Step 1 to implement GC. Finding garbage PtNodes.

Bug: 6669677
Change-Id: I3551fe2f16a09d2bf7761f4e1d73ebd4a03380e7

7 files changed, 347 insertions, 67 deletions

diff --git a/native/jni/Android.mk b/native/jni/Android.mk
index ee93b4248..c2070327e 100644
--- a/native/jni/Android.mk
+++ b/native/jni/Android.mk
@@ -73,6 +73,7 @@ LATIN_IME_CORE_SRC_FILES := \
         header/header_read_write_utils.cpp \
         shortcut/shortcut_list_reading_utils.cpp \
         dictionary_structure_with_buffer_policy_factory.cpp \
+        dynamic_patricia_trie_gc_event_listeners.cpp \
         dynamic_patricia_trie_node_reader.cpp \
         dynamic_patricia_trie_policy.cpp \
         dynamic_patricia_trie_reading_helper.cpp \
diff --git a/native/jni/src/suggest/policyimpl/dictionary/dynamic_patricia_trie_gc_event_listeners.cpp b/native/jni/src/suggest/policyimpl/dictionary/dynamic_patricia_trie_gc_event_listeners.cpp
new file mode 100644
index 000000000..9b826733f
--- /dev/null
+++ b/native/jni/src/suggest/policyimpl/dictionary/dynamic_patricia_trie_gc_event_listeners.cpp
@@ -0,0 +1,48 @@
+/*
+ * Copyright (C) 2013 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "suggest/policyimpl/dictionary/dynamic_patricia_trie_gc_event_listeners.h"
+
+namespace latinime {
+
+bool DynamicPatriciaTrieGcEventListeners
+        ::ListenerForUpdatingUnigramProbabilityAndMarkingUselessPtNodesAsDeleted
+                ::onVisitingPtNode(const DynamicPatriciaTrieNodeReader *const node) {
+    // PtNode is useless when the PtNode is not a terminal and doesn't have any not useless
+    // children.
+    bool isUselessPtNode = !node->isTerminal();
+    if (mChildrenValue > 0) {
+        isUselessPtNode = false;
+    } else if (node->isTerminal()) {
+        // Remove children as all children are useless.
+        int writingPos = node->getChildrenPosFieldPos();
+        if (!DynamicPatriciaTrieWritingUtils::writeChildrenPositionAndAdvancePosition(
+                mBuffer, NOT_A_DICT_POS /* childrenPosition */, &writingPos)) {
+            return false;
+        }
+    }
+    if (isUselessPtNode) {
+        // Current PtNode is no longer needed. Mark it as deleted.
+        if (!mWritingHelper->markNodeAsDeleted(node)) {
+            return false;
+        }
+    } else {
+        valueStack.back() += 1;
+    }
+    return true;
+}
+
+} // namespace latinime
diff --git a/native/jni/src/suggest/policyimpl/dictionary/dynamic_patricia_trie_gc_event_listeners.h b/native/jni/src/suggest/policyimpl/dictionary/dynamic_patricia_trie_gc_event_listeners.h
new file mode 100644
index 000000000..8569a29cc
--- /dev/null
+++ b/native/jni/src/suggest/policyimpl/dictionary/dynamic_patricia_trie_gc_event_listeners.h
@@ -0,0 +1,75 @@
+/*
+ * Copyright (C) 2013, The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef LATINIME_DYNAMIC_PATRICIA_TRIE_GC_EVENT_LISTENERS_H
+#define LATINIME_DYNAMIC_PATRICIA_TRIE_GC_EVENT_LISTENERS_H
+
+#include <vector>
+
+#include "defines.h"
+#include "suggest/policyimpl/dictionary/dynamic_patricia_trie_reading_helper.h"
+#include "suggest/policyimpl/dictionary/dynamic_patricia_trie_writing_helper.h"
+#include "suggest/policyimpl/dictionary/dynamic_patricia_trie_writing_utils.h"
+
+namespace latinime {
+
+class DynamicPatriciaTrieGcEventListeners {
+ public:
+    // Updates all PtNodes that can be reached from the root. Checks if each PtNode is useless or
+    // not and marks useless PtNodes as deleted. Such deleted PtNodes will be discarded in the GC.
+    // TODO: Concatenate non-terminal PtNodes.
+    class ListenerForUpdatingUnigramProbabilityAndMarkingUselessPtNodesAsDeleted
+        : public DynamicPatriciaTrieReadingHelper::TraversingEventListener {
