diff --git a/source/common/rbbi_cache.cpp b/source/common/rbbi_cache.cpp
new file mode 100644
index 0000000..0a1e1a7
--- /dev/null
+++ b/source/common/rbbi_cache.cpp
@@ -0,0 +1,622 @@
+// Copyright (C) 2016 and later: Unicode, Inc. and others.
+// License & terms of use: http://www.unicode.org/copyright.html
+
+// file: rbbi_cache.cpp
+
+#include "unicode/utypes.h"
+#include "unicode/ubrk.h"
+#include "unicode/rbbi.h"
+
+#include "rbbi_cache.h"
+
+#include "brkeng.h"
+#include "cmemory.h"
+#include "rbbidata.h"
+#include "uassert.h"
+#include "uvectr32.h"
+
+U_NAMESPACE_BEGIN
+
+/*
+ * DictionaryCache implementation
+ */
+
+RuleBasedBreakIterator::DictionaryCache::DictionaryCache(RuleBasedBreakIterator *bi, UErrorCode &status) :
+        fBI(bi), fBreaks(NULL), fPositionInCache(-1),
+        fStart(0), fLimit(0), fFirstRuleStatusIndex(0), fOtherRuleStatusIndex(0) {
+    fBreaks = new UVector32(status);
+}
+
+RuleBasedBreakIterator::DictionaryCache::~DictionaryCache() {
+    delete fBreaks;
+    fBreaks = NULL;
+}
+
+void RuleBasedBreakIterator::DictionaryCache::reset() {
+    fPositionInCache = -1;
+    fStart = 0;
+    fLimit = 0;
+    fFirstRuleStatusIndex = 0;
+    fOtherRuleStatusIndex = 0;
+    fBreaks->removeAllElements();
+}
+
+UBool RuleBasedBreakIterator::DictionaryCache::following(int32_t fromPos, int32_t *result, int32_t *statusIndex) {
+    if (fromPos >= fLimit || fromPos < fStart) {
+        fPositionInCache = -1;
+        return FALSE;
+    }
+
+    // Sequential iteration, move from previous boundary to the following
+
+    int32_t r = 0;
+    if (fPositionInCache >= 0 && fPositionInCache < fBreaks->size() && fBreaks->elementAti(fPositionInCache) == fromPos) {
+        ++fPositionInCache;
+        if (fPositionInCache >= fBreaks->size()) {
+            fPositionInCache = -1;
+            return FALSE;
+        }
+        r = fBreaks->elementAti(fPositionInCache);
+        U_ASSERT(r > fromPos);
+        *result = r;
+        *statusIndex = fOtherRuleStatusIndex;
+        return TRUE;
+    }
+
+    // Random indexing. Linear search for the boundary following the given position.
+
+    for (fPositionInCache = 0; fPositionInCache < fBreaks->size(); ++fPositionInCache) {
+        r= fBreaks->elementAti(fPositionInCache);
+        if (r > fromPos) {
+            *result = r;
+            *statusIndex = fOtherRuleStatusIndex;
+            return TRUE;
+        }
+    }
+    U_ASSERT(FALSE);
+    fPositionInCache = -1;
+    return FALSE;
+}
+
+
+UBool RuleBasedBreakIterator::DictionaryCache::preceding(int32_t fromPos, int32_t *result, int32_t *statusIndex) {
+    if (fromPos <= fStart || fromPos > fLimit) {
+        fPositionInCache = -1;
+        return FALSE;
+    }
+
+    if (fromPos == fLimit) {
+        fPositionInCache = fBreaks->size() - 1;
+        if (fPositionInCache >= 0) {
+            U_ASSERT(fBreaks->elementAti(fPositionInCache) == fromPos);
+        }
+    }
+
+    int32_t r;
+    if (fPositionInCache > 0 && fPositionInCache < fBreaks->size() && fBreaks->elementAti(fPositionInCache) == fromPos) {
+        --fPositionInCache;
+        r = fBreaks->elementAti(fPositionInCache);
+        U_ASSERT(r < fromPos);
+        *result = r;
+        *statusIndex = ( r== fStart) ? fFirstRuleStatusIndex : fOtherRuleStatusIndex;
+        return TRUE;
+    }
+
+    if (fPositionInCache == 0) {
+        fPositionInCache = -1;
+        return FALSE;
+    }
+
+    for (fPositionInCache = fBreaks->size()-1; fPositionInCache >= 0; --fPositionInCache) {
+        r = fBreaks->elementAti(fPositionInCache);
+        if (r < fromPos) {
+            *result = r;
