// © 2016 and later: Unicode, Inc. and others.
// License & terms of use: http://www.unicode.org/copyright.html
/*
*******************************************************************************
* Copyright (C) 2012-2015, International Business Machines
* Corporation and others.  All Rights Reserved.
*******************************************************************************
* collationdatabuilder.cpp
*
* (replaced the former ucol_elm.cpp)
*
* created on: 2012apr01
* created by: Markus W. Scherer
*/

#include "unicode/utypes.h"

#if !UCONFIG_NO_COLLATION

#include "unicode/localpointer.h"
#include "unicode/uchar.h"
#include "unicode/ucharstrie.h"
#include "unicode/ucharstriebuilder.h"
#include "unicode/uniset.h"
#include "unicode/unistr.h"
#include "unicode/usetiter.h"
#include "unicode/utf16.h"
#include "cmemory.h"
#include "collation.h"
#include "collationdata.h"
#include "collationdatabuilder.h"
#include "collationfastlatinbuilder.h"
#include "collationiterator.h"
#include "normalizer2impl.h"
#include "utrie2.h"
#include "uvectr32.h"
#include "uvectr64.h"
#include "uvector.h"

U_NAMESPACE_BEGIN

CollationDataBuilder::CEModifier::~CEModifier() {}

/**
 * Build-time context and CE32 for a code point.
 * If a code point has contextual mappings, then the default (no-context) mapping
 * and all conditional mappings are stored in a singly-linked list
 * of ConditionalCE32, sorted by context strings.
 *
 * Context strings sort by prefix length, then by prefix, then by contraction suffix.
 * Context strings must be unique and in ascending order.
 */
struct ConditionalCE32 : public UMemory {
    ConditionalCE32()
            : context(),
              ce32(0), defaultCE32(Collation::NO_CE32), builtCE32(Collation::NO_CE32),
              next(-1) {}
    ConditionalCE32(const UnicodeString &ct, uint32_t ce)
            : context(ct),
              ce32(ce), defaultCE32(Collation::NO_CE32), builtCE32(Collation::NO_CE32),
              next(-1) {}

    inline UBool hasContext() const { return context.length() > 1; }
    inline int32_t prefixLength() const { return context.charAt(0); }

    /**
     * "\0" for the first entry for any code point, with its default CE32.
     *
     * Otherwise one unit with the length of the prefix string,
     * then the prefix string, then the contraction suffix.
     */
    UnicodeString context;
    /**
     * CE32 for the code point and its context.
     * Can be special (e.g., for an expansion) but not contextual (prefix or contraction tag).
     */
    uint32_t ce32;
    /**
     * Default CE32 for all contexts with this same prefix.
     * Initially NO_CE32. Set only while building runtime data structures,
     * and only on one of the nodes of a sub-list with the same prefix.
     */
    uint32_t defaultCE32;
    /**
     * CE32 for the built contexts.
     * When fetching CEs from the builder, the contexts are built into their runtime form
     * so that the normal collation implementation can process them.
     * The result is cached in the list head. It is reset when the contexts are modified.
     * All of these builtCE32 are invalidated by clearContexts(),
     * via incrementing the contextsEra.
     */
    uint32_t builtCE32;
    /**
     * The "era" of building intermediate contexts when the above builtCE32 was set.
     * When the array of cached, temporary contexts overflows, then clearContexts()
     * removes them all and invalidates the builtCE32 that used to point to built tries.
     */
    int32_t era = -1;
    /**
     * Index of the next ConditionalCE32.
     * Negative for the end of the list.
     */
    int32_t next;
    // Note: We could create a separate class for all of the contextual mappings for
    // a code point, with the builtCE32, the era, and a list of the actual mappings.
    // The class that represents one mapping would then not need to
    // store those fields in each element.
};

U_CDECL_BEGIN

void U_CALLCONV
uprv_deleteConditionalCE32(void *obj) {
    delete static_cast<ConditionalCE32 *>(obj);
}

U_CDECL_END

/**
 * Build-time collation element and character iterator.
 * Uses the runtime CollationIterator for fetching CEs for a string
 * but reads from the builder's unfinished data structures.
 * In particular, this class reads from the unfinished trie
 * and has to avoid CollationIterator::nextCE() and redirect other
 * calls to data->getCE32() and data->getCE32FromSupplementary().
 *
 * We do this so that we need not implement the collation algorithm
 * again for the builder and make it behave exactly like the runtime code.
 * That would be more difficult to test and maintain than this indirection.
 *
 * Some CE32 tags (for example, the DIGIT_TAG) do not occur in the builder data,
 * so the data accesses from those code paths need not be modified.
 *
 * This class iterates directly over whole code points
 * so that the CollationIterator does not need the finished trie
 * for handling the LEAD_SURROGATE_TAG.
 */
class DataBuilderCollationIterator : public CollationIterator {
public:
    DataBuilderCollationIterator(CollationDataBuilder &b);

    virtual ~DataBuilderCollationIterator();

    int32_t fetchCEs(const UnicodeString &str, int32_t start, int64_t ces[], int32_t cesLength);

    virtual void resetToOffset(int32_t newOffset) override;
    virtual int32_t getOffset() const override;

    virtual UChar32 nextCodePoint(UErrorCode &errorCode) override;
    virtual UChar32 previousCodePoint(UErrorCode &errorCode) override;

protected:
    virtual void forwardNumCodePoints(int32_t num, UErrorCode &errorCode) override;
    virtual void backwardNumCodePoints(int32_t num, UErrorCode &errorCode) override;

    virtual uint32_t getDataCE32(UChar32 c) const override;
    virtual uint32_t getCE32FromBuilderData(uint32_t ce32, UErrorCode &errorCode) override;

    CollationDataBuilder &builder;
    CollationData builderData;
    uint32_t jamoCE32s[CollationData::JAMO_CE32S_LENGTH];
    const UnicodeString *s;
    int32_t pos;
};

DataBuilderCollationIterator::DataBuilderCollationIterator(CollationDataBuilder &b)
        : CollationIterator(&builderData, /*numeric=*/ false),
          builder(b), builderData(b.nfcImpl),
          s(nullptr), pos(0) {
    builderData.base = builder.base;
    // Set all of the jamoCE32s[] to indirection CE32s.
    for(int32_t j = 0; j < CollationData::JAMO_CE32S_LENGTH; ++j) {  // Count across Jamo types.
        UChar32 jamo = CollationDataBuilder::jamoCpFromIndex(j);
        jamoCE32s[j] = Collation::makeCE32FromTagAndIndex(Collation::BUILDER_DATA_TAG, jamo) |
                CollationDataBuilder::IS_BUILDER_JAMO_CE32;
    }
    builderData.jamoCE32s = jamoCE32s;
}

DataBuilderCollationIterator::~DataBuilderCollationIterator() {}

int32_t
DataBuilderCollationIterator::fetchCEs(const UnicodeString &str, int32_t start,
                                       int64_t ces[], int32_t cesLength) {
    // Set the pointers each time, in case they changed due to reallocation.
    builderData.ce32s = reinterpret_cast<const uint32_t *>(builder.ce32s.getBuffer());
    builderData.ces = builder.ce64s.getBuffer();
    builderData.contexts = builder.contexts.getBuffer();
    // Modified copy of CollationIterator::nextCE() and CollationIterator::nextCEFromCE32().
    reset();
    s = &str;
    pos = start;
    UErrorCode errorCode = U_ZERO_ERROR;
    while(U_SUCCESS(errorCode) && pos < s->length()) {
        // No need to keep all CEs in the iterator buffer.
        clearCEs();
        UChar32 c = s->char32At(pos);
        pos += U16_LENGTH(c);
        uint32_t ce32 = utrie2_get32(builder.trie, c);
        const CollationData *d;
        if(ce32 == Collation::FALLBACK_CE32) {
            d = builder.base;
            ce32 = builder.base->getCE32(c);
        } else {
            d = &builderData;
        }
        appendCEsFromCE32(d, c, ce32, /*forward=*/ true, errorCode);
        U_ASSERT(U_SUCCESS(errorCode));
        for(int32_t i = 0; i < getCEsLength(); ++i) {
            int64_t ce = getCE(i);
            if(ce != 0) {
                if(cesLength < Collation::MAX_EXPANSION_LENGTH) {
                    ces[cesLength] = ce;
                }
                ++cesLength;
            }
        }
    }
    return cesLength;
}

void
DataBuilderCollationIterator::resetToOffset(int32_t newOffset) {
    reset();
    pos = newOffset;
}

int32_t
DataBuilderCollationIterator::getOffset() const {
    return pos;
}

UChar32
DataBuilderCollationIterator::nextCodePoint(UErrorCode & /*errorCode*/) {
    if(pos == s->length()) {
        return U_SENTINEL;
    }
    UChar32 c = s->char32At(pos);
    pos += U16_LENGTH(c);
    return c;
}

