diff --git a/BUILD.gn b/BUILD.gn
index 53236f5..73d0494 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1728,6 +1728,7 @@
   include_dirs = []
   if (pdf_enable_xfa) {
     sources += [
+      "xfa/fde/cfde_txtedtbuf_unittest.cpp",
       "xfa/fde/css/fde_cssdatatable_unittest.cpp",
       "xfa/fde/xml/fde_xml_imp_unittest.cpp",
       "xfa/fxbarcode/pdf417/BC_PDF417HighLevelEncoder_unittest.cpp",
diff --git a/xfa/fde/cfde_txtedtbuf.cpp b/xfa/fde/cfde_txtedtbuf.cpp
index 94383c2..36fc47e 100644
--- a/xfa/fde/cfde_txtedtbuf.cpp
+++ b/xfa/fde/cfde_txtedtbuf.cpp
@@ -37,6 +37,7 @@
 
 void CFDE_TxtEdtBuf::SetText(const CFX_WideString& wsText) {
   ASSERT(!wsText.IsEmpty());
+
   Clear(false);
   int32_t nTextLength = wsText.GetLength();
   int32_t nNeedCount =
@@ -89,6 +90,12 @@
 }
 
 CFX_WideString CFDE_TxtEdtBuf::GetRange(int32_t nBegin, int32_t nLength) const {
+  if (nLength == 0 || GetTextLength() == 0)
+    return CFX_WideString();
+
+  ASSERT(nBegin >= 0 && nLength > 0 && nBegin < GetTextLength() &&
+         nBegin + nLength <= GetTextLength());
+
   FDE_CHUNKPLACE cp;
   Index2CP(nBegin, cp);
   int32_t nLeave = nLength;
@@ -124,6 +131,8 @@
                             const FX_WCHAR* lpText,
                             int32_t nLength) {
   ASSERT(nPos >= 0 && nPos <= m_nTotal);
+  ASSERT(nLength > 0);
+
   FDE_CHUNKPLACE cp;
   Index2CP(nPos, cp);
   int32_t nLengthTemp = nLength;
@@ -220,52 +229,6 @@
   m_bChanged = true;
 }
 
-bool CFDE_TxtEdtBuf::Optimize(IFX_Pause* pPause) {
-  if (m_bChanged == false) {
-    return true;
-  }
-  if (m_nTotal == 0) {
-    return true;
-  }
-  int32_t nCount = m_Chunks.GetSize();
-  if (nCount == 0) {
-    return true;
-  }
-  int32_t i = 0;
-  for (; i < nCount; i++) {
-    FDE_CHUNKHEADER* lpChunk = m_Chunks[i];
-    if (lpChunk->nUsed == 0) {
-      m_pAllocator->Free(lpChunk);
-      m_Chunks.RemoveAt(i);
-      --i;
-      --nCount;
-    }
-  }
-  if (pPause && pPause->NeedToPauseNow())
-    return false;
-
-  FDE_CHUNKHEADER* lpPreChunk = m_Chunks[0];
-  FDE_CHUNKHEADER* lpCurChunk = nullptr;
-  for (i = 1; i < nCount; i++) {
-    lpCurChunk = m_Chunks[i];
-    if (lpPreChunk->nUsed + lpCurChunk->nUsed <= m_nChunkSize) {
-      FXSYS_memcpy(lpPreChunk->wChars + lpPreChunk->nUsed, lpCurChunk->wChars,
-                   lpCurChunk->nUsed * sizeof(FX_WCHAR));
-      lpPreChunk->nUsed += lpCurChunk->nUsed;
-      m_pAllocator->Free(lpCurChunk);
-      m_Chunks.RemoveAt(i);
-      --i;
-      --nCount;
-    } else {
-      lpPreChunk = lpCurChunk;
-    }
-    if (pPause && pPause->NeedToPauseNow())
-      return false;
-  }
-  m_bChanged = false;
-  return true;
-}
-
 void CFDE_TxtEdtBuf::ResetChunkBuffer(int32_t nDefChunkCount,
                                       int32_t nChunkSize) {
   ASSERT(nChunkSize);
@@ -284,6 +247,11 @@
   m_nTotal = 0;
 }
 