+     public:
+        ListenerForUpdatingUnigramProbabilityAndMarkingUselessPtNodesAsDeleted(
+                DynamicPatriciaTrieWritingHelper *const writingHelper,
+                BufferWithExtendableBuffer *const buffer)
+                : mWritingHelper(writingHelper), mBuffer(buffer), valueStack(),
+                  mChildrenValue(0) {}
+
+        ~ListenerForUpdatingUnigramProbabilityAndMarkingUselessPtNodesAsDeleted() {};
+
+        bool onAscend() {
+            if (valueStack.empty()) {
+                return false;
+            }
+            mChildrenValue = valueStack.back();
+            valueStack.pop_back();
+            return true;
+        }
+
+        bool onDescend() {
+            valueStack.push_back(0);
+            return true;
+        }
+
+        bool onVisitingPtNode(const DynamicPatriciaTrieNodeReader *const node);
+
+     private:
+        DISALLOW_IMPLICIT_CONSTRUCTORS(
+                ListenerForUpdatingUnigramProbabilityAndMarkingUselessPtNodesAsDeleted);
+
+        DynamicPatriciaTrieWritingHelper *const mWritingHelper;
+        BufferWithExtendableBuffer *const mBuffer;
+        std::vector<int> valueStack;
+        int mChildrenValue;
+    };
+
+ private:
+    DISALLOW_IMPLICIT_CONSTRUCTORS(DynamicPatriciaTrieGcEventListeners);
+};
+} // namespace latinime
+#endif /* LATINIME_DYNAMIC_PATRICIA_TRIE_GC_EVENT_LISTENERS_H */
diff --git a/native/jni/src/suggest/policyimpl/dictionary/dynamic_patricia_trie_reading_helper.cpp b/native/jni/src/suggest/policyimpl/dictionary/dynamic_patricia_trie_reading_helper.cpp
index a0b5be6a4..59e39a551 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/dynamic_patricia_trie_reading_helper.cpp
+++ b/native/jni/src/suggest/policyimpl/dictionary/dynamic_patricia_trie_reading_helper.cpp
@@ -23,36 +23,85 @@ namespace latinime {
 // To avoid infinite loop caused by invalid or malicious forward links.
 const int DynamicPatriciaTrieReadingHelper::MAX_CHILD_COUNT_TO_AVOID_INFINITE_LOOP = 100000;
 const int DynamicPatriciaTrieReadingHelper::MAX_NODE_ARRAY_COUNT_TO_AVOID_INFINITE_LOOP = 100000;
+const size_t DynamicPatriciaTrieReadingHelper::MAX_READING_STATE_STACK_SIZE = MAX_WORD_LENGTH;
+
+bool DynamicPatriciaTrieReadingHelper::traverseAllPtNodesInPostorderDepthFirstManner(
+        TraversingEventListener *const listener) {
+    bool alreadyVisitedChildren = false;
+    // Descend from the root to the root PtNode array.
+    if (!listener->onDescend()) {
+        return false;
+    }
+    while (!isEnd()) {
+        if (!alreadyVisitedChildren) {
+            if (mNodeReader.hasChildren()) {
+                // Move to the first child.
+                if (!listener->onDescend()) {
+                    return false;
+                }
+                pushReadingStateToStack();
+                readChildNode();
+            } else {
+                alreadyVisitedChildren = true;
+            }
+        } else {
+            if (!listener->onVisitingPtNode(&mNodeReader)) {
+                return false;
+            }
+            readNextSiblingNode();
+            if (isEnd()) {
+                // All PtNodes in current linked PtNode arrays have been visited.
+                // Return to the parent.
+                if (!listener->onAscend()) {
+                    return false;
+                }
+                popReadingStateFromStack();
+                alreadyVisitedChildren = true;
+            } else {
+                // Process sibling PtNode.
+                alreadyVisitedChildren = false;
+            }
+        }
+    }
+    // Ascend from the root PtNode array to the root.
+    if (!listener->onAscend()) {
+        return false;
+    }
+    return !isError();
+}
 