UChar32
DataBuilderCollationIterator::previousCodePoint(UErrorCode & /*errorCode*/) {
    if(pos == 0) {
        return U_SENTINEL;
    }
    UChar32 c = s->char32At(pos - 1);
    pos -= U16_LENGTH(c);
    return c;
}

void
DataBuilderCollationIterator::forwardNumCodePoints(int32_t num, UErrorCode & /*errorCode*/) {
    pos = s->moveIndex32(pos, num);
}

void
DataBuilderCollationIterator::backwardNumCodePoints(int32_t num, UErrorCode & /*errorCode*/) {
    pos = s->moveIndex32(pos, -num);
}

uint32_t
DataBuilderCollationIterator::getDataCE32(UChar32 c) const {
    return utrie2_get32(builder.trie, c);
}

uint32_t
DataBuilderCollationIterator::getCE32FromBuilderData(uint32_t ce32, UErrorCode &errorCode) {
    if (U_FAILURE(errorCode)) { return 0; }
    U_ASSERT(Collation::hasCE32Tag(ce32, Collation::BUILDER_DATA_TAG));
    if((ce32 & CollationDataBuilder::IS_BUILDER_JAMO_CE32) != 0) {
        UChar32 jamo = Collation::indexFromCE32(ce32);
        return utrie2_get32(builder.trie, jamo);
    } else {
        ConditionalCE32 *cond = builder.getConditionalCE32ForCE32(ce32);
        if (cond == nullptr) {
            errorCode = U_INTERNAL_PROGRAM_ERROR;
            // TODO: ICU-21531 figure out why this happens.
            return 0;
        }
        if(cond->builtCE32 == Collation::NO_CE32 || cond->era != builder.contextsEra) {
            // Build the context-sensitive mappings into their runtime form and cache the result.
            cond->builtCE32 = builder.buildContext(cond, errorCode);
            if(errorCode == U_BUFFER_OVERFLOW_ERROR) {
                errorCode = U_ZERO_ERROR;
                builder.clearContexts();
                cond->builtCE32 = builder.buildContext(cond, errorCode);
            }
            cond->era = builder.contextsEra;
            builderData.contexts = builder.contexts.getBuffer();
        }
        return cond->builtCE32;
    }
}

// ------------------------------------------------------------------------- ***

CollationDataBuilder::CollationDataBuilder(UBool icu4xMode, UErrorCode &errorCode)
        : nfcImpl(*Normalizer2Factory::getNFCImpl(errorCode)),
          base(nullptr), baseSettings(nullptr),
          trie(nullptr),
          ce32s(errorCode), ce64s(errorCode), conditionalCE32s(errorCode),
          modified(false),
          icu4xMode(icu4xMode),
          fastLatinEnabled(false), fastLatinBuilder(nullptr),
          collIter(nullptr) {
    // Reserve the first CE32 for U+0000.
    if (!icu4xMode) {
        ce32s.addElement(0, errorCode);
    }
    conditionalCE32s.setDeleter(uprv_deleteConditionalCE32);
}

CollationDataBuilder::~CollationDataBuilder() {
    utrie2_close(trie);
    delete fastLatinBuilder;
    delete collIter;
}

void
CollationDataBuilder::initForTailoring(const CollationData *b, UErrorCode &errorCode) {
    if(U_FAILURE(errorCode)) { return; }
    if(trie != nullptr) {
        errorCode = U_INVALID_STATE_ERROR;
        return;
    }
    if(b == nullptr) {
        errorCode = U_ILLEGAL_ARGUMENT_ERROR;
        return;
    }
    base = b;

    // For a tailoring, the default is to fall back to the base.
    // For ICU4X, use the same value for fallback as for the default
    // to avoid having to have different blocks for the two.
    trie = utrie2_open(Collation::FALLBACK_CE32, icu4xMode ? Collation::FALLBACK_CE32 : Collation::FFFD_CE32, &errorCode);

    if (!icu4xMode) {
        // Set the Latin-1 letters block so that it is allocated first in the data array,
        // to try to improve locality of reference when sorting Latin-1 text.
        // Do not use utrie2_setRange32() since that will not actually allocate blocks
        // that are filled with the default value.
        // ASCII (0..7F) is already preallocated anyway.
        for(UChar32 c = 0xc0; c <= 0xff; ++c) {
            utrie2_set32(trie, c, Collation::FALLBACK_CE32, &errorCode);
        }

        // Hangul syllables are not tailorable (except via tailoring Jamos).
        // Always set the Hangul tag to help performance.
        // Do this here, rather than in buildMappings(),
        // so that we see the HANGUL_TAG in various assertions.
        uint32_t hangulCE32 = Collation::makeCE32FromTagAndIndex(Collation::HANGUL_TAG, 0);
        utrie2_setRange32(trie, Hangul::HANGUL_BASE, Hangul::HANGUL_END, hangulCE32, true, &errorCode);

        // Copy the set contents but don't copy/clone the set as a whole because
        // that would copy the isFrozen state too.
        unsafeBackwardSet.addAll(*b->unsafeBackwardSet);
    }

    if(U_FAILURE(errorCode)) { return; }
}

UBool
CollationDataBuilder::maybeSetPrimaryRange(UChar32 start, UChar32 end,
                                           uint32_t primary, int32_t step,
                                           UErrorCode &errorCode) {
    if(U_FAILURE(errorCode)) { return false; }
    U_ASSERT(start <= end);
    // TODO: Do we need to check what values are currently set for start..end?
    // An offset range is worth it only if we can achieve an overlap between
    // adjacent UTrie2 blocks of 32 code points each.
    // An offset CE is also a little more expensive to look up and compute
    // than a simple CE.
    // If the range spans at least three UTrie2 block boundaries (> 64 code points),
    // then we take it.
    // If the range spans one or two block boundaries and there are
    // at least 4 code points on either side, then we take it.
    // (We could additionally require a minimum range length of, say, 16.)
    int32_t blockDelta = (end >> 5) - (start >> 5);
    if(2 <= step && step <= 0x7f &&
            (blockDelta >= 3 ||
            (blockDelta > 0 && (start & 0x1f) <= 0x1c && (end & 0x1f) >= 3))) {
        int64_t dataCE = (static_cast<int64_t>(primary) << 32) | (start << 8) | step;
        if(isCompressiblePrimary(primary)) { dataCE |= 0x80; }
        int32_t index = addCE(dataCE, errorCode);
        if(U_FAILURE(errorCode)) { return 0; }
        if(index > Collation::MAX_INDEX) {
            errorCode = U_BUFFER_OVERFLOW_ERROR;
            return 0;
        }
        uint32_t offsetCE32 = Collation::makeCE32FromTagAndIndex(Collation::OFFSET_TAG, index);
        utrie2_setRange32(trie, start, end, offsetCE32, true, &errorCode);
        modified = true;
        return true;
    } else {
        return false;
    }
}

uint32_t
CollationDataBuilder::setPrimaryRangeAndReturnNext(UChar32 start, UChar32 end,
                                                   uint32_t primary, int32_t step,
                                                   UErrorCode &errorCode) {
    if(U_FAILURE(errorCode)) { return 0; }
    UBool isCompressible = isCompressiblePrimary(primary);
    if(maybeSetPrimaryRange(start, end, primary, step, errorCode)) {
        return Collation::incThreeBytePrimaryByOffset(primary, isCompressible,
                                                      (end - start + 1) * step);
    } else {
        // Short range: Set individual CE32s.
        for(;;) {
            utrie2_set32(trie, start, Collation::makeLongPrimaryCE32(primary), &errorCode);
            ++start;
            primary = Collation::incThreeBytePrimaryByOffset(primary, isCompressible, step);
            if(start > end) { return primary; }
        }
        modified = true;
    }
}

uint32_t
CollationDataBuilder::getCE32FromOffsetCE32(UBool fromBase, UChar32 c, uint32_t ce32) const {
    int32_t i = Collation::indexFromCE32(ce32);
    int64_t dataCE = fromBase ? base->ces[i] : ce64s.elementAti(i);
    uint32_t p = Collation::getThreeBytePrimaryForOffsetData(c, dataCE);
    return Collation::makeLongPrimaryCE32(p);
}

UBool
CollationDataBuilder::isCompressibleLeadByte(uint32_t b) const {
    return base->isCompressibleLeadByte(b);
}

UBool
CollationDataBuilder::isAssigned(UChar32 c) const {
    return Collation::isAssignedCE32(utrie2_get32(trie, c));
}

uint32_t
CollationDataBuilder::getLongPrimaryIfSingleCE(UChar32 c) const {
    uint32_t ce32 = utrie2_get32(trie, c);
    if(Collation::isLongPrimaryCE32(ce32)) {
        return Collation::primaryFromLongPrimaryCE32(ce32);
    } else {
        return 0;
    }
}