+void CFDE_TxtEdtBuf::SetChunkSizeForTesting(size_t size) {
+  Clear(true);
+  ResetChunkBuffer(kDefaultChunkCount, size);
+}
+
 int32_t CFDE_TxtEdtBuf::CP2Index(const FDE_CHUNKPLACE& cp) const {
   int32_t nTotal = cp.nCharIndex;
   int32_t i = 0;
diff --git a/xfa/fde/cfde_txtedtbuf.h b/xfa/fde/cfde_txtedtbuf.h
index ac25a08..63edbf6 100644
--- a/xfa/fde/cfde_txtedtbuf.h
+++ b/xfa/fde/cfde_txtedtbuf.h
@@ -25,16 +25,15 @@
   void SetText(const CFX_WideString& wsText);
   CFX_WideString GetText() const;
   FX_WCHAR GetCharByIndex(int32_t nIndex) const;
-  CFX_WideString GetRange(int32_t nBegin, int32_t nCount = -1) const;
+  CFX_WideString GetRange(int32_t nBegin, int32_t nCount) const;
 
-  void Insert(int32_t nPos, const FX_WCHAR* lpText, int32_t nLength = 1);
-  void Delete(int32_t nIndex, int32_t nLength = 1);
-  void Clear(bool bRelease = true);
-
-  bool Optimize(IFX_Pause* pPause = nullptr);
+  void Insert(int32_t nPos, const FX_WCHAR* lpText, int32_t nLength);
+  void Delete(int32_t nIndex, int32_t nLength);
+  void Clear(bool bRelease);
 
  private:
   friend class CFDE_TxtEdtBufIter;
+  friend class CFDE_TxtEdtBufTest;
 
   struct FDE_CHUNKHEADER {
     int32_t nUsed;
@@ -50,6 +49,8 @@
   int32_t CP2Index(const FDE_CHUNKPLACE& cp) const;
   void Index2CP(int32_t nIndex, FDE_CHUNKPLACE& cp) const;
 
+  void SetChunkSizeForTesting(size_t size);
+
   int32_t m_nChunkSize;
 