 // Read node array size and process empty node arrays. Nodes and arrays are counted up in this
 // method to avoid an infinite loop.
 void DynamicPatriciaTrieReadingHelper::nextNodeArray() {
-    const bool usesAdditionalBuffer = mBuffer->isInAdditionalBuffer(mPos);
+    mReadingState.mPosOfLastPtNodeArrayHead = mReadingState.mPos;
+    const bool usesAdditionalBuffer = mBuffer->isInAdditionalBuffer(mReadingState.mPos);
     const uint8_t *const dictBuf = mBuffer->getBuffer(usesAdditionalBuffer);
     if (usesAdditionalBuffer) {
-        mPos -= mBuffer->getOriginalBufferSize();
+        mReadingState.mPos -= mBuffer->getOriginalBufferSize();
     }
-    mNodeCount = PatriciaTrieReadingUtils::getPtNodeArraySizeAndAdvancePosition(dictBuf,
-            &mPos);
+    mReadingState.mNodeCount = PatriciaTrieReadingUtils::getPtNodeArraySizeAndAdvancePosition(
+            dictBuf, &mReadingState.mPos);
     if (usesAdditionalBuffer) {
-        mPos += mBuffer->getOriginalBufferSize();
+        mReadingState.mPos += mBuffer->getOriginalBufferSize();
     }
     // Count up nodes and node arrays to avoid infinite loop.
-    mTotalNodeCount += mNodeCount;
-    mNodeArrayCount++;
-    if (mNodeCount < 0 || mTotalNodeCount > MAX_CHILD_COUNT_TO_AVOID_INFINITE_LOOP
-            || mNodeArrayCount > MAX_NODE_ARRAY_COUNT_TO_AVOID_INFINITE_LOOP) {
+    mReadingState.mTotalNodeCount += mReadingState.mNodeCount;
+    mReadingState.mNodeArrayCount++;
+    if (mReadingState.mNodeCount < 0
+            || mReadingState.mTotalNodeCount > MAX_CHILD_COUNT_TO_AVOID_INFINITE_LOOP
+            || mReadingState.mNodeArrayCount > MAX_NODE_ARRAY_COUNT_TO_AVOID_INFINITE_LOOP) {
         // Invalid dictionary.
         AKLOGI("Invalid dictionary. nodeCount: %d, totalNodeCount: %d, MAX_CHILD_COUNT: %d"
                 "nodeArrayCount: %d, MAX_NODE_ARRAY_COUNT: %d",
-                mNodeCount, mTotalNodeCount, MAX_CHILD_COUNT_TO_AVOID_INFINITE_LOOP,
-                mNodeArrayCount, MAX_NODE_ARRAY_COUNT_TO_AVOID_INFINITE_LOOP);
+                mReadingState.mNodeCount, mReadingState.mTotalNodeCount,
+                MAX_CHILD_COUNT_TO_AVOID_INFINITE_LOOP, mReadingState.mNodeArrayCount,
+                MAX_NODE_ARRAY_COUNT_TO_AVOID_INFINITE_LOOP);
         ASSERT(false);
         mIsError = true;
-        mPos = NOT_A_DICT_POS;
+        mReadingState.mPos = NOT_A_DICT_POS;
         return;
     }
-    if (mNodeCount == 0) {
+    if (mReadingState.mNodeCount == 0) {
         // Empty node array. Try following forward link.
         followForwardLink();
     }
@@ -60,24 +109,24 @@ void DynamicPatriciaTrieReadingHelper::nextNodeArray() {
 