int64_t
CollationDataBuilder::getSingleCE(UChar32 c, UErrorCode &errorCode) const {
    if(U_FAILURE(errorCode)) { return 0; }
    // Keep parallel with CollationData::getSingleCE().
    UBool fromBase = false;
    uint32_t ce32 = utrie2_get32(trie, c);
    if(ce32 == Collation::FALLBACK_CE32) {
        fromBase = true;
        ce32 = base->getCE32(c);
    }
    while(Collation::isSpecialCE32(ce32)) {
        switch(Collation::tagFromCE32(ce32)) {
        case Collation::LATIN_EXPANSION_TAG:
        case Collation::BUILDER_DATA_TAG:
        case Collation::PREFIX_TAG:
        case Collation::CONTRACTION_TAG:
        case Collation::HANGUL_TAG:
        case Collation::LEAD_SURROGATE_TAG:
            errorCode = U_UNSUPPORTED_ERROR;
            return 0;
        case Collation::FALLBACK_TAG:
        case Collation::RESERVED_TAG_3:
            errorCode = U_INTERNAL_PROGRAM_ERROR;
            return 0;
        case Collation::LONG_PRIMARY_TAG:
            return Collation::ceFromLongPrimaryCE32(ce32);
        case Collation::LONG_SECONDARY_TAG:
            return Collation::ceFromLongSecondaryCE32(ce32);
        case Collation::EXPANSION32_TAG:
            if(Collation::lengthFromCE32(ce32) == 1) {
                int32_t i = Collation::indexFromCE32(ce32);
                ce32 = fromBase ? base->ce32s[i] : ce32s.elementAti(i);
                break;
            } else {
                errorCode = U_UNSUPPORTED_ERROR;
                return 0;
            }
        case Collation::EXPANSION_TAG: {
            if(Collation::lengthFromCE32(ce32) == 1) {
                int32_t i = Collation::indexFromCE32(ce32);
                return fromBase ? base->ces[i] : ce64s.elementAti(i);
            } else {
                errorCode = U_UNSUPPORTED_ERROR;
                return 0;
            }
        }
        case Collation::DIGIT_TAG:
            // Fetch the non-numeric-collation CE32 and continue.
            ce32 = ce32s.elementAti(Collation::indexFromCE32(ce32));
            break;
        case Collation::U0000_TAG:
            U_ASSERT(c == 0);
            // Fetch the normal ce32 for U+0000 and continue.
            ce32 = fromBase ? base->ce32s[0] : ce32s.elementAti(0);
            break;
        case Collation::OFFSET_TAG:
            ce32 = getCE32FromOffsetCE32(fromBase, c, ce32);
            break;
        case Collation::IMPLICIT_TAG:
            return Collation::unassignedCEFromCodePoint(c);
        }
    }
    return Collation::ceFromSimpleCE32(ce32);
}

int32_t
CollationDataBuilder::addCE(int64_t ce, UErrorCode &errorCode) {
    int32_t length = ce64s.size();
    for(int32_t i = 0; i < length; ++i) {
        if(ce == ce64s.elementAti(i)) { return i; }
    }
    ce64s.addElement(ce, errorCode);
    return length;
}

int32_t
CollationDataBuilder::addCE32(uint32_t ce32, UErrorCode &errorCode) {
    int32_t length = ce32s.size();
    for(int32_t i = 0; i < length; ++i) {
        if (ce32 == static_cast<uint32_t>(ce32s.elementAti(i))) { return i; }
    }
    ce32s.addElement(static_cast<int32_t>(ce32), errorCode);
    return length;
}

int32_t
CollationDataBuilder::addConditionalCE32(const UnicodeString &context, uint32_t ce32,
                                         UErrorCode &errorCode) {
    if(U_FAILURE(errorCode)) { return -1; }
    U_ASSERT(!context.isEmpty());
    int32_t index = conditionalCE32s.size();
    if(index > Collation::MAX_INDEX) {
        errorCode = U_BUFFER_OVERFLOW_ERROR;
        return -1;
    }
    LocalPointer<ConditionalCE32> cond(new ConditionalCE32(context, ce32), errorCode);
    conditionalCE32s.adoptElement(cond.orphan(), errorCode);
    if(U_FAILURE(errorCode)) {
        return -1;
    }
    return index;
}

void
CollationDataBuilder::add(const UnicodeString &prefix, const UnicodeString &s,
                          const int64_t ces[], int32_t cesLength,
                          UErrorCode &errorCode) {
    uint32_t ce32 = encodeCEs(ces, cesLength, errorCode);
    addCE32(prefix, s, ce32, errorCode);
}

void
CollationDataBuilder::addCE32(const UnicodeString &prefix, const UnicodeString &s,
                              uint32_t ce32, UErrorCode &errorCode) {
    if(U_FAILURE(errorCode)) { return; }
    if(s.isEmpty()) {
        errorCode = U_ILLEGAL_ARGUMENT_ERROR;
        return;
    }
    if(trie == nullptr || utrie2_isFrozen(trie)) {
        errorCode = U_INVALID_STATE_ERROR;
        return;
    }
    UChar32 c = s.char32At(0);
    int32_t cLength = U16_LENGTH(c);
    uint32_t oldCE32 = utrie2_get32(trie, c);
    UBool hasContext = !prefix.isEmpty() || s.length() > cLength;

    if (icu4xMode) {
        if (base && c >= 0x1100 && c < 0x1200) {
            // Omit jamo tailorings.
            // TODO(https://github.com/unicode-org/icu4x/issues/1941).
        }
        const Normalizer2* nfdNormalizer = Normalizer2::getNFDInstance(errorCode);
        UnicodeString sInNfd;
        nfdNormalizer->normalize(s, sInNfd, errorCode);
        if (s != sInNfd) {
            // s is not in NFD, so it cannot match in ICU4X, since ICU4X only
            // does NFD lookups.

            // As of Unicode 16 alpha, the cases that come here are:
            //
            // 1. The second character is a special decomposing Tibetan vowel
            //    sign. These are OK to ignore in the precomposed form, since
            //    the decomposed form is added also.
            // 2. Likewise for KIRAT RAI VOWEL SIGN AA followed by KIRAT RAI VOWEL SIGN AI
            //    and other such cases.
            //    For details see the normalization section of
            //    https://www.unicode.org/review/pri497/pri497-background.html
            // 3. U+FDD1 followed by U+AC00 is a marker for the alphabetical
            //    index feature of ICU4C, which at this time does not have
            //    a counterpart in ICU4X.
            return;
        }

        if (!prefix.isEmpty()) {
            UnicodeString prefixInNfd;
            nfdNormalizer->normalize(prefix, prefixInNfd, errorCode);
            if (prefix != prefixInNfd) {
                errorCode = U_UNSUPPORTED_ERROR;
                return;
            }

            int32_t count = prefix.countChar32();
            if (count > 2) {
                // Prefix too long for ICU4X.
                errorCode = U_UNSUPPORTED_ERROR;
                return;
            }
            UChar32 utf32[4];
            int32_t len = prefix.toUTF32(utf32, 4, errorCode);
            if (len != count) {
                errorCode = U_INVALID_STATE_ERROR;
                return;
            }
            UChar32 c = utf32[0];
            if (u_getCombiningClass(c)) {
                // Prefix must start with as starter for ICU4X.
                errorCode = U_UNSUPPORTED_ERROR;
                return;
            }
            // XXX: Korean searchjl has jamo in prefix, so commenting out this
            // check for now. ICU4X currently ignores non-root jamo tables anyway.
            // searchjl was added in
            // https://unicode-org.atlassian.net/browse/CLDR-3560
            // Contractions were changed to prefixes in
            // https://unicode-org.atlassian.net/browse/CLDR-6546
            //
            // if ((c >= 0x1100 && c < 0x1200) || (c >= 0xAC00 && c < 0xD7A4)) {
            //     errorCode = U_UNSUPPORTED_ERROR;
            //     return;
            // }
            if ((len > 1) && !(utf32[1] == 0x3099 || utf32[1] == 0x309A)) {
                // Second character in prefix, if present, must be a kana voicing mark for ICU4X.
                errorCode = U_UNSUPPORTED_ERROR;
                return;
            }
        }

        if (s.length() > cLength) {
            // Check that there's no modern Hangul in contractions.
            for (int32_t i = 0; i < s.length(); ++i) {
                char16_t c = s.charAt(i);
                if ((c >= 0x1100 && c < 0x1100 + 19) || (c >= 0x1161 && c < 0x1161 + 21) || (c >= 0x11A7 && c < 0x11A7 + 28) || (c >= 0xAC00 && c < 0xD7A4)) {
                    errorCode = U_UNSUPPORTED_ERROR;
                    return;
                }
            }
            int32_t sCount = s.countChar32();
            UChar32 sUtf32[32];
            int32_t sLen = s.toUTF32(sUtf32, 32, errorCode);
            if (sLen != sCount) {
                // If this error is ever reached, just increase the buffer
                // size above.
                errorCode = U_UNSUPPORTED_ERROR;
                return;
            }
            for (int32_t i = 1; i < sLen - 1; ++i) {
                if (u_getCombiningClass(sUtf32[i]) == 0) {
                    contractionMiddleStarter.add(sUtf32[i]);
                }
            }
        }
    }