+            *statusIndex = ( r == fStart) ? fFirstRuleStatusIndex : fOtherRuleStatusIndex;
+            return TRUE;
+        }
+    }
+    U_ASSERT(FALSE);
+    fPositionInCache = -1;
+    return FALSE;
+}
+
+void RuleBasedBreakIterator::DictionaryCache::populateDictionary(int32_t startPos, int32_t endPos,
+                                       int32_t firstRuleStatus, int32_t otherRuleStatus) {
+    if ((endPos - startPos) <= 1) {
+        return;
+    }
+
+    reset();
+    fFirstRuleStatusIndex = firstRuleStatus;
+    fOtherRuleStatusIndex = otherRuleStatus;
+
+    int32_t rangeStart = startPos;
+    int32_t rangeEnd = endPos;
+
+    uint16_t    category;
+    int32_t     current;
+    UErrorCode  status = U_ZERO_ERROR;
+    int32_t     foundBreakCount = 0;
+    UText      *text = fBI->fText;
+
+    // Loop through the text, looking for ranges of dictionary characters.
+    // For each span, find the appropriate break engine, and ask it to find
+    // any breaks within the span.
+
+    utext_setNativeIndex(text, rangeStart);
+    UChar32     c = utext_current32(text);
+    category = UTRIE2_GET16(fBI->fData->fTrie, c);
+
+    while(U_SUCCESS(status)) {
+        while((current = (int32_t)UTEXT_GETNATIVEINDEX(text)) < rangeEnd && (category & 0x4000) == 0) {
+            utext_next32(text);           // TODO: cleaner loop structure.
+            c = utext_current32(text);
+            category = UTRIE2_GET16(fBI->fData->fTrie, c);
+        }
+        if (current >= rangeEnd) {
+            break;
+        }
+
+        // We now have a dictionary character. Get the appropriate language object
+        // to deal with it.
+        const LanguageBreakEngine *lbe = fBI->getLanguageBreakEngine(c);
+
+        // Ask the language object if there are any breaks. It will add them to the cache and
+        // leave the text pointer on the other side of its range, ready to search for the next one.
+        if (lbe != NULL) {
+            foundBreakCount += lbe->findBreaks(text, rangeStart, rangeEnd, fBI->fBreakType, *fBreaks);
+        }
+
+        // Reload the loop variables for the next go-round
+        c = utext_current32(text);
+        category = UTRIE2_GET16(fBI->fData->fTrie, c);
+    }
+
+    // If we found breaks, ensure that the first and last entries are
+    // the original starting and ending position. And initialize the
+    // cache iteration position to the first entry.
+
+    // printf("foundBreakCount = %d\n", foundBreakCount);
+    if (foundBreakCount > 0) {
+        U_ASSERT(foundBreakCount == fBreaks->size());
+        if (startPos < fBreaks->elementAti(0)) {
+            // The dictionary did not place a boundary at the start of the segment of text.
+            // Add one now. This should not commonly happen, but it would be easy for interactions
+            // of the rules for dictionary segments and the break engine implementations to
+            // inadvertently cause it. Cover it here, just in case.
+            fBreaks->insertElementAt(startPos, 0, status);
+        }
+        if (endPos > fBreaks->peeki()) {
+            fBreaks->push(endPos, status);
+        }
+        fPositionInCache = 0;
+        // Note: Dictionary matching may extend beyond the original limit.
+        fStart = fBreaks->elementAti(0);
+        fLimit = fBreaks->peeki();
+    } else {
+        // there were no language-based breaks, even though the segment contained
+        // dictionary characters. Subsequent attempts to fetch boundaries from the dictionary cache
+        // for this range will fail, and the calling code will fall back to the rule based boundaries.