 // Follow the forward link and read the next node array if exists.
 void DynamicPatriciaTrieReadingHelper::followForwardLink() {
-    const bool usesAdditionalBuffer = mBuffer->isInAdditionalBuffer(mPos);
+    const bool usesAdditionalBuffer = mBuffer->isInAdditionalBuffer(mReadingState.mPos);
     const uint8_t *const dictBuf = mBuffer->getBuffer(usesAdditionalBuffer);
     if (usesAdditionalBuffer) {
-        mPos -= mBuffer->getOriginalBufferSize();
+        mReadingState.mPos -= mBuffer->getOriginalBufferSize();
     }
     const int forwardLinkPosition =
-            DynamicPatriciaTrieReadingUtils::getForwardLinkPosition(dictBuf, mPos);
+            DynamicPatriciaTrieReadingUtils::getForwardLinkPosition(dictBuf, mReadingState.mPos);
     if (usesAdditionalBuffer) {
-        mPos += mBuffer->getOriginalBufferSize();
+        mReadingState.mPos += mBuffer->getOriginalBufferSize();
     }
-    mPosOfLastForwardLinkField = mPos;
+    mReadingState.mPosOfLastForwardLinkField = mReadingState.mPos;
     if (DynamicPatriciaTrieReadingUtils::isValidForwardLinkPosition(forwardLinkPosition)) {
         // Follow the forward link.
-        mPos += forwardLinkPosition;
+        mReadingState.mPos += forwardLinkPosition;
         nextNodeArray();
     } else {
         // All node arrays have been read.
-        mPos = NOT_A_DICT_POS;
+        mReadingState.mPos = NOT_A_DICT_POS;
     }
 }
 
diff --git a/native/jni/src/suggest/policyimpl/dictionary/dynamic_patricia_trie_reading_helper.h b/native/jni/src/suggest/policyimpl/dictionary/dynamic_patricia_trie_reading_helper.h
index 120fd7699..08ab07267 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/dynamic_patricia_trie_reading_helper.h
+++ b/native/jni/src/suggest/policyimpl/dictionary/dynamic_patricia_trie_reading_helper.h
@@ -17,6 +17,9 @@
 #ifndef LATINIME_DYNAMIC_PATRICIA_TRIE_READING_HELPER_H
 #define LATINIME_DYNAMIC_PATRICIA_TRIE_READING_HELPER_H
 
+#include <cstddef>
+#include <vector>
+
 #include "defines.h"
 #include "suggest/policyimpl/dictionary/dynamic_patricia_trie_node_reader.h"
 #include "suggest/policyimpl/dictionary/dynamic_patricia_trie_reading_utils.h"
@@ -34,12 +37,31 @@ class DictionaryShortcutsStructurePolicy;
  */
 class DynamicPatriciaTrieReadingHelper {
  public:
+    class TraversingEventListener {
+     public:
+        virtual ~TraversingEventListener() {};
+
+        // Returns whether the event handling was succeeded or not.
+        virtual bool onAscend() = 0;
+
+        // Returns whether the event handling was succeeded or not.
+        virtual bool onDescend() = 0;
+
+        // Returns whether the event handling was succeeded or not.
+        virtual bool onVisitingPtNode(const DynamicPatriciaTrieNodeReader *const node) = 0;
+
+     protected:
+        TraversingEventListener() {};
+
+     private:
+        DISALLOW_COPY_AND_ASSIGN(TraversingEventListener);
+    };
+
     DynamicPatriciaTrieReadingHelper(const BufferWithExtendableBuffer *const buffer,
             const DictionaryBigramsStructurePolicy *const bigramsPolicy,
             const DictionaryShortcutsStructurePolicy *const shortcutsPolicy)
-            : mIsError(false), mPos(NOT_A_DICT_POS), mNodeCount(0), mPrevTotalCodePointCount(0),
-              mTotalNodeCount(0), mNodeArrayCount(0), mPosOfLastForwardLinkField(NOT_A_DICT_POS),
-              mBuffer(buffer), mNodeReader(mBuffer, bigramsPolicy, shortcutsPolicy) {}
+            : mIsError(false), mReadingState(), mBuffer(buffer),
+              mNodeReader(mBuffer, bigramsPolicy, shortcutsPolicy), mReadingStateStack() {}
 