    if(oldCE32 == Collation::FALLBACK_CE32) {
        // First tailoring for c.
        // If c has contextual base mappings or if we add a contextual mapping,
        // then copy the base mappings.
        // Otherwise we just override the base mapping.
        uint32_t baseCE32 = base->getFinalCE32(base->getCE32(c));
        if(hasContext || Collation::ce32HasContext(baseCE32)) {
            oldCE32 = copyFromBaseCE32(c, baseCE32, true, errorCode);
            utrie2_set32(trie, c, oldCE32, &errorCode);
            if(U_FAILURE(errorCode)) { return; }
        }
    }
    if(!hasContext) {
        // No prefix, no contraction.
        if(!isBuilderContextCE32(oldCE32)) {
            utrie2_set32(trie, c, ce32, &errorCode);
        } else {
            ConditionalCE32 *cond = getConditionalCE32ForCE32(oldCE32);
            cond->builtCE32 = Collation::NO_CE32;
            cond->ce32 = ce32;
        }
    } else {
        ConditionalCE32 *cond;
        if(!isBuilderContextCE32(oldCE32)) {
            // Replace the simple oldCE32 with a builder context CE32
            // pointing to a new ConditionalCE32 list head.
            int32_t index = addConditionalCE32(UnicodeString(static_cast<char16_t>(0)), oldCE32, errorCode);
            if(U_FAILURE(errorCode)) { return; }
            uint32_t contextCE32 = makeBuilderContextCE32(index);
            utrie2_set32(trie, c, contextCE32, &errorCode);
            contextChars.add(c);
            cond = getConditionalCE32(index);
        } else {
            cond = getConditionalCE32ForCE32(oldCE32);
            cond->builtCE32 = Collation::NO_CE32;
        }
        UnicodeString suffix(s, cLength);
        UnicodeString context(static_cast<char16_t>(prefix.length()));
        context.append(prefix).append(suffix);
        if (icu4xMode && !suffix.isEmpty() && !prefix.isEmpty()) {
            // ICU4X does not support the combination of prefix and contraction.
            // This combination is supported by LDML but does not occur in the
            // root or any tailorings in CLDR as of February 2025.
            // If support for this case becomes necessary, a practical change
            // would be allocating a flag on prefix ce32 and setting the
            // flag on a prefix ce32 if any ce32 that can be found under
            // the prefix ce32 (either the default or any UCharsTrie value) is
            // a contraction ce32 or if the prefix ce32 is the utrie2 value
            // for a character that is a starter that occurs in a middle
            // (neither first nor last) position in a contraction.
            errorCode = U_UNSUPPORTED_ERROR;
            return;
        }
        unsafeBackwardSet.addAll(suffix);
        for(;;) {
            // invariant: context > cond->context
            int32_t next = cond->next;
            if(next < 0) {
                // Append a new ConditionalCE32 after cond.
                int32_t index = addConditionalCE32(context, ce32, errorCode);
                if(U_FAILURE(errorCode)) { return; }
                cond->next = index;
                break;
            }
            ConditionalCE32 *nextCond = getConditionalCE32(next);
            int8_t cmp = context.compare(nextCond->context);
            if(cmp < 0) {
                // Insert a new ConditionalCE32 between cond and nextCond.
                int32_t index = addConditionalCE32(context, ce32, errorCode);
                if(U_FAILURE(errorCode)) { return; }
                cond->next = index;
                getConditionalCE32(index)->next = next;
                break;
            } else if(cmp == 0) {
                // Same context as before, overwrite its ce32.
                nextCond->ce32 = ce32;
                break;
            }
            cond = nextCond;
        }
    }
    modified = true;
}

uint32_t
CollationDataBuilder::encodeOneCEAsCE32(int64_t ce) {
    uint32_t p = static_cast<uint32_t>(ce >> 32);
    uint32_t lower32 = static_cast<uint32_t>(ce);
    uint32_t t = static_cast<uint32_t>(ce & 0xffff);
    U_ASSERT((t & 0xc000) != 0xc000);  // Impossible case bits 11 mark special CE32s.
    if((ce & INT64_C(0xffff00ff00ff)) == 0) {
        // normal form ppppsstt
        return p | (lower32 >> 16) | (t >> 8);
    } else if((ce & INT64_C(0xffffffffff)) == Collation::COMMON_SEC_AND_TER_CE) {
        // long-primary form ppppppC1
        return Collation::makeLongPrimaryCE32(p);
    } else if(p == 0 && (t & 0xff) == 0) {
        // long-secondary form ssssttC2
        return Collation::makeLongSecondaryCE32(lower32);
    }
    return Collation::NO_CE32;
}

uint32_t
CollationDataBuilder::encodeOneCE(int64_t ce, UErrorCode &errorCode) {
    // Try to encode one CE as one CE32.
    uint32_t ce32 = encodeOneCEAsCE32(ce);
    if(ce32 != Collation::NO_CE32) { return ce32; }
    int32_t index = addCE(ce, errorCode);
    if(U_FAILURE(errorCode)) { return 0; }
    if(index > Collation::MAX_INDEX) {
        errorCode = U_BUFFER_OVERFLOW_ERROR;
        return 0;
    }
    return Collation::makeCE32FromTagIndexAndLength(Collation::EXPANSION_TAG, index, 1);
}

uint32_t
CollationDataBuilder::encodeCEs(const int64_t ces[], int32_t cesLength,
                                UErrorCode &errorCode) {
    if(U_FAILURE(errorCode)) { return 0; }
    if(cesLength < 0 || cesLength > Collation::MAX_EXPANSION_LENGTH) {
        errorCode = U_ILLEGAL_ARGUMENT_ERROR;
        return 0;
    }
    if(trie == nullptr || utrie2_isFrozen(trie)) {
        errorCode = U_INVALID_STATE_ERROR;
        return 0;
    }
    if(cesLength == 0) {
        // Convenience: We cannot map to nothing, but we can map to a completely ignorable CE.
        // Do this here so that callers need not do it.
        return encodeOneCEAsCE32(0);
    } else if(cesLength == 1) {
        return encodeOneCE(ces[0], errorCode);
    } else if(cesLength == 2 && !icu4xMode) {
        // Try to encode two CEs as one CE32.
        // Turn this off for ICU4X, because without the canonical closure
        // these are so rare that it doesn't make sense to spend a branch
        // on checking this tag when using the data.
        int64_t ce0 = ces[0];
        int64_t ce1 = ces[1];
        uint32_t p0 = static_cast<uint32_t>(ce0 >> 32);
        if((ce0 & INT64_C(0xffffffffff00ff)) == Collation::COMMON_SECONDARY_CE &&
                (ce1 & INT64_C(0xffffffff00ffffff)) == Collation::COMMON_TERTIARY_CE &&
                p0 != 0) {
            // Latin mini expansion
            return
                p0 |
                ((static_cast<uint32_t>(ce0) & 0xff00u) << 8) |
                static_cast<uint32_t>(ce1 >> 16) |
                Collation::SPECIAL_CE32_LOW_BYTE |
                Collation::LATIN_EXPANSION_TAG;
        }
    }
    // Try to encode two or more CEs as CE32s.
    int32_t newCE32s[Collation::MAX_EXPANSION_LENGTH];
    for(int32_t i = 0;; ++i) {
        if(i == cesLength) {
            return encodeExpansion32(newCE32s, cesLength, errorCode);
        }
        uint32_t ce32 = encodeOneCEAsCE32(ces[i]);
        if(ce32 == Collation::NO_CE32) { break; }
        newCE32s[i] = static_cast<int32_t>(ce32);
    }
    return encodeExpansion(ces, cesLength, errorCode);
}

uint32_t
CollationDataBuilder::encodeExpansion(const int64_t ces[], int32_t length, UErrorCode &errorCode) {
    if(U_FAILURE(errorCode)) { return 0; }
    // See if this sequence of CEs has already been stored.
    int64_t first = ces[0];
    int32_t ce64sMax = ce64s.size() - length;
    for(int32_t i = 0; i <= ce64sMax; ++i) {
        if(first == ce64s.elementAti(i)) {
            if(i > Collation::MAX_INDEX) {
                errorCode = U_BUFFER_OVERFLOW_ERROR;
                return 0;
            }
            for(int32_t j = 1;; ++j) {
                if(j == length) {
                    return Collation::makeCE32FromTagIndexAndLength(
                            Collation::EXPANSION_TAG, i, length);
                }
                if(ce64s.elementAti(i + j) != ces[j]) { break; }
            }
        }
    }
    // Store the new sequence.
    int32_t i = ce64s.size();
    if(i > Collation::MAX_INDEX) {
        errorCode = U_BUFFER_OVERFLOW_ERROR;
        return 0;
    }
    for(int32_t j = 0; j < length; ++j) {
        ce64s.addElement(ces[j], errorCode);
    }
    return Collation::makeCE32FromTagIndexAndLength(Collation::EXPANSION_TAG, i, length);
}

uint32_t
CollationDataBuilder::encodeExpansion32(const int32_t newCE32s[], int32_t length,
                                        UErrorCode &errorCode) {
    if(U_FAILURE(errorCode)) { return 0; }
    // See if this sequence of CE32s has already been stored.
    int32_t first = newCE32s[0];
    int32_t ce32sMax = ce32s.size() - length;
    for(int32_t i = 0; i <= ce32sMax; ++i) {
        if(first == ce32s.elementAti(i)) {
            if(i > Collation::MAX_INDEX) {
                errorCode = U_BUFFER_OVERFLOW_ERROR;
                return 0;
            }
            for(int32_t j = 1;; ++j) {
                if(j == length) {
                    return Collation::makeCE32FromTagIndexAndLength(
                            Collation::EXPANSION32_TAG, i, length);
                }
                if(ce32s.elementAti(i + j) != newCE32s[j]) { break; }
            }
        }
    }
    // Store the new sequence.
    int32_t i = ce32s.size();
    if(i > Collation::MAX_INDEX) {
        errorCode = U_BUFFER_OVERFLOW_ERROR;
        return 0;
    }
    for(int32_t j = 0; j < length; ++j) {
        ce32s.addElement(newCE32s[j], errorCode);
    }
    return Collation::makeCE32FromTagIndexAndLength(Collation::EXPANSION32_TAG, i, length);
}