+    }
+}
+
+
+/*
+ *   BreakCache implemetation
+ */
+
+RuleBasedBreakIterator::BreakCache::BreakCache(RuleBasedBreakIterator *bi, UErrorCode &status) : 
+        fBI(bi), fSideBuffer(status) {
+    reset();
+}
+
+
+RuleBasedBreakIterator::BreakCache::~BreakCache() {
+}
+
+
+void RuleBasedBreakIterator::BreakCache::reset(int32_t pos, int32_t ruleStatus) {
+    fStartBufIdx = 0;
+    fEndBufIdx = 0;
+    fTextIdx = pos;
+    fBufIdx = 0;
+    fBoundaries[0] = pos;
+    fStatuses[0] = (uint16_t)ruleStatus;
+}
+
+
+int32_t  RuleBasedBreakIterator::BreakCache::current() {
+    fBI->fPosition = fTextIdx;
+    fBI->fRuleStatusIndex = fStatuses[fBufIdx];
+    fBI->fDone = FALSE;
+    return fTextIdx;
+}
+
+
+void RuleBasedBreakIterator::BreakCache::following(int32_t startPos, UErrorCode &status) {
+    if (U_FAILURE(status)) {
+        return;
+    }
+    if (startPos == fTextIdx || seek(startPos) || populateNear(startPos, status)) {
+        // startPos is in the cache. Do a next() from that position.
+        // TODO: an awkward set of interactions with bi->fDone
+        //       seek() does not clear it; it can't because of interactions with populateNear().
+        //       next() does not clear it in the fast-path case, where everything matters. Maybe it should.
+        //       So clear it here, for the case where seek() succeeded on an iterator that had previously run off the end.
+        fBI->fDone = false;
+        next();
+    }
+    return;
+}
+
+
+void RuleBasedBreakIterator::BreakCache::preceding(int32_t startPos, UErrorCode &status) {
+    if (U_FAILURE(status)) {
+        return;
+    }
+    if (startPos == fTextIdx || seek(startPos) || populateNear(startPos, status)) {
+        if (startPos == fTextIdx) {
+            previous(status);
+        } else {
+            // seek() leaves the BreakCache positioned at the preceding boundary
+            //        if the requested position is between two bounaries.
+            // current() pushes the BreakCache position out to the BreakIterator itself.
+            U_ASSERT(startPos > fTextIdx);
+            current();
+        }
+    }
+    return;
+}
+
+
+/*
+ * Out-of-line code for BreakCache::next().
+ * Cache does not already contain the boundary
+ */
+void RuleBasedBreakIterator::BreakCache::nextOL() {
+    fBI->fDone = !populateFollowing();
+    fBI->fPosition = fTextIdx;
+    fBI->fRuleStatusIndex = fStatuses[fBufIdx];
+    return;
+}
+
+
+void RuleBasedBreakIterator::BreakCache::previous(UErrorCode &status) {
+    if (U_FAILURE(status)) {
+        return;
+    }
+    int32_t initialBufIdx = fBufIdx;
+    if (fBufIdx == fStartBufIdx) {
+        // At start of cache. Prepend to it.
+        populatePreceding(status);
+    } else {
+        // Cache already holds the next boundary
+        fBufIdx = modChunkSize(fBufIdx - 1);
+        fTextIdx = fBoundaries[fBufIdx];
+    }
+    fBI->fDone = (fBufIdx == initialBufIdx);
+    fBI->fPosition = fTextIdx;
+    fBI->fRuleStatusIndex = fStatuses[fBufIdx];
+    return;
+}    
+
+
+UBool RuleBasedBreakIterator::BreakCache::seek(int32_t pos) {
+    if (pos < fBoundaries[fStartBufIdx] || pos > fBoundaries[fEndBufIdx]) {
+        return FALSE;
+    }
+    if (pos == fBoundaries[fStartBufIdx]) {
+        // Common case: seek(0), from BreakIterator::first()
+        fBufIdx = fStartBufIdx;
+        fTextIdx = fBoundaries[fBufIdx];
+        return TRUE;
+    }
+    if (pos == fBoundaries[fEndBufIdx]) {
+        fBufIdx = fEndBufIdx;
+        fTextIdx = fBoundaries[fBufIdx];
+        return TRUE;
+    }
+    
+    int32_t min = fStartBufIdx;
+    int32_t max = fEndBufIdx;
+    while (min != max) {
+        int32_t probe = (min + max + (min>max ? CACHE_SIZE : 0)) / 2;
+        probe = modChunkSize(probe);
+        if (fBoundaries[probe] > pos) {
+            max = probe;
+        } else {
+            min = modChunkSize(probe + 1);
+        }
+    }
+    U_ASSERT(fBoundaries[max] > pos);
+    fBufIdx = modChunkSize(max - 1);
+    fTextIdx = fBoundaries[fBufIdx];
+    U_ASSERT(fTextIdx <= pos);
+    return TRUE;
+}
+
+
+UBool RuleBasedBreakIterator::BreakCache::populateNear(int32_t position, UErrorCode &status) {
+    if (U_FAILURE(status)) {
+        return FALSE;
+    }
+    U_ASSERT(position < fBoundaries[fStartBufIdx] || position > fBoundaries[fEndBufIdx]);
+
+    // Find a boundary somewhere in the vicinity of the requested position.