     ~DynamicPatriciaTrieReadingHelper() {}
 
@@ -48,21 +70,21 @@ class DynamicPatriciaTrieReadingHelper {
     }
 
     AK_FORCE_INLINE bool isEnd() const {
-        return mPos == NOT_A_DICT_POS;
+        return mReadingState.mPos == NOT_A_DICT_POS;
     }
 
     // Initialize reading state with the head position of a node array.
     AK_FORCE_INLINE void initWithNodeArrayPos(const int nodeArrayPos) {
         if (nodeArrayPos == NOT_A_DICT_POS) {
-            mPos = NOT_A_DICT_POS;
+            mReadingState.mPos = NOT_A_DICT_POS;
         } else {
             mIsError = false;
-            mPos = nodeArrayPos;
-            mNodeCount = 0;
-            mPrevTotalCodePointCount = 0;
-            mTotalNodeCount = 0;
-            mNodeArrayCount = 0;
-            mPosOfLastForwardLinkField = NOT_A_DICT_POS;
+            mReadingState.mPos = nodeArrayPos;
+            mReadingState.mPrevTotalCodePointCount = 0;
+            mReadingState.mTotalNodeCount = 0;
+            mReadingState.mNodeArrayCount = 0;
+            mReadingState.mPosOfLastForwardLinkField = NOT_A_DICT_POS;
+            mReadingStateStack.clear();
             nextNodeArray();
             if (!isEnd()) {
                 fetchNodeInfo();
@@ -73,15 +95,17 @@ class DynamicPatriciaTrieReadingHelper {
     // Initialize reading state with the head position of a node.
     AK_FORCE_INLINE void initWithNodePos(const int nodePos) {
         if (nodePos == NOT_A_DICT_POS) {
-            mPos = NOT_A_DICT_POS;
+            mReadingState.mPos = NOT_A_DICT_POS;
         } else {
             mIsError = false;
-            mPos = nodePos;
-            mNodeCount = 1;
-            mPrevTotalCodePointCount = 0;
-            mTotalNodeCount = 1;
-            mNodeArrayCount = 1;
-            mPosOfLastForwardLinkField = NOT_A_DICT_POS;
+            mReadingState.mPos = nodePos;
+            mReadingState.mNodeCount = 1;
+            mReadingState.mPrevTotalCodePointCount = 0;
+            mReadingState.mTotalNodeCount = 1;
+            mReadingState.mNodeArrayCount = 1;
+            mReadingState.mPosOfLastForwardLinkField = NOT_A_DICT_POS;
+            mReadingState.mPosOfLastPtNodeArrayHead = NOT_A_DICT_POS;
+            mReadingStateStack.clear();
             fetchNodeInfo();
         }
     }
@@ -100,12 +124,12 @@ class DynamicPatriciaTrieReadingHelper {
 
     // Return code point count exclude the last read node's code points.
     AK_FORCE_INLINE int getPrevTotalCodePointCount() const {
-        return mPrevTotalCodePointCount;
+        return mReadingState.mPrevTotalCodePointCount;
     }
 
     // Return code point count include the last read node's code points.
     AK_FORCE_INLINE int getTotalCodePointCount() const {
-        return mPrevTotalCodePointCount + mNodeReader.getCodePointCount();
+        return mReadingState.mPrevTotalCodePointCount + mNodeReader.getCodePointCount();
     }
 
     AK_FORCE_INLINE void fetchMergedNodeCodePointsInReverseOrder(
@@ -121,9 +145,9 @@ class DynamicPatriciaTrieReadingHelper {
     }
 