uint32_t
CollationDataBuilder::copyFromBaseCE32(UChar32 c, uint32_t ce32, UBool withContext,
                                       UErrorCode &errorCode) {
    if(U_FAILURE(errorCode)) { return 0; }
    if(!Collation::isSpecialCE32(ce32)) { return ce32; }
    switch(Collation::tagFromCE32(ce32)) {
    case Collation::LONG_PRIMARY_TAG:
    case Collation::LONG_SECONDARY_TAG:
    case Collation::LATIN_EXPANSION_TAG:
        // copy as is
        break;
    case Collation::EXPANSION32_TAG: {
        const uint32_t *baseCE32s = base->ce32s + Collation::indexFromCE32(ce32);
        int32_t length = Collation::lengthFromCE32(ce32);
        ce32 = encodeExpansion32(
            reinterpret_cast<const int32_t *>(baseCE32s), length, errorCode);
        break;
    }
    case Collation::EXPANSION_TAG: {
        const int64_t *baseCEs = base->ces + Collation::indexFromCE32(ce32);
        int32_t length = Collation::lengthFromCE32(ce32);
        ce32 = encodeExpansion(baseCEs, length, errorCode);
        break;
    }
    case Collation::PREFIX_TAG: {
        // Flatten prefixes and nested suffixes (contractions)
        // into a linear list of ConditionalCE32.
        const char16_t *p = base->contexts + Collation::indexFromCE32(ce32);
        ce32 = CollationData::readCE32(p);  // Default if no prefix match.
        if(!withContext) {
            return copyFromBaseCE32(c, ce32, false, errorCode);
        }
        ConditionalCE32 head;
        UnicodeString context(static_cast<char16_t>(0));
        int32_t index;
        if(Collation::isContractionCE32(ce32)) {
            index = copyContractionsFromBaseCE32(context, c, ce32, &head, errorCode);
        } else {
            ce32 = copyFromBaseCE32(c, ce32, true, errorCode);
            head.next = index = addConditionalCE32(context, ce32, errorCode);
        }
        if(U_FAILURE(errorCode)) { return 0; }
        ConditionalCE32 *cond = getConditionalCE32(index);  // the last ConditionalCE32 so far
        UCharsTrie::Iterator prefixes(p + 2, 0, errorCode);
        while(prefixes.next(errorCode)) {
            context = prefixes.getString();
            context.reverse();
            context.insert(0, static_cast<char16_t>(context.length()));
            ce32 = static_cast<uint32_t>(prefixes.getValue());
            if(Collation::isContractionCE32(ce32)) {
                index = copyContractionsFromBaseCE32(context, c, ce32, cond, errorCode);
            } else {
                ce32 = copyFromBaseCE32(c, ce32, true, errorCode);
                cond->next = index = addConditionalCE32(context, ce32, errorCode);
            }
            if(U_FAILURE(errorCode)) { return 0; }
            cond = getConditionalCE32(index);
        }
        ce32 = makeBuilderContextCE32(head.next);
        contextChars.add(c);
        break;
    }
    case Collation::CONTRACTION_TAG: {
        if(!withContext) {
            const char16_t *p = base->contexts + Collation::indexFromCE32(ce32);
            ce32 = CollationData::readCE32(p);  // Default if no suffix match.
            return copyFromBaseCE32(c, ce32, false, errorCode);
        }
        ConditionalCE32 head;
        UnicodeString context(static_cast<char16_t>(0));
        copyContractionsFromBaseCE32(context, c, ce32, &head, errorCode);
        ce32 = makeBuilderContextCE32(head.next);
        contextChars.add(c);
        break;
    }
    case Collation::HANGUL_TAG:
        errorCode = U_UNSUPPORTED_ERROR;  // We forbid tailoring of Hangul syllables.
        break;
    case Collation::OFFSET_TAG:
        ce32 = getCE32FromOffsetCE32(true, c, ce32);
        break;
    case Collation::IMPLICIT_TAG:
        ce32 = encodeOneCE(Collation::unassignedCEFromCodePoint(c), errorCode);
        break;
    default:
        UPRV_UNREACHABLE_EXIT;  // require ce32 == base->getFinalCE32(ce32)
    }
    return ce32;
}

int32_t
CollationDataBuilder::copyContractionsFromBaseCE32(UnicodeString &context, UChar32 c, uint32_t ce32,
                                                   ConditionalCE32 *cond, UErrorCode &errorCode) {
    if(U_FAILURE(errorCode)) { return 0; }
    const char16_t *p = base->contexts + Collation::indexFromCE32(ce32);
    int32_t index;
    if((ce32 & Collation::CONTRACT_SINGLE_CP_NO_MATCH) != 0) {
        // No match on the single code point.
        // We are underneath a prefix, and the default mapping is just
        // a fallback to the mappings for a shorter prefix.
        U_ASSERT(context.length() > 1);
        index = -1;
    } else {
        ce32 = CollationData::readCE32(p);  // Default if no suffix match.
        U_ASSERT(!Collation::isContractionCE32(ce32));
        ce32 = copyFromBaseCE32(c, ce32, true, errorCode);
        cond->next = index = addConditionalCE32(context, ce32, errorCode);
        if(U_FAILURE(errorCode)) { return 0; }
        cond = getConditionalCE32(index);
    }

    int32_t suffixStart = context.length();
    UCharsTrie::Iterator suffixes(p + 2, 0, errorCode);
    while(suffixes.next(errorCode)) {
        context.append(suffixes.getString());
        ce32 = copyFromBaseCE32(c, static_cast<uint32_t>(suffixes.getValue()), true, errorCode);
        cond->next = index = addConditionalCE32(context, ce32, errorCode);
        if(U_FAILURE(errorCode)) { return 0; }
        // No need to update the unsafeBackwardSet because the tailoring set
        // is already a copy of the base set.
        cond = getConditionalCE32(index);
        context.truncate(suffixStart);
    }
    U_ASSERT(index >= 0);
    return index;
}

class CopyHelper {
public:
    CopyHelper(const CollationDataBuilder &s, CollationDataBuilder &d,
               const CollationDataBuilder::CEModifier &m, UErrorCode &initialErrorCode)
            : src(s), dest(d), modifier(m),
              errorCode(initialErrorCode) {}

    UBool copyRangeCE32(UChar32 start, UChar32 end, uint32_t ce32) {
        ce32 = copyCE32(ce32);
        utrie2_setRange32(dest.trie, start, end, ce32, true, &errorCode);
        if(CollationDataBuilder::isBuilderContextCE32(ce32)) {
            dest.contextChars.add(start, end);
        }
        return U_SUCCESS(errorCode);
    }

    uint32_t copyCE32(uint32_t ce32) {
        if(!Collation::isSpecialCE32(ce32)) {
            int64_t ce = modifier.modifyCE32(ce32);
            if(ce != Collation::NO_CE) {
                ce32 = dest.encodeOneCE(ce, errorCode);
            }
        } else {
            int32_t tag = Collation::tagFromCE32(ce32);
            if(tag == Collation::EXPANSION32_TAG) {
                const uint32_t *srcCE32s = reinterpret_cast<uint32_t *>(src.ce32s.getBuffer());
                srcCE32s += Collation::indexFromCE32(ce32);
                int32_t length = Collation::lengthFromCE32(ce32);
                // Inspect the source CE32s. Just copy them if none are modified.
                // Otherwise copy to modifiedCEs, with modifications.
                UBool isModified = false;
                for(int32_t i = 0; i < length; ++i) {
                    ce32 = srcCE32s[i];
                    int64_t ce;
                    if(Collation::isSpecialCE32(ce32) ||
                            (ce = modifier.modifyCE32(ce32)) == Collation::NO_CE) {
                        if(isModified) {
                            modifiedCEs[i] = Collation::ceFromCE32(ce32);
                        }
                    } else {
                        if(!isModified) {
                            for(int32_t j = 0; j < i; ++j) {
                                modifiedCEs[j] = Collation::ceFromCE32(srcCE32s[j]);
                            }
                            isModified = true;
                        }
                        modifiedCEs[i] = ce;
                    }
                }
                if(isModified) {
                    ce32 = dest.encodeCEs(modifiedCEs, length, errorCode);
                } else {
                    ce32 = dest.encodeExpansion32(
                        reinterpret_cast<const int32_t *>(srcCE32s), length, errorCode);
                }
            } else if(tag == Collation::EXPANSION_TAG) {
                const int64_t *srcCEs = src.ce64s.getBuffer();
                srcCEs += Collation::indexFromCE32(ce32);
                int32_t length = Collation::lengthFromCE32(ce32);
                // Inspect the source CEs. Just copy them if none are modified.
                // Otherwise copy to modifiedCEs, with modifications.
                UBool isModified = false;
                for(int32_t i = 0; i < length; ++i) {
                    int64_t srcCE = srcCEs[i];
                    int64_t ce = modifier.modifyCE(srcCE);
                    if(ce == Collation::NO_CE) {
                        if(isModified) {
                            modifiedCEs[i] = srcCE;
                        }
                    } else {
                        if(!isModified) {
                            for(int32_t j = 0; j < i; ++j) {
                                modifiedCEs[j] = srcCEs[j];
                            }
                            isModified = true;
                        }
                        modifiedCEs[i] = ce;
                    }
                }
                if(isModified) {
                    ce32 = dest.encodeCEs(modifiedCEs, length, errorCode);
                } else {
                    ce32 = dest.encodeExpansion(srcCEs, length, errorCode);
                }
            } else if(tag == Collation::BUILDER_DATA_TAG) {
                // Copy the list of ConditionalCE32.
                ConditionalCE32 *cond = src.getConditionalCE32ForCE32(ce32);
                U_ASSERT(!cond->hasContext());
                int32_t destIndex = dest.addConditionalCE32(
                        cond->context, copyCE32(cond->ce32), errorCode);
                ce32 = CollationDataBuilder::makeBuilderContextCE32(destIndex);
                while(cond->next >= 0) {
                    cond = src.getConditionalCE32(cond->next);
                    ConditionalCE32 *prevDestCond = dest.getConditionalCE32(destIndex);
                    destIndex = dest.addConditionalCE32(
                            cond->context, copyCE32(cond->ce32), errorCode);
                    int32_t suffixStart = cond->prefixLength() + 1;
                    dest.unsafeBackwardSet.addAll(cond->context.tempSubString(suffixStart));
                    prevDestCond->next = destIndex;
                }
            } else {
                // Just copy long CEs and Latin mini expansions (and other expected values) as is,
                // assuming that the modifier would not modify them.
                U_ASSERT(tag == Collation::LONG_PRIMARY_TAG ||
                        tag == Collation::LONG_SECONDARY_TAG ||
                        tag == Collation::LATIN_EXPANSION_TAG ||
                        tag == Collation::HANGUL_TAG);
            }
        }
        return ce32;
    }

    const CollationDataBuilder &src;
    CollationDataBuilder &dest;
    const CollationDataBuilder::CEModifier &modifier;
    int64_t modifiedCEs[Collation::MAX_EXPANSION_LENGTH];
    UErrorCode errorCode;
};