   int32_t m_nTotal;
diff --git a/xfa/fde/cfde_txtedtbuf_unittest.cpp b/xfa/fde/cfde_txtedtbuf_unittest.cpp
new file mode 100644
index 0000000..7228b2a
--- /dev/null
+++ b/xfa/fde/cfde_txtedtbuf_unittest.cpp
@@ -0,0 +1,199 @@
+// Copyright 2017 PDFium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "xfa/fde/cfde_txtedtbuf.h"
+
+#include "testing/gtest/include/gtest/gtest.h"
+#include "testing/test_support.h"
+#include "third_party/base/ptr_util.h"
+
+class CFDE_TxtEdtBufTest : public testing::Test {
+ public:
+  void SetUp() override {
+    buf_ = pdfium::MakeUnique<CFDE_TxtEdtBuf>();
+    buf_->SetChunkSizeForTesting(5);
+  }
+  size_t ChunkCount() const { return buf_->m_Chunks.GetSize(); }
+
+  std::unique_ptr<CFDE_TxtEdtBuf> buf_;
+};
+
+TEST_F(CFDE_TxtEdtBufTest, SetTextLessThenChunkSize) {
+  buf_->SetText(L"Hi");
+  EXPECT_EQ(1UL, ChunkCount());
+  EXPECT_EQ(2, buf_->GetTextLength());
+
+  CFX_WideString res = buf_->GetText();
+  EXPECT_EQ(2, res.GetLength());
+  EXPECT_EQ(L"Hi", res);
+}
+
+TEST_F(CFDE_TxtEdtBufTest, InsertAppendChunk) {
+  buf_->SetText(L"Hi");
+
+  CFX_WideString end = L" World";
+  buf_->Insert(2, end.c_str(), end.GetLength());
+  EXPECT_EQ(3UL, ChunkCount());
+  EXPECT_EQ(8, buf_->GetTextLength());
+
+  CFX_WideString res = buf_->GetText();
+  EXPECT_EQ(8, res.GetLength());
+  EXPECT_EQ(L"Hi World", res);
+}
+
+TEST_F(CFDE_TxtEdtBufTest, InsertPrependChunk) {
+  buf_->SetText(L"Hi");
+
+  CFX_WideString end = L"World ";
+  buf_->Insert(0, end.c_str(), end.GetLength());
+  EXPECT_EQ(3UL, ChunkCount());
+  EXPECT_EQ(8, buf_->GetTextLength());
+
+  CFX_WideString res = buf_->GetText();
+  EXPECT_EQ(L"World Hi", res);
+  EXPECT_EQ(8, res.GetLength());
+}
+
+TEST_F(CFDE_TxtEdtBufTest, InsertBetweenChunks) {
+  buf_->SetText(L"Hello World");
+  EXPECT_EQ(3UL, ChunkCount());
+
+  CFX_WideString inst = L"there ";
+  buf_->Insert(6, inst.c_str(), inst.GetLength());
+  EXPECT_EQ(5UL, ChunkCount());
+  EXPECT_EQ(17, buf_->GetTextLength());
+
+  CFX_WideString res = buf_->GetText();
+  EXPECT_EQ(L"Hello there World", res);
+  EXPECT_EQ(17, res.GetLength());
+}
+
+TEST_F(CFDE_TxtEdtBufTest, SetText) {
+  buf_->SetText(L"Hello World");
+  EXPECT_EQ(11, buf_->GetTextLength());
+
+  buf_->SetText(L"Hi");
+  // Don't remove chunks on setting shorter text.
+  EXPECT_EQ(3UL, ChunkCount());
+  EXPECT_EQ(2, buf_->GetTextLength());
+
+  CFX_WideString res = buf_->GetText();
+  EXPECT_EQ(L"Hi", res);
+  EXPECT_EQ(2, res.GetLength());
+}
+
+TEST_F(CFDE_TxtEdtBufTest, DeleteMiddleText) {
+  buf_->SetText(L"Hello there World");
+  buf_->Delete(6, 6);
+  EXPECT_EQ(4UL, ChunkCount());
+  EXPECT_EQ(11, buf_->GetTextLength());
+
+  CFX_WideString res = buf_->GetText();
+  EXPECT_EQ(L"Hello World", res);
+  EXPECT_EQ(11, res.GetLength());
+}
+
+TEST_F(CFDE_TxtEdtBufTest, DeleteEndText) {
+  buf_->SetText(L"Hello World");
+  buf_->Delete(5, 6);
+  EXPECT_EQ(1UL, ChunkCount());
+  EXPECT_EQ(5, buf_->GetTextLength());
+
+  CFX_WideString res = buf_->GetText();
+  EXPECT_EQ(L"Hello", res);
+  EXPECT_EQ(5, res.GetLength());
+}
+
+TEST_F(CFDE_TxtEdtBufTest, DeleteStartText) {
+  buf_->SetText(L"Hello World");
+  buf_->Delete(0, 6);
+  EXPECT_EQ(2UL, ChunkCount());
+  EXPECT_EQ(5, buf_->GetTextLength());
+
+  CFX_WideString res = buf_->GetText();
+  EXPECT_EQ(L"World", res);