     AK_FORCE_INLINE void readNextSiblingNode() {
-        mNodeCount -= 1;
-        mPos = mNodeReader.getSiblingNodePos();
-        if (mNodeCount <= 0) {
+        mReadingState.mNodeCount -= 1;
+        mReadingState.mPos = mNodeReader.getSiblingNodePos();
+        if (mReadingState.mNodeCount <= 0) {
             // All nodes in the current node array have been read.
             followForwardLink();
             if (!isEnd()) {
@@ -137,69 +161,117 @@ class DynamicPatriciaTrieReadingHelper {
     // Read the first child node of the current node.
     AK_FORCE_INLINE void readChildNode() {
         if (mNodeReader.hasChildren()) {
-            mPrevTotalCodePointCount += mNodeReader.getCodePointCount();
-            mTotalNodeCount = 0;
-            mNodeArrayCount = 0;
-            mPos = mNodeReader.getChildrenPos();
-            mPosOfLastForwardLinkField = NOT_A_DICT_POS;
+            mReadingState.mPrevTotalCodePointCount += mNodeReader.getCodePointCount();
+            mReadingState.mTotalNodeCount = 0;
+            mReadingState.mNodeArrayCount = 0;
+            mReadingState.mPos = mNodeReader.getChildrenPos();
+            mReadingState.mPosOfLastForwardLinkField = NOT_A_DICT_POS;
             // Read children node array.
             nextNodeArray();
             if (!isEnd()) {
                 fetchNodeInfo();
             }
         } else {
-            mPos = NOT_A_DICT_POS;
+            mReadingState.mPos = NOT_A_DICT_POS;
         }
     }
 
     // Read the parent node of the current node.
     AK_FORCE_INLINE void readParentNode() {
         if (mNodeReader.getParentPos() != NOT_A_DICT_POS) {
-            mPrevTotalCodePointCount += mNodeReader.getCodePointCount();
-            mTotalNodeCount = 1;
-            mNodeArrayCount = 1;
-            mNodeCount = 1;
-            mPos = mNodeReader.getParentPos();
-            mPosOfLastForwardLinkField = NOT_A_DICT_POS;
+            mReadingState.mPrevTotalCodePointCount += mNodeReader.getCodePointCount();
+            mReadingState.mTotalNodeCount = 1;
+            mReadingState.mNodeArrayCount = 1;
+            mReadingState.mNodeCount = 1;
+            mReadingState.mPos = mNodeReader.getParentPos();
+            mReadingState.mPosOfLastForwardLinkField = NOT_A_DICT_POS;
+            mReadingState.mPosOfLastPtNodeArrayHead = NOT_A_DICT_POS;
             fetchNodeInfo();
         } else {
-            mPos = NOT_A_DICT_POS;
+            mReadingState.mPos = NOT_A_DICT_POS;
         }
     }
 
     AK_FORCE_INLINE int getPosOfLastForwardLinkField() const {
-        return mPosOfLastForwardLinkField;
+        return mReadingState.mPosOfLastForwardLinkField;
+    }
+
+    AK_FORCE_INLINE int getPosOfLastPtNodeArrayHead() const {
+        return mReadingState.mPosOfLastPtNodeArrayHead;
+    }
+
+    AK_FORCE_INLINE void reloadCurrentPtNodeInfo() {
+        if (!isEnd()) {
+            fetchNodeInfo();
+        }
     }
 
+    bool traverseAllPtNodesInPostorderDepthFirstManner(TraversingEventListener *const listener);
+
  private:
     DISALLOW_COPY_AND_ASSIGN(DynamicPatriciaTrieReadingHelper);
 