U_CDECL_BEGIN

static UBool U_CALLCONV
enumRangeForCopy(const void *context, UChar32 start, UChar32 end, uint32_t value) {
    return
        value == Collation::UNASSIGNED_CE32 || value == Collation::FALLBACK_CE32 ||
        ((CopyHelper *)context)->copyRangeCE32(start, end, value);
}

U_CDECL_END

void
CollationDataBuilder::copyFrom(const CollationDataBuilder &src, const CEModifier &modifier,
                               UErrorCode &errorCode) {
    if(U_FAILURE(errorCode)) { return; }
    if(trie == nullptr || utrie2_isFrozen(trie)) {
        errorCode = U_INVALID_STATE_ERROR;
        return;
    }
    CopyHelper helper(src, *this, modifier, errorCode);
    utrie2_enum(src.trie, nullptr, enumRangeForCopy, &helper);
    errorCode = helper.errorCode;
    // Update the contextChars and the unsafeBackwardSet while copying,
    // in case a character had conditional mappings in the source builder
    // and they were removed later.
    modified |= src.modified;
}

void
CollationDataBuilder::optimize(const UnicodeSet &set, UErrorCode &errorCode) {
    if(U_FAILURE(errorCode) || set.isEmpty()) { return; }
    UnicodeSetIterator iter(set);
    while(iter.next() && !iter.isString()) {
        UChar32 c = iter.getCodepoint();
        uint32_t ce32 = utrie2_get32(trie, c);
        if(ce32 == Collation::FALLBACK_CE32) {
            ce32 = base->getFinalCE32(base->getCE32(c));
            ce32 = copyFromBaseCE32(c, ce32, true, errorCode);
            utrie2_set32(trie, c, ce32, &errorCode);
        }
    }
    modified = true;
}

void
CollationDataBuilder::suppressContractions(const UnicodeSet &set, UErrorCode &errorCode) {
    if(U_FAILURE(errorCode) || set.isEmpty()) { return; }
    UnicodeSetIterator iter(set);
    while(iter.next() && !iter.isString()) {
        UChar32 c = iter.getCodepoint();
        uint32_t ce32 = utrie2_get32(trie, c);
        if(ce32 == Collation::FALLBACK_CE32) {
            ce32 = base->getFinalCE32(base->getCE32(c));
            if(Collation::ce32HasContext(ce32)) {
                ce32 = copyFromBaseCE32(c, ce32, false /* without context */, errorCode);
                utrie2_set32(trie, c, ce32, &errorCode);
            }
        } else if(isBuilderContextCE32(ce32)) {
            ce32 = getConditionalCE32ForCE32(ce32)->ce32;
            // Simply abandon the list of ConditionalCE32.
            // The caller will copy this builder in the end,
            // eliminating unreachable data.
            utrie2_set32(trie, c, ce32, &errorCode);
            contextChars.remove(c);
        }
    }
    modified = true;
}

UBool
CollationDataBuilder::getJamoCE32s(uint32_t jamoCE32s[], UErrorCode &errorCode) {
    if(U_FAILURE(errorCode)) { return false; }
    UBool anyJamoAssigned = base == nullptr;  // always set jamoCE32s in the base data
    UBool needToCopyFromBase = false;
    for(int32_t j = 0; j < CollationData::JAMO_CE32S_LENGTH; ++j) {  // Count across Jamo types.
        UChar32 jamo = jamoCpFromIndex(j);
        UBool fromBase = false;
        uint32_t ce32 = utrie2_get32(trie, jamo);
        anyJamoAssigned |= Collation::isAssignedCE32(ce32);
        // TODO: Try to prevent [optimize [Jamo]] from counting as anyJamoAssigned.
        // (As of CLDR 24 [2013] the Korean tailoring does not optimize conjoining Jamo.)
        if(ce32 == Collation::FALLBACK_CE32) {
            fromBase = true;
            ce32 = base->getCE32(jamo);
        }
        if(Collation::isSpecialCE32(ce32)) {
            switch(Collation::tagFromCE32(ce32)) {
            case Collation::LONG_PRIMARY_TAG:
            case Collation::LONG_SECONDARY_TAG:
            case Collation::LATIN_EXPANSION_TAG:
                // Copy the ce32 as-is.
                break;
            case Collation::EXPANSION32_TAG:
            case Collation::EXPANSION_TAG:
            case Collation::PREFIX_TAG:
            case Collation::CONTRACTION_TAG:
                if(fromBase) {
                    // Defer copying until we know if anyJamoAssigned.
                    ce32 = Collation::FALLBACK_CE32;
                    needToCopyFromBase = true;
                }
                break;
            case Collation::IMPLICIT_TAG:
                // An unassigned Jamo should only occur in tests with incomplete bases.
                U_ASSERT(fromBase);
                ce32 = Collation::FALLBACK_CE32;
                needToCopyFromBase = true;
                break;
            case Collation::OFFSET_TAG:
                ce32 = getCE32FromOffsetCE32(fromBase, jamo, ce32);
                break;
            case Collation::FALLBACK_TAG:
            case Collation::RESERVED_TAG_3:
            case Collation::BUILDER_DATA_TAG:
            case Collation::DIGIT_TAG:
            case Collation::U0000_TAG:
            case Collation::HANGUL_TAG:
            case Collation::LEAD_SURROGATE_TAG:
                errorCode = U_INTERNAL_PROGRAM_ERROR;
                return false;
            }
        }
        jamoCE32s[j] = ce32;
    }
    if(anyJamoAssigned && needToCopyFromBase) {
        for(int32_t j = 0; j < CollationData::JAMO_CE32S_LENGTH; ++j) {
            if(jamoCE32s[j] == Collation::FALLBACK_CE32) {
                UChar32 jamo = jamoCpFromIndex(j);
                jamoCE32s[j] = copyFromBaseCE32(jamo, base->getCE32(jamo),
                                                /*withContext=*/ true, errorCode);
            }
        }
    }
    return anyJamoAssigned && U_SUCCESS(errorCode);
}

void
CollationDataBuilder::setDigitTags(UErrorCode &errorCode) {
    UnicodeSet digits(UNICODE_STRING_SIMPLE("[:Nd:]"), errorCode);
    if(U_FAILURE(errorCode)) { return; }
    UnicodeSetIterator iter(digits);
    while(iter.next()) {
        U_ASSERT(!iter.isString());
        UChar32 c = iter.getCodepoint();
        uint32_t ce32 = utrie2_get32(trie, c);
        if(ce32 != Collation::FALLBACK_CE32 && ce32 != Collation::UNASSIGNED_CE32) {
            int32_t index = addCE32(ce32, errorCode);
            if(U_FAILURE(errorCode)) { return; }
            if(index > Collation::MAX_INDEX) {
                errorCode = U_BUFFER_OVERFLOW_ERROR;
                return;
            }
            ce32 = Collation::makeCE32FromTagIndexAndLength(
                    Collation::DIGIT_TAG, index, u_charDigitValue(c));
            utrie2_set32(trie, c, ce32, &errorCode);
        }
    }
}

U_CDECL_BEGIN

static UBool U_CALLCONV
enumRangeLeadValue(const void *context, UChar32 /*start*/, UChar32 /*end*/, uint32_t value) {
    int32_t *pValue = (int32_t *)context;
    if(value == Collation::UNASSIGNED_CE32) {
        value = Collation::LEAD_ALL_UNASSIGNED;
    } else if(value == Collation::FALLBACK_CE32) {
        value = Collation::LEAD_ALL_FALLBACK;
    } else {
        *pValue = Collation::LEAD_MIXED;
        return false;
    }
    if(*pValue < 0) {
        *pValue = (int32_t)value;
    } else if(*pValue != (int32_t)value) {
        *pValue = Collation::LEAD_MIXED;
        return false;
    }
    return true;
}

