors are featured
      *  @return length of the resulting string
      *  @see ucol_openFromShortString
      *  @see ucol_normalizeShortDefinitionString
      *  @see ucol_getShortDefinitionString
      *  @internal
      */
     virtual int32_t internalGetShortDefinitionString(const char *locale,
                                                      char *buffer,
                                                      int32_t capacity,
                                                      UErrorCode &status) const;
 
     /**
      * Implements ucol_strcollUTF8().
      * @internal
      */
     virtual UCollationResult internalCompareUTF8(
             const char *left, int32_t leftLength,
             const char *right, int32_t rightLength,
             UErrorCode &errorCode) const;
 
     /**
      * Implements ucol_nextSortKeyPart().
      * @internal
      */
     virtual int32_t
     internalNextSortKeyPart(
             UCharIterator *iter, uint32_t state[2],
             uint8_t *dest, int32_t count, UErrorCode &errorCode) const;
 
 #ifndef U_HIDE_INTERNAL_API
     /** @internal */
     static inline Collator *fromUCollator(UCollator *uc) {
         return reinterpret_cast<Collator *>(uc);
     }
     /** @internal */
     static inline const Collator *fromUCollator(const UCollator *uc) {
         return reinterpret_cast<const Collator *>(uc);
     }
     /** @internal */
     inline UCollator *toUCollator() {
         return reinterpret_cast<UCollator *>(this);
     }
     /** @internal */
     inline const UCollator *toUCollator() const {
         return reinterpret_cast<const UCollator *>(this);
     }
 #endif  // U_HIDE_INTERNAL_API
 
 private:
     /**
      * Assignment operator. Private for now.
      */
     Collator& operator=(const Collator& other) = delete;
 
     friend class CFactory;
     friend class SimpleCFactory;
     friend class ICUCollatorFactory;
     friend class ICUCollatorService;
     static Collator* makeInstance(const Locale& desiredLocale,
                                   UErrorCode& status);
 
 #ifndef U_HIDE_DRAFT_API
     /**
      * Function object for performing comparisons using a Collator.
      * @internal
      */
     template <template <typename...> typename Compare, UCollationResult result>
     class Predicate {
       public:
         explicit Predicate(const Collator& parent) : collator(parent) {}
 
         template <
             typename T, typename U,
             typename = std::enable_if_t<ConvertibleToU16StringView<T> && ConvertibleToU16StringView<U>>>
         bool operator()(const T& lhs, const U& rhs) const {
             UErrorCode status = U_ZERO_ERROR;
             return compare(
                 collator.compare(
                     UnicodeString::readOnlyAlias(lhs),
                     UnicodeString::readOnlyAlias(rhs),
                     status),
                 result);
         }
 
         bool operator()(std::string_view lhs, std::string_view rhs) const {
             UErrorCode status = U_ZERO_ERROR;
             return compare(collator.compareUTF8(lhs, rhs, status), result);
         }
 
 #if defined(__cpp_char8_t)
         bool operator()(std::u8string_view lhs, std::u8string_view rhs) const {
             UErrorCode status = U_ZERO_ERROR;
             return compare(collator.compareUTF8(lhs, rhs, status), result);
         }
 #endif
 
       private:
         const Collator& collator;
         static constexpr Compare<UCollationResult> compare{};
     };
 #endif  // U_HIDE_DRAFT_API
 };
 
 #if !UCONFIG_NO_SERVICE
 /**
  * A factory, used with registerFactory, the creates multiple collators and provides
  * display names for them.  A factory supports some number of locales-- these are the
  * locales for which it can create collators.  The factory can be visible, in which
  * case the supported locales will be enumerated by getAvailableLocales, or invisible,
  * in which they are not.  Invisible locales are still supported, they are just not
  * listed by getAvailableLocales.
  * <p>
  * If standard locale display names are sufficient, Collator instances can
  * be registered using registerInstance instead.</p>
  * <p>
  * Note: if the collators are to be used from C APIs, they must be instances
  * of RuleBasedCollator.</p>
  *
  * @stable ICU 2.6
  */
 class U_I18N_API CollatorFactory : public UObject {
 public:
 
     /**
      * Destructor
      * @stable ICU 3.0
      */
     virtual ~CollatorFactory();
 
     /**
      * Return true if this factory is visible.  Default is true.
      * If not visible, the locales supported by this factory will not
      * be listed by getAvailableLocales.
      * @return true if the factory is visible.
      * @stable ICU 2.6
      */
     virtual UBool visible() const;
 
     /**
      * Return a collator for the provided locale.  If the locale
      * is not supported, return nullptr.
      * @param loc the locale identifying the collator to be created.
      * @return a new collator if the locale is supported, otherwise nullptr.
      * @stable ICU 2.6
      */
     virtual Collator* createCollator(const Locale& loc) = 0;
 
     /**
      * Return the name of the collator for the objectLocale, localized for the displayLocale.
      * If objectLocale is not supported, or the factory is not visible, set the result string
      * to bogus.
      * @param objectLocale the locale identifying the collator
      * @param displayLocale the locale for which the display name of the collator should be localized
      * @param result an output parameter for the display name, set to bogus if not supported.
      * @return the display name
      * @stable ICU 2.6
      */
     virtual  UnicodeString& getDisplayName(const Locale& objectLocale,
                                            const Locale& displayLocale,
                                            UnicodeString& result);
 
     /**
      * Return an array of all the locale names directly supported by this factory.
      * The number of names is returned in count.  This array is owned by the factory.
      * Its contents must never change.
      * @param count output parameter for the number of locales supported by the factory
      * @param status the in/out error code
      * @return a pointer to an array of count UnicodeStrings.
      * @stable ICU 2.6
      */
     virtual const UnicodeString * getSupportedIDs(int32_t &count, UErrorCode& status) = 0;
 };
 #endif /* UCONFIG_NO_SERVICE */
 
 // Collator inline methods -----------------------------------------------
 
 U_NAMESPACE_END
 
 #endif /* #if !UCONFIG_NO_COLLATION */
 
 #endif /* U_SHOW_CPLUSPLUS_API */
 
 #endif

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