+    // Depending on the safe rules and the text data, it could be either before, at, or after
+    // the requested position.
+
+
+    // If the requested position is not near already cached positions, clear the existing cache,
+    // find a near-by boundary and begin new cache contents there.
+
+    if ((position < fBoundaries[fStartBufIdx] - 15) || position > (fBoundaries[fEndBufIdx] + 15)) {
+        int32_t aBoundary = 0;
+        int32_t ruleStatusIndex = 0;
+        // TODO: check for position == length of text. Although may still need to back up to get rule status.
+        if (position > 20) {
+            int32_t backupPos = fBI->handlePrevious(position);
+            fBI->fPosition = backupPos;
+            aBoundary = fBI->handleNext();                // Ignore dictionary, just finding a rule based boundary.
+            ruleStatusIndex = fBI->fRuleStatusIndex;
+        }
+        reset(aBoundary, ruleStatusIndex);               // Reset cache to hold aBoundary as a single starting point.
+    }
+    
+    // Fill in boundaries between existing cache content and the new requested position.
+
+    if (fBoundaries[fEndBufIdx] < position) {
+        // The last position in the cache precedes the requested position.
+        // Add following position(s) to the cache.
+        while (fBoundaries[fEndBufIdx] < position) {
+            if (!populateFollowing()) {
+                U_ASSERT(false);
+                return false;
+            }
+        }
+        fBufIdx = fEndBufIdx;                      // Set iterator position to the end of the buffer.
+        fTextIdx = fBoundaries[fBufIdx];           // Required because populateFollowing may add extra boundaries.
+        while (fTextIdx > position) {              // Move backwards to a position at or preceding the requested pos.
+            previous(status);
+        }
+        return true;
+    }
+
+    if (fBoundaries[fStartBufIdx] > position) {
+        // The first position in the cache is beyond the requested position.
+        // back up more until we get a boundary <= the requested position.
+        while (fBoundaries[fStartBufIdx] > position) {
+            populatePreceding(status);
+        }
+        fBufIdx = fStartBufIdx;                    // Set iterator position to the start of the buffer.
+        fTextIdx = fBoundaries[fBufIdx];           // Required because populatePreceding may add extra boundaries.
+        while (fTextIdx < position) {              // Move forwards to a position at or following the requested pos.
+            next();
+        }
+        if (fTextIdx > position) {
+            // If position is not itself a boundary, the next() loop above will overshoot.
+            // Back up one, leaving cache position at the boundary preceding the requested position.
+            previous(status);
+        }
+        return true;
+    }
+
+    U_ASSERT(fTextIdx == position);
+    return true;
+}
+
+
+
+UBool RuleBasedBreakIterator::BreakCache::populateFollowing() {
+    int32_t fromPosition = fBoundaries[fEndBufIdx];
+    int32_t fromRuleStatusIdx = fStatuses[fEndBufIdx];
+    int32_t pos = 0;
+    int32_t ruleStatusIdx = 0;
+
+    if (fBI->fDictionaryCache->following(fromPosition, &pos, &ruleStatusIdx)) {
+        addFollowing(pos, ruleStatusIdx, UpdateCachePosition);
+        return TRUE;
+    }
+
+    fBI->fPosition = fromPosition;
+    pos = fBI->handleNext();
+    if (pos == UBRK_DONE) {
+        return FALSE;
+    }
+
+    ruleStatusIdx = fBI->fRuleStatusIndex;
+    if (fBI->fDictionaryCharCount > 0) {
+        // The text segment obtained from the rules includes dictionary characters.