U_CDECL_END

void
CollationDataBuilder::setLeadSurrogates(UErrorCode &errorCode) {
    for(char16_t lead = 0xd800; lead < 0xdc00; ++lead) {
        int32_t value = -1;
        utrie2_enumForLeadSurrogate(trie, lead, nullptr, enumRangeLeadValue, &value);
        utrie2_set32ForLeadSurrogateCodeUnit(
            trie, lead,
            Collation::makeCE32FromTagAndIndex(Collation::LEAD_SURROGATE_TAG, 0) | static_cast<uint32_t>(value),
            &errorCode);
    }
}

void
CollationDataBuilder::build(CollationData &data, UErrorCode &errorCode) {
    buildMappings(data, errorCode);
    if(base != nullptr) {
        data.numericPrimary = base->numericPrimary;
        data.compressibleBytes = base->compressibleBytes;
        data.numScripts = base->numScripts;
        data.scriptsIndex = base->scriptsIndex;
        data.scriptStarts = base->scriptStarts;
        data.scriptStartsLength = base->scriptStartsLength;
    }
    buildFastLatinTable(data, errorCode);
}

void
CollationDataBuilder::buildMappings(CollationData &data, UErrorCode &errorCode) {
    if(U_FAILURE(errorCode)) { return; }
    if(trie == nullptr || utrie2_isFrozen(trie)) {
        errorCode = U_INVALID_STATE_ERROR;
        return;
    }

    buildContexts(errorCode);

    uint32_t jamoCE32s[CollationData::JAMO_CE32S_LENGTH];
    int32_t jamoIndex = -1;
    if(getJamoCE32s(jamoCE32s, errorCode)) {
        jamoIndex = ce32s.size();
        for(int32_t i = 0; i < CollationData::JAMO_CE32S_LENGTH; ++i) {
            ce32s.addElement(static_cast<int32_t>(jamoCE32s[i]), errorCode);
        }
        // Small optimization: Use a bit in the Hangul ce32
        // to indicate that none of the Jamo CE32s are isSpecialCE32()
        // (as it should be in the root collator).
        // It allows CollationIterator to avoid recursive function calls and per-Jamo tests.
        // In order to still have good trie compression and keep this code simple,
        // we only set this flag if a whole block of 588 Hangul syllables starting with
        // a common leading consonant (Jamo L) has this property.
        UBool isAnyJamoVTSpecial = false;
        for(int32_t i = Hangul::JAMO_L_COUNT; i < CollationData::JAMO_CE32S_LENGTH; ++i) {
            if(Collation::isSpecialCE32(jamoCE32s[i])) {
                isAnyJamoVTSpecial = true;
                break;
            }
        }
        uint32_t hangulCE32 = Collation::makeCE32FromTagAndIndex(Collation::HANGUL_TAG, 0);
        UChar32 c = Hangul::HANGUL_BASE;
        for(int32_t i = 0; i < Hangul::JAMO_L_COUNT; ++i) {  // iterate over the Jamo L
            uint32_t ce32 = hangulCE32;
            if(!isAnyJamoVTSpecial && !Collation::isSpecialCE32(jamoCE32s[i])) {
                ce32 |= Collation::HANGUL_NO_SPECIAL_JAMO;
            }
            UChar32 limit = c + Hangul::JAMO_VT_COUNT;
            utrie2_setRange32(trie, c, limit - 1, ce32, true, &errorCode);
            c = limit;
        }
    } else {
        // Copy the Hangul CE32s from the base in blocks per Jamo L,
        // assuming that HANGUL_NO_SPECIAL_JAMO is set or not set for whole blocks.
        for(UChar32 c = Hangul::HANGUL_BASE; c < Hangul::HANGUL_LIMIT;) {
            uint32_t ce32 = base->getCE32(c);
            U_ASSERT(Collation::hasCE32Tag(ce32, Collation::HANGUL_TAG));
            UChar32 limit = c + Hangul::JAMO_VT_COUNT;
            utrie2_setRange32(trie, c, limit - 1, ce32, true, &errorCode);
            c = limit;
        }
    }

    setDigitTags(errorCode);
    setLeadSurrogates(errorCode);

    if (icu4xMode) {
        // Make sure that starters that occur is the middle of a
        // contraction have contraction ce32 with the
        // `CONTRACT_HAS_STARTER` flag set so that starters that
        // can occur in a non-final position in a contraction can
        // be easily recognized from having a contraction ce32
        // that has the `CONTRACT_HAS_STARTER` flag set.

        UCharsTrieBuilder contractionBuilder(errorCode);
        // Intentionally unpaired low surrogate to make it never
        // match well-formed UTF-16 which ICU4X feeds to the
        // matcher.
        UnicodeString placeholder(0xDC00);

        for (UChar32 c : contractionMiddleStarter.codePoints()) {
            uint32_t ce32 = utrie2_get32(trie, c);
            UBool fromBase = false;
            if(ce32 == Collation::FALLBACK_CE32) {
                fromBase = true;
                ce32 = base->getCE32(c);
            }
            if (!(Collation::hasCE32Tag(ce32, Collation::CONTRACTION_TAG) && (ce32 & Collation::CONTRACT_HAS_STARTER))) {
                if (fromBase) {
                    // This case does not actually happen as of February 2025.
                    ce32 = copyFromBaseCE32(c, ce32, true, errorCode);
                }
                if (Collation::hasCE32Tag(ce32, Collation::CONTRACTION_TAG)) {
                    // This middle starter is also the first character of another
                    // contraction, but that contraction does not have the
                    // CONTRACT_HAS_STARTER flag. Let's add the flag to
                    // mark this at the expense of pessimizing the matching
                    // of this contraction.
                    // As of February 2025, this case does not actually occur
                    // in CLDR.
                    ce32 |= Collation::CONTRACT_HAS_STARTER;
                } else {
                    // This middle starter is not also the first character
                    // in another contraction.

                    // The UCharsTrie needs to contain some placeholder
                    // because it cannot be empty. We build a trie
                    // that never actually matches anything that ICU4X can try to
                    // match, since ICU4X always passes well-formed UTF-16 to the
                    // matcher and we put an unpaired low surrogate into the trie.
                    // This pessimizes the character to CE mapping of the `c`,
                    // since useless trie matching will be attempted but as of
                    // February 2025, only two relatively rare characters are affected.
                    contractionBuilder.clear();
                    contractionBuilder.add(placeholder, static_cast<int32_t>(ce32), errorCode);

                    int32_t index = addContextTrie(ce32, contractionBuilder, errorCode);
                    if(U_FAILURE(errorCode)) { return; }
                    if(index > Collation::MAX_INDEX) {
                        errorCode = U_BUFFER_OVERFLOW_ERROR;
                        return;
                    }
                    // Set CONTRACT_HAS_STARTER to make identical prefix matching able to catch this.
                    ce32 = Collation::makeCE32FromTagAndIndex(Collation::CONTRACTION_TAG, index) | Collation::CONTRACT_HAS_STARTER;
                }
                utrie2_set32(trie, c, ce32, &errorCode);
            }
        }
    } else {
        // For U+0000, move its normal ce32 into CE32s[0] and set U0000_TAG.
        ce32s.setElementAt(static_cast<int32_t>(utrie2_get32(trie, 0)), 0);
        utrie2_set32(trie, 0, Collation::makeCE32FromTagAndIndex(Collation::U0000_TAG, 0), &errorCode);
    }

    utrie2_freeze(trie, UTRIE2_32_VALUE_BITS, &errorCode);
    if(U_FAILURE(errorCode)) { return; }

    // Mark each lead surrogate as "unsafe"
    // if any of its 1024 associated supplementary code points is "unsafe".
    UChar32 c = 0x10000;
    for(char16_t lead = 0xd800; lead < 0xdc00; ++lead, c += 0x400) {
        if(unsafeBackwardSet.containsSome(c, c + 0x3ff)) {
            unsafeBackwardSet.add(lead);
        }
    }
    unsafeBackwardSet.freeze();

    data.trie = trie;
    data.ce32s = reinterpret_cast<const uint32_t *>(ce32s.getBuffer());
    data.ces = ce64s.getBuffer();
    data.contexts = contexts.getBuffer();

    data.ce32sLength = ce32s.size();
    data.cesLength = ce64s.size();
    data.contextsLength = contexts.length();

    data.base = base;
    if(jamoIndex >= 0) {
        data.jamoCE32s = data.ce32s + jamoIndex;
    } else {
        data.jamoCE32s = base->jamoCE32s;
    }
    data.unsafeBackwardSet = &unsafeBackwardSet;
}

void
CollationDataBuilder::clearContexts() {
    contexts.remove();
    // Incrementing the contexts build "era" invalidates all of the builtCE32
    // from before this clearContexts() call.
    // Simpler than finding and resetting all of those fields.
    ++contextsEra;
}

void
CollationDataBuilder::buildContexts(UErrorCode &errorCode) {
    if(U_FAILURE(errorCode)) { return; }
    // Ignore abandoned lists and the cached builtCE32,
    // and build all contexts from scratch.
    clearContexts();
    UnicodeSetIterator iter(contextChars);
    while(U_SUCCESS(errorCode) && iter.next()) {
        U_ASSERT(!iter.isString());
        UChar32 c = iter.getCodepoint();
        uint32_t ce32 = utrie2_get32(trie, c);
        if(!isBuilderContextCE32(ce32)) {
            // Impossible: No context data for c in contextChars.
            errorCode = U_INTERNAL_PROGRAM_ERROR;
            return;
        }
        ConditionalCE32 *cond = getConditionalCE32ForCE32(ce32);
        ce32 = buildContext(cond, errorCode);
        utrie2_set32(trie, c, ce32, &errorCode);
    }
}