+  EXPECT_EQ(5, res.GetLength());
+}
+
+TEST_F(CFDE_TxtEdtBufTest, DeleteAllText) {
+  buf_->SetText(L"Hello World");
+  buf_->Delete(0, 11);
+  EXPECT_EQ(0UL, ChunkCount());
+  EXPECT_EQ(0, buf_->GetTextLength());
+
+  CFX_WideString res = buf_->GetText();
+  EXPECT_EQ(L"", res);
+  EXPECT_EQ(0, res.GetLength());
+}
+
+TEST_F(CFDE_TxtEdtBufTest, ClearWithRelease) {
+  buf_->SetText(L"Hello World");
+  buf_->Clear(true);
+  EXPECT_EQ(0UL, ChunkCount());
+  EXPECT_EQ(0, buf_->GetTextLength());
+
+  CFX_WideString res = buf_->GetText();
+  EXPECT_EQ(L"", res);
+  EXPECT_EQ(0, res.GetLength());
+}
+
+TEST_F(CFDE_TxtEdtBufTest, ClearWithoutRelease) {
+  buf_->SetText(L"Hello World");
+  buf_->Clear(false);
+  EXPECT_EQ(3UL, ChunkCount());
+  EXPECT_EQ(0, buf_->GetTextLength());
+
+  CFX_WideString res = buf_->GetText();
+  EXPECT_EQ(L"", res);
+  EXPECT_EQ(0, res.GetLength());
+}
+
+TEST_F(CFDE_TxtEdtBufTest, GetCharByIndex) {
+  buf_->SetText(L"Hello world");
+  EXPECT_EQ(L"e", CFX_WideString(buf_->GetCharByIndex(1)));
+  EXPECT_EQ(L"o", CFX_WideString(buf_->GetCharByIndex(7)));
+}
+
+TEST_F(CFDE_TxtEdtBufTest, GetRange) {
+  buf_->SetText(L"Hello World");
+  EXPECT_EQ(L"", buf_->GetRange(1, 0));
+  EXPECT_EQ(L"ello", buf_->GetRange(1, 4));
+  EXPECT_EQ(L"lo Wo", buf_->GetRange(3, 5));
+}
+
+#ifndef NDEBUG
+using CFDE_TxtEdtBufTestDeathTest = CFDE_TxtEdtBufTest;
+
+TEST_F(CFDE_TxtEdtBufTestDeathTest, InsertBadIndexes) {
+  CFX_WideString inst = L"there ";
+
+  buf_->SetText(L"Hi");
+  EXPECT_DEATH(buf_->Insert(-4, inst.c_str(), inst.GetLength()), "Assertion");
+  EXPECT_DEATH(buf_->Insert(9999, inst.c_str(), inst.GetLength()), "Assertion");
+  EXPECT_DEATH(buf_->Insert(1, inst.c_str(), -6), "Assertion");
+}
+
+TEST_F(CFDE_TxtEdtBufTestDeathTest, DeleteWithBadIdx) {
+  buf_->SetText(L"Hi");
+  EXPECT_DEATH(buf_->Delete(-10, 4), "Assertion");
+  EXPECT_DEATH(buf_->Delete(1, -5), "Assertion");
+  EXPECT_DEATH(buf_->Delete(5, 1), "Assertion");
+  EXPECT_DEATH(buf_->Delete(0, 10000), "Assertion");
+}
+
+TEST_F(CFDE_TxtEdtBufTestDeathTest, GetCharByIndex) {
+  buf_->SetText(L"Hi");
+  EXPECT_DEATH(buf_->GetCharByIndex(-1), "Assertion");
+  EXPECT_DEATH(buf_->GetCharByIndex(100), "Assertion");
+}
+
+TEST_F(CFDE_TxtEdtBufTestDeathTest, GetRange) {
+  buf_->SetText(L"Hi");
+  EXPECT_DEATH(buf_->GetRange(1, -1), "Assertion");
+  EXPECT_DEATH(buf_->GetRange(-1, 1), "Assertion");
+  EXPECT_DEATH(buf_->GetRange(10, 1), "Assertion");
+  EXPECT_DEATH(buf_->GetRange(1, 100), "Assertion");
+}
+
+#endif  // NDEBUG
diff --git a/xfa/fde/cfde_txtedtengine.cpp b/xfa/fde/cfde_txtedtengine.cpp
index 342913d..555df20 100644
--- a/xfa/fde/cfde_txtedtengine.cpp
+++ b/xfa/fde/cfde_txtedtengine.cpp
@@ -675,10 +675,6 @@
   Unlock();
 }
 
-bool CFDE_TxtEdtEngine::Optimize(IFX_Pause* pPause) {
-  return m_pTxtBuf->Optimize(pPause);
-}
-
 CFDE_TxtEdtBuf* CFDE_TxtEdtEngine::GetTextBuf() const {
   return m_pTxtBuf.get();
 }
diff --git a/xfa/fde/cfde_txtedtengine.h b/xfa/fde/cfde_txtedtengine.h
index 755a820..6918d63 100644
--- a/xfa/fde/cfde_txtedtengine.h
+++ b/xfa/fde/cfde_txtedtengine.h
@@ -69,7 +69,6 @@
   int32_t DoLayout(IFX_Pause* pPause);
   void EndLayout();
 
-  bool Optimize(IFX_Pause* pPause = nullptr);
   int32_t CountParags() const;
   CFDE_TxtEdtParag* GetParag(int32_t nParagIndex) const;
   IFX_CharIter* CreateCharIter();