+        // Subdivide it, with subdivided results going into the dictionary cache.
+        fBI->fDictionaryCache->populateDictionary(fromPosition, pos, fromRuleStatusIdx, ruleStatusIdx);
+        if (fBI->fDictionaryCache->following(fromPosition, &pos, &ruleStatusIdx)) {
+            addFollowing(pos, ruleStatusIdx, UpdateCachePosition);
+            return TRUE;
+            // TODO: may want to move a sizable chunk of dictionary cache to break cache at this point.
+            //       But be careful with interactions with populateNear().
+        }
+    }
+
+    // Rule based segment did not include dictionary characters.
+    // Or, it did contain dictionary chars, but the dictionary segmenter didn't handle them,
+    //    meaning that we didn't take the return, above.
+    // Add its end point to the cache.
+    addFollowing(pos, ruleStatusIdx, UpdateCachePosition);
+
+    // Add several non-dictionary boundaries at this point, to optimize straight forward iteration.
+    //    (subsequent calls to BreakIterator::next() will take the fast path, getting cached results.
+    //
+    for (int count=0; count<6; ++count) {
+        pos = fBI->handleNext();
+        if (pos == UBRK_DONE || fBI->fDictionaryCharCount > 0) {
+            break;
+        }
+        addFollowing(pos, fBI->fRuleStatusIndex, RetainCachePosition);
+    }
+
+    return TRUE;
+}
+
+
+UBool RuleBasedBreakIterator::BreakCache::populatePreceding(UErrorCode &status) {
+    if (U_FAILURE(status)) {
+        return FALSE;
+    }
+
+    int32_t fromPosition = fBoundaries[fStartBufIdx];
+    if (fromPosition == 0) {
+        return FALSE;
+    }
+
+    int32_t position = 0;
+    int32_t positionStatusIdx = 0;
+
+    if (fBI->fDictionaryCache->preceding(fromPosition, &position, &positionStatusIdx)) {
+        addPreceding(position, positionStatusIdx, UpdateCachePosition);
+        return TRUE;
+    }
+
+    int32_t backupPosition = fromPosition;
+
+    // Find a boundary somewhere preceding the first already-cached boundary
+    do {
+        backupPosition = backupPosition - 30;
+        if (backupPosition <= 0) {
+            backupPosition = 0;
+        } else {
+            backupPosition = fBI->handlePrevious(backupPosition);
+        }
+        if (backupPosition == UBRK_DONE || backupPosition == 0) {
+            position = 0;
+            positionStatusIdx = 0;
+        } else {
+            fBI->fPosition = backupPosition;  // TODO: pass starting position in a clearer way.
+            position = fBI->handleNext();
+            positionStatusIdx = fBI->fRuleStatusIndex;
+
+        }
+    } while (position >= fromPosition);
+
+    // Find boundaries between the one we just located and the first already-cached boundary
+    // Put them in a side buffer, because we don't yet know where they will fall in the circular cache buffer..
+
+    fSideBuffer.removeAllElements();
+    fSideBuffer.addElement(position, status);
+    fSideBuffer.addElement(positionStatusIdx, status);
+
+    do {
+        int32_t prevPosition = fBI->fPosition = position;
+        int32_t prevStatusIdx = positionStatusIdx;
+        position = fBI->handleNext();
+        positionStatusIdx = fBI->fRuleStatusIndex;
+        if (position == UBRK_DONE) {
+            break;
+        }
+
+        UBool segmentHandledByDictionary = FALSE;
+        if (fBI->fDictionaryCharCount != 0) {
+            // Segment from the rules includes dictionary characters.
+            // Subdivide it, with subdivided results going into the dictionary cache.