+    class ReadingState {
+     public:
+        // Note that copy constructor and assignment operator are used for this class to use
+        // std::vector.
+        ReadingState() : mPos(NOT_A_DICT_POS), mNodeCount(0), mPrevTotalCodePointCount(0),
+                mTotalNodeCount(0), mNodeArrayCount(0), mPosOfLastForwardLinkField(NOT_A_DICT_POS),
+                mPosOfLastPtNodeArrayHead(NOT_A_DICT_POS) {}
+
+        int mPos;
+        // Node count of a node array.
+        int mNodeCount;
+        int mPrevTotalCodePointCount;
+        int mTotalNodeCount;
+        int mNodeArrayCount;
+        int mPosOfLastForwardLinkField;
+        int mPosOfLastPtNodeArrayHead;
+    };
+
     static const int MAX_CHILD_COUNT_TO_AVOID_INFINITE_LOOP;
     static const int MAX_NODE_ARRAY_COUNT_TO_AVOID_INFINITE_LOOP;
+    static const size_t MAX_READING_STATE_STACK_SIZE;
 
     bool mIsError;
-    int mPos;
-    // Node count of a node array.
-    int mNodeCount;
-    int mPrevTotalCodePointCount;
-    int mTotalNodeCount;
-    int mNodeArrayCount;
-    int mPosOfLastForwardLinkField;
+    ReadingState mReadingState;
     const BufferWithExtendableBuffer *const mBuffer;
     DynamicPatriciaTrieNodeReader mNodeReader;
     int mMergedNodeCodePoints[MAX_WORD_LENGTH];
+    std::vector<ReadingState> mReadingStateStack;
 
     void nextNodeArray();
 
     void followForwardLink();
 
     AK_FORCE_INLINE void fetchNodeInfo() {
-        mNodeReader.fetchNodeInfoFromBufferAndGetNodeCodePoints(mPos, MAX_WORD_LENGTH,
-                mMergedNodeCodePoints);
+        mNodeReader.fetchNodeInfoFromBufferAndGetNodeCodePoints(mReadingState.mPos,
+                MAX_WORD_LENGTH, mMergedNodeCodePoints);
         if (mNodeReader.getCodePointCount() <= 0) {
             // Empty node is not allowed.
             mIsError = true;
-            mPos = NOT_A_DICT_POS;
+            mReadingState.mPos = NOT_A_DICT_POS;
+        }
+    }
+
+    AK_FORCE_INLINE void pushReadingStateToStack() {
+        if (mReadingStateStack.size() > MAX_READING_STATE_STACK_SIZE) {
+            AKLOGI("Reading state stack overflow. Max size: %d", MAX_READING_STATE_STACK_SIZE);
+            ASSERT(false);
+            mIsError = true;
+            mReadingState.mPos = NOT_A_DICT_POS;
+        } else {
+            mReadingStateStack.push_back(mReadingState);
+        }
+    }
+
+    AK_FORCE_INLINE void popReadingStateFromStack() {
+        if (mReadingStateStack.empty()) {
+            mReadingState.mPos = NOT_A_DICT_POS;
+        } else {
+            mReadingState = mReadingStateStack.back();
+            mReadingStateStack.pop_back();
+            fetchNodeInfo();
         }
     }
 };
diff --git a/native/jni/src/suggest/policyimpl/dictionary/dynamic_patricia_trie_writing_helper.cpp b/native/jni/src/suggest/policyimpl/dictionary/dynamic_patricia_trie_writing_helper.cpp
index f4c98d848..45575471f 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/dynamic_patricia_trie_writing_helper.cpp
+++ b/native/jni/src/suggest/policyimpl/dictionary/dynamic_patricia_trie_writing_helper.cpp
@@ -20,6 +20,7 @@
 #include <cstring>
 
 #include "suggest/policyimpl/dictionary/bigram/dynamic_bigram_list_policy.h"
+#include "suggest/policyimpl/dictionary/dynamic_patricia_trie_gc_event_listeners.h"
 #include "suggest/policyimpl/dictionary/dynamic_patricia_trie_node_reader.h"
 #include "suggest/policyimpl/dictionary/dynamic_patricia_trie_reading_helper.h"
 #include "suggest/policyimpl/dictionary/dynamic_patricia_trie_reading_utils.h"
@@ -159,6 +160,26 @@ void DynamicPatriciaTrieWritingHelper::writeToDictFileWithGC(const int rootPtNod
     flushAllToFile(fileName, &headerBuffer, &newDictBuffer);
 }
 