uint32_t
CollationDataBuilder::buildContext(ConditionalCE32 *head, UErrorCode &errorCode) {
    if(U_FAILURE(errorCode)) { return 0; }
    // The list head must have no context.
    U_ASSERT(!head->hasContext());
    // The list head must be followed by one or more nodes that all do have context.
    U_ASSERT(head->next >= 0);
    UCharsTrieBuilder prefixBuilder(errorCode);
    UCharsTrieBuilder contractionBuilder(errorCode);
    // This outer loop goes from each prefix to the next.
    // For each prefix it finds the one or more same-prefix entries (firstCond..lastCond).
    // If there are multiple suffixes for the same prefix,
    // then an inner loop builds a contraction trie for them.
    for(ConditionalCE32 *cond = head;; cond = getConditionalCE32(cond->next)) {
        if(U_FAILURE(errorCode)) { return 0; }  // early out for memory allocation errors
        // After the list head, the prefix or suffix can be empty, but not both.
        U_ASSERT(cond == head || cond->hasContext());
        int32_t prefixLength = cond->prefixLength();
        UnicodeString prefix(cond->context, 0, prefixLength + 1);
        // Collect all contraction suffixes for one prefix.
        ConditionalCE32 *firstCond = cond;
        ConditionalCE32 *lastCond;
        do {
            lastCond = cond;
            // Clear the defaultCE32 fields as we go.
            // They are left over from building a previous version of this list of contexts.
            //
            // One of the code paths below may copy a preceding defaultCE32
            // into its emptySuffixCE32.
            // If a new suffix has been inserted before what used to be
            // the firstCond for its prefix, then that previous firstCond could still
            // contain an outdated defaultCE32 from an earlier buildContext() and
            // result in an incorrect emptySuffixCE32.
            // So we reset all defaultCE32 before reading and setting new values.
            cond->defaultCE32 = Collation::NO_CE32;
        } while(cond->next >= 0 &&
                (cond = getConditionalCE32(cond->next))->context.startsWith(prefix));
        uint32_t ce32;
        int32_t suffixStart = prefixLength + 1;  // == prefix.length()
        if(lastCond->context.length() == suffixStart) {
            // One prefix without contraction suffix.
            U_ASSERT(firstCond == lastCond);
            ce32 = lastCond->ce32;
            cond = lastCond;
        } else {
            // Build the contractions trie.
            contractionBuilder.clear();
            // Entry for an empty suffix, to be stored before the trie.
            uint32_t emptySuffixCE32 = 0;
            uint32_t flags = 0;
            if(firstCond->context.length() == suffixStart) {
                // There is a mapping for the prefix and the single character c. (p|c)
                // If no other suffix matches, then we return this value.
                emptySuffixCE32 = firstCond->ce32;
                cond = getConditionalCE32(firstCond->next);
            } else {
                // There is no mapping for the prefix and just the single character.
                // (There is no p|c, only p|cd, p|ce etc.)
                flags |= Collation::CONTRACT_SINGLE_CP_NO_MATCH;
                // When the prefix matches but none of the prefix-specific suffixes,
                // then we fall back to the mappings with the next-longest prefix,
                // and ultimately to mappings with no prefix.
                // Each fallback might be another set of contractions.
                // For example, if there are mappings for ch, p|cd, p|ce, but not for p|c,
                // then in text "pch" we find the ch contraction.
                for(cond = head;; cond = getConditionalCE32(cond->next)) {
                    int32_t length = cond->prefixLength();
                    if(length == prefixLength) { break; }
                    if(cond->defaultCE32 != Collation::NO_CE32 &&
                            (length==0 || prefix.endsWith(cond->context, 1, length))) {
                        emptySuffixCE32 = cond->defaultCE32;
                    }
                }
                cond = firstCond;
            }
            // Optimization: Set a flag when
            // the first character of every contraction suffix has lccc!=0.
            // Short-circuits contraction matching when a normal letter follows.
            flags |= Collation::CONTRACT_NEXT_CCC;
            // Add all of the non-empty suffixes into the contraction trie.
            for(;;) {
                UnicodeString suffix(cond->context, suffixStart);
                uint16_t fcd16 = nfcImpl.getFCD16(suffix.char32At(0));
                if(fcd16 <= 0xff) {
                    flags &= ~Collation::CONTRACT_NEXT_CCC;
                }
                fcd16 = nfcImpl.getFCD16(suffix.char32At(suffix.length() - 1));
                if(fcd16 > 0xff) {
                    // The last suffix character has lccc!=0, allowing for discontiguous contractions.
                    flags |= Collation::CONTRACT_TRAILING_CCC;
                }
                if (icu4xMode && (flags & Collation::CONTRACT_HAS_STARTER) == 0) {
                    for (int32_t i = 0; i < suffix.length();) {
                        UChar32 c = suffix.char32At(i);
                            if (!u_getCombiningClass(c)) {
                                flags |= Collation::CONTRACT_HAS_STARTER;
                                break;
                            }
                        if (c > 0xFFFF) {
                            i += 2;
                        } else {
                            ++i;
                        }
                    }
                }
                contractionBuilder.add(suffix, static_cast<int32_t>(cond->ce32), errorCode);
                if(cond == lastCond) { break; }
                cond = getConditionalCE32(cond->next);
            }
            int32_t index = addContextTrie(emptySuffixCE32, contractionBuilder, errorCode);
            if(U_FAILURE(errorCode)) { return 0; }
            if(index > Collation::MAX_INDEX) {
                errorCode = U_BUFFER_OVERFLOW_ERROR;
                return 0;
            }
            ce32 = Collation::makeCE32FromTagAndIndex(Collation::CONTRACTION_TAG, index) | flags;
        }
        U_ASSERT(cond == lastCond);
        firstCond->defaultCE32 = ce32;
        if(prefixLength == 0) {
            if(cond->next < 0) {
                // No non-empty prefixes, only contractions.
                return ce32;
            }
        } else {
            prefix.remove(0, 1);  // Remove the length unit.
            prefix.reverse();
            prefixBuilder.add(prefix, static_cast<int32_t>(ce32), errorCode);
            if(cond->next < 0) { break; }
        }
    }
    U_ASSERT(head->defaultCE32 != Collation::NO_CE32);
    int32_t index = addContextTrie(head->defaultCE32, prefixBuilder, errorCode);
    if(U_FAILURE(errorCode)) { return 0; }
    if(index > Collation::MAX_INDEX) {
        errorCode = U_BUFFER_OVERFLOW_ERROR;
        return 0;
    }
    return Collation::makeCE32FromTagAndIndex(Collation::PREFIX_TAG, index);
}

int32_t
CollationDataBuilder::addContextTrie(uint32_t defaultCE32, UCharsTrieBuilder &trieBuilder,
                                     UErrorCode &errorCode) {
    UnicodeString context;
    context.append(static_cast<char16_t>(defaultCE32 >> 16)).append(static_cast<char16_t>(defaultCE32));
    UnicodeString trieString;
    context.append(trieBuilder.buildUnicodeString(USTRINGTRIE_BUILD_SMALL, trieString, errorCode));
    if(U_FAILURE(errorCode)) { return -1; }
    int32_t index = contexts.indexOf(context);
    if(index < 0) {
        index = contexts.length();
        contexts.append(context);
    }
    return index;
}

void
CollationDataBuilder::buildFastLatinTable(CollationData &data, UErrorCode &errorCode) {
    if(U_FAILURE(errorCode) || !fastLatinEnabled) { return; }

    delete fastLatinBuilder;
    fastLatinBuilder = new CollationFastLatinBuilder(errorCode);
    if(fastLatinBuilder == nullptr) {
        errorCode = U_MEMORY_ALLOCATION_ERROR;
        return;
    }
    if(fastLatinBuilder->forData(data, errorCode)) {
        const uint16_t *table = fastLatinBuilder->getTable();
        int32_t length = fastLatinBuilder->lengthOfTable();
        if(base != nullptr && length == base->fastLatinTableLength &&
                uprv_memcmp(table, base->fastLatinTable, length * 2) == 0) {
            // Same fast Latin table as in the base, use that one instead.
            delete fastLatinBuilder;
            fastLatinBuilder = nullptr;
            table = base->fastLatinTable;
        }
        data.fastLatinTable = table;
        data.fastLatinTableLength = length;
    } else {
        delete fastLatinBuilder;
        fastLatinBuilder = nullptr;
    }
}

int32_t
CollationDataBuilder::getCEs(const UnicodeString &s, int64_t ces[], int32_t cesLength) {
    return getCEs(s, 0, ces, cesLength);
}

int32_t
CollationDataBuilder::getCEs(const UnicodeString &prefix, const UnicodeString &s,
                             int64_t ces[], int32_t cesLength) {
    int32_t prefixLength = prefix.length();
    if(prefixLength == 0) {
        return getCEs(s, 0, ces, cesLength);
    } else {
        return getCEs(prefix + s, prefixLength, ces, cesLength);
    }
}

int32_t
CollationDataBuilder::getCEs(const UnicodeString &s, int32_t start,
                             int64_t ces[], int32_t cesLength) {
    if(collIter == nullptr) {
        collIter = new DataBuilderCollationIterator(*this);
        if(collIter == nullptr) { return 0; }
    }
    return collIter->fetchCEs(s, start, ces, cesLength);
}

U_NAMESPACE_END

#endif  // !UCONFIG_NO_COLLATION