+            int32_t dictSegEndPosition = position;
+            fBI->fDictionaryCache->populateDictionary(prevPosition, dictSegEndPosition, prevStatusIdx, positionStatusIdx);
+            while (fBI->fDictionaryCache->following(prevPosition, &position, &positionStatusIdx)) {
+                segmentHandledByDictionary = true;
+                U_ASSERT(position > prevPosition);
+                if (position >= fromPosition) {
+                    break;
+                }
+                U_ASSERT(position <= dictSegEndPosition);
+                fSideBuffer.addElement(position, status);
+                fSideBuffer.addElement(positionStatusIdx, status);
+                prevPosition = position;
+            }
+            U_ASSERT(position==dictSegEndPosition || position>=fromPosition);
+        }
+            
+        if (!segmentHandledByDictionary && position < fromPosition) {
+            fSideBuffer.addElement(position, status);
+            fSideBuffer.addElement(positionStatusIdx, status);
+        }
+    } while (position < fromPosition);
+
+    // Move boundaries from the side buffer to the main circular buffer.
+    UBool success = FALSE;
+    if (!fSideBuffer.isEmpty()) {
+        positionStatusIdx = fSideBuffer.popi();
+        position = fSideBuffer.popi();
+        addPreceding(position, positionStatusIdx, UpdateCachePosition);
+        success = TRUE;
+    }
+
+    while (!fSideBuffer.isEmpty()) {
+        positionStatusIdx = fSideBuffer.popi();
+        position = fSideBuffer.popi();
+        if (!addPreceding(position, positionStatusIdx, RetainCachePosition)) {
+            // No space in circular buffer to hold a new preceding result while
+            // also retaining the current cache (iteration) position.
+            // Bailing out is safe; the cache will refill again if needed.
+            break;
+        }
+    }
+      
+    return success;
+}
+
+
+void RuleBasedBreakIterator::BreakCache::addFollowing(int32_t position, int32_t ruleStatusIdx, UpdatePositionValues update) {
+    U_ASSERT(position > fBoundaries[fEndBufIdx]);
+    U_ASSERT(ruleStatusIdx <= UINT16_MAX);
+    int32_t nextIdx = modChunkSize(fEndBufIdx + 1);
+    if (nextIdx == fStartBufIdx) {
+        fStartBufIdx = modChunkSize(fStartBufIdx + 6);    // TODO: experiment. Probably revert to 1.
+    }
+    fBoundaries[nextIdx] = position;
+    fStatuses[nextIdx] = ruleStatusIdx;
+    fEndBufIdx = nextIdx;
+    if (update == UpdateCachePosition) {
+        // Set current position to the newly added boundary.
+        fBufIdx = nextIdx;
+        fTextIdx = position;
+    } else {
+        // Retaining the original cache position.
+        // Check if the added boundary wraps around the buffer, and would over-write the original position.
+        // It's the responsibility of callers of this function to not add too many.
+        U_ASSERT(nextIdx != fBufIdx);
+    }
+}
+
+bool RuleBasedBreakIterator::BreakCache::addPreceding(int32_t position, int32_t ruleStatusIdx, UpdatePositionValues update) {
+    U_ASSERT(position < fBoundaries[fStartBufIdx]);
+    U_ASSERT(ruleStatusIdx <= UINT16_MAX);
+    int32_t nextIdx = modChunkSize(fStartBufIdx - 1);
+    if (nextIdx == fEndBufIdx) {
+        if (fBufIdx == fEndBufIdx && update == RetainCachePosition) {
+            // Failure. The insertion of the new boundary would claim the buffer position that is the
+            // current iteration position. And we also want to retain the current iteration position.
+            // (The buffer is already completely full of entries that precede the iteration position.)