+bool DynamicPatriciaTrieWritingHelper::markNodeAsDeleted(
+        const DynamicPatriciaTrieNodeReader *const nodeToUpdate) {
+    int pos = nodeToUpdate->getHeadPos();
+    const bool usesAdditionalBuffer = mBuffer->isInAdditionalBuffer(pos);
+    const uint8_t *const dictBuf = mBuffer->getBuffer(usesAdditionalBuffer);
+    if (usesAdditionalBuffer) {
+        pos -= mBuffer->getOriginalBufferSize();
+    }
+    // Read original flags
+    const PatriciaTrieReadingUtils::NodeFlags originalFlags =
+            PatriciaTrieReadingUtils::getFlagsAndAdvancePosition(dictBuf, &pos);
+    const PatriciaTrieReadingUtils::NodeFlags updatedFlags =
+            DynamicPatriciaTrieReadingUtils::updateAndGetFlags(originalFlags, false /* isMoved */,
+                    true /* isDeleted */);
+    int writingPos = nodeToUpdate->getHeadPos();
+    // Update flags.
+    return DynamicPatriciaTrieWritingUtils::writeFlagsAndAdvancePosition(mBuffer, updatedFlags,
+            &writingPos);
+}
+
 bool DynamicPatriciaTrieWritingHelper::markNodeAsMovedAndSetPosition(
         const DynamicPatriciaTrieNodeReader *const originalNode, const int movedPos,
         const int bigramLinkedNodePos) {
@@ -497,6 +518,16 @@ bool DynamicPatriciaTrieWritingHelper::writeBufferToFilePointer(FILE *const file
 
 bool DynamicPatriciaTrieWritingHelper::runGC(const int rootPtNodeArrayPos,
         BufferWithExtendableBuffer *const bufferToWrite) {
+    DynamicPatriciaTrieReadingHelper readingHelper(mBuffer, mBigramPolicy, mShortcutPolicy);
+    readingHelper.initWithNodeArrayPos(rootPtNodeArrayPos);
+    DynamicPatriciaTrieGcEventListeners
+            ::ListenerForUpdatingUnigramProbabilityAndMarkingUselessPtNodesAsDeleted
+                    listenerForUpdatingUnigramProbabilityAndMarkingUselessPtNodesAsDeleted(
+                            this, mBuffer);
+    if (!readingHelper.traverseAllPtNodesInPostorderDepthFirstManner(
+            &listenerForUpdatingUnigramProbabilityAndMarkingUselessPtNodesAsDeleted)) {
+        return false;
+    }
     // TODO: Implement.
     return false;
 }
diff --git a/native/jni/src/suggest/policyimpl/dictionary/dynamic_patricia_trie_writing_helper.h b/native/jni/src/suggest/policyimpl/dictionary/dynamic_patricia_trie_writing_helper.h
index 8f78d84d0..d164e4331 100644
--- a/native/jni/src/suggest/policyimpl/dictionary/dynamic_patricia_trie_writing_helper.h
+++ b/native/jni/src/suggest/policyimpl/dictionary/dynamic_patricia_trie_writing_helper.h
@@ -55,6 +55,10 @@ class DynamicPatriciaTrieWritingHelper {
     void writeToDictFileWithGC(const int rootPtNodeArrayPos, const char *const fileName,
             const HeaderPolicy *const headerPolicy);
 
+    // CAVEAT: This method must be called only from inner classes of
+    // DynamicPatriciaTrieGcEventListeners.
+    bool markNodeAsDeleted(const DynamicPatriciaTrieNodeReader *const nodeToUpdate);
+
  private:
     DISALLOW_IMPLICIT_CONSTRUCTORS(DynamicPatriciaTrieWritingHelper);