+            return false;
+        }
+        fEndBufIdx = modChunkSize(fEndBufIdx - 1);
+    }
+    fBoundaries[nextIdx] = position;
+    fStatuses[nextIdx] = ruleStatusIdx;
+    fStartBufIdx = nextIdx;
+    if (update == UpdateCachePosition) {
+        fBufIdx = nextIdx;
+        fTextIdx = position;
+    }
+    return true;
+}
+
+
+void RuleBasedBreakIterator::BreakCache::dumpCache() {
+    printf("fTextIdx:%d   fBufIdx:%d\n", fTextIdx, fBufIdx);
+    for (int32_t i=fStartBufIdx; ; i=modChunkSize(i+1)) {
+        printf("%d  %d\n", i, fBoundaries[i]);
+        if (i == fEndBufIdx) {
+            break;
+        }
+    }
+}
+
+U_NAMESPACE_END
diff --git a/source/common/rbbi_cache.h b/source/common/rbbi_cache.h
new file mode 100644
index 0000000..72576e5
--- /dev/null
+++ b/source/common/rbbi_cache.h
@@ -0,0 +1,199 @@
+// Copyright (C) 2016 and later: Unicode, Inc. and others.
+// License & terms of use: http://www.unicode.org/copyright.html
+
+// file: rbbi_cache.h
+//
+#ifndef RBBI_CACHE_H
+#define RBBI_CACHE_H
+
+#include "unicode/utypes.h"
+
+#include "unicode/rbbi.h"
+#include "unicode/uobject.h"
+
+#include "uvectr32.h"
+
+U_NAMESPACE_BEGIN
+
+/* DictionaryCache  stores the boundaries obtained from a run of dictionary characters.
+ *                 Dictionary boundaries are moved first to this cache, then from here
+ *                 to the main BreakCache, where they may inter-leave with non-dictionary
+ *                 boundaries. The public BreakIterator API always fetches directly
+ *                 from the main BreakCache, not from here.
+ *
+ *                 In common situations, the number of boundaries in a single dictionary run
+ *                 should be quite small, it will be terminated by punctuation, spaces,
+ *                 or any other non-dictionary characters. The main BreakCache may end
+ *                 up with boundaries from multiple dictionary based runs.
+ *
+ *                 The boundaries are stored in a simple ArrayList (vector), with the
+ *                 assumption that they will be accessed sequentially.
+ */                 
+class RuleBasedBreakIterator::DictionaryCache: public UMemory {
+  public:
+     DictionaryCache(RuleBasedBreakIterator *bi, UErrorCode &status);
+     ~DictionaryCache();
+
+     void reset();
+
+     UBool following(int32_t fromPos, int32_t *pos, int32_t *statusIndex);
+     UBool preceding(int32_t fromPos, int32_t *pos, int32_t *statusIndex);
+
+    /**
+     * Populate the cache with the dictionary based boundaries within a region of text.
+     * @param startPos  The start position of a range of text
+     * @param endPos    The end position of a range of text
+     * @param firstRuleStatus The rule status index that applies to the break at startPos
+     * @param otherRuleStatus The rule status index that applies to boundaries other than startPos
+     * @internal
+     */
+    void populateDictionary(int32_t startPos, int32_t endPos,
+                         int32_t firstRuleStatus, int32_t otherRuleStatus);
+
+
+
+    RuleBasedBreakIterator *fBI;
+    
+    UVector32          *fBreaks;                // A vector containing the boundaries.
+    int32_t             fPositionInCache;       // Index in fBreaks of last boundary returned by following()
+                                                //    or preceding(). Optimizes sequential access.
+    int32_t             fStart;                 // Text position of first boundary in cache.
+    int32_t             fLimit;                 // Last boundary in cache. Which is the limit of the
+                                                //    text segment being handled by the dictionary.
+    int32_t             fFirstRuleStatusIndex;  // Rule status info for first boundary.
+    int32_t             fOtherRuleStatusIndex;  // Rule status info for 2nd through last boundaries.
+};
+
+
+/*
+ * class BreakCache
+ *
+ * Cache of break boundary positions and rule status values.
+ * Break iterator API functions, next(), previous(), etc., will use cached results
+ * when possible, and otherwise cache new results as they are obtained.
+ *
+ * Uniformly caches both dictionary and rule based (non-dictionary) boundaries.
+ *
+ * The cache is implemented as a single circular buffer.
+ */
+
+/*
+ * size of the circular cache buffer.
+ */
+
+class RuleBasedBreakIterator::BreakCache: public UMemory {
+  public:
+                BreakCache(RuleBasedBreakIterator *bi, UErrorCode &status);
+    virtual     ~BreakCache();
+    void        reset(int32_t pos = 0, int32_t ruleStatus = 0);
+    void        next() {    if (fBufIdx == fEndBufIdx) {
+                                nextOL();
+                            } else {
+                                fBufIdx = modChunkSize(fBufIdx + 1);
+                                fTextIdx = fBI->fPosition = fBoundaries[fBufIdx];
+                                fBI->fRuleStatusIndex = fStatuses[fBufIdx];
+                            }
+                };
+
+
+    void        nextOL();
+    void        previous(UErrorCode &status);
+
+    // Move the iteration state to the position following the startPosition.
+    // Input position must be pinned to the input length.
+    void        following(int32_t startPosition, UErrorCode &status);
+
+    void        preceding(int32_t startPosition, UErrorCode &status);
+
+    /*
+     * Update the state of the public BreakIterator (fBI) to reflect the
+     * current state of the break iterator cache (this).
+     */
+    int32_t     current();
+
+    /**
+     * Add boundaries to the cache near the specified position.
+     * The given position need not be a boundary itself.
+     * The input position must be within the range of the text, and
+     * on a code point boundary.
+     * If the requested position is a break boundary, leave the iteration
+     * position on it.
+     * If the requested position is not a boundary, leave the iteration
+     * position on the preceding boundary and include both the the
+     * preceding and following boundaries in the cache.
+     * Additional boundaries, either preceding or following, may be added
+     * to the cache as a side effect.
+     *
+     * Return FALSE if the operation failed.
+     */
+    UBool populateNear(int32_t position, UErrorCode &status);
+
+    /**
+     *  Add boundary(s) to the cache following the current last boundary.
+     *  Return FALSE if at the end of the text, and no more boundaries can be added.
+     *  Leave iteration position at the first newly added boundary, or unchanged if no boundary was added.
+     */
+    UBool populateFollowing();
+
+    /**
+     *  Add one or more boundaries to the cache preceding the first currently cached boundary.
+     *  Leave the iteration position on the first added boundary.
+     *  Return false if no boundaries could be added (if at the start of the text.)
+     */
+    UBool populatePreceding(UErrorCode &status);
+
+    enum UpdatePositionValues {
+        RetainCachePosition = 0,
+        UpdateCachePosition = 1
+    };
+
+    /*
+     * Add the boundary following the current position.
+     * The current position can be left as it was, or changed to the newly added boundary,
+     * as specified by the update parameter.
+     */
+    void addFollowing(int32_t position, int32_t ruleStatusIdx, UpdatePositionValues update);
+
+
+    /*
+     * Add the boundary preceding the current position.
+     * The current position can be left as it was, or changed to the newly added boundary,
+     * as specified by the update parameter.
+     */
+    bool addPreceding(int32_t position, int32_t ruleStatusIdx, UpdatePositionValues update);
+
+    /**
+     *  Set the cache position to the specified position, or, if the position
+     *  falls between to cached boundaries, to the preceding boundary.
+     *  Fails if the requested position is outside of the range of boundaries currently held by the cache.
+     *  The startPosition must be on a code point boundary.
+     *
+     *  Return TRUE if successful, FALSE if the specified position is after
+     *  the last cached boundary or before the first.
+     */
+    UBool                   seek(int32_t startPosition);
+
+    void dumpCache();
+
+  private:
+    static inline int32_t   modChunkSize(int index) { return index & (CACHE_SIZE - 1); };
+
+    static constexpr int32_t CACHE_SIZE = 128;
+    static_assert((CACHE_SIZE & (CACHE_SIZE-1)) == 0, "CACHE_SIZE must be power of two.");
+
+    RuleBasedBreakIterator *fBI;
+    int32_t                 fStartBufIdx;
+    int32_t                 fEndBufIdx;    // inclusive
+
+    int32_t                 fTextIdx;
+    int32_t                 fBufIdx;
+
+    int32_t                 fBoundaries[CACHE_SIZE];
+    uint16_t                fStatuses[CACHE_SIZE];
+
+    UVector32               fSideBuffer;
+};
+
+U_NAMESPACE_END
+
+#endif // RBBI_CACHE_H