4 files changed, 315 insertions, 409 deletions

diff --git a/xfa/fgas/crt/fgas_codepage.cpp b/xfa/fgas/crt/fgas_codepage.cpp
index cae2dd7234..c1a4322883 100644
--- a/xfa/fgas/crt/fgas_codepage.cpp
+++ b/xfa/fgas/crt/fgas_codepage.cpp
@@ -68,127 +68,3 @@ uint16_t FX_GetCodePageFromCharset(uint8_t charset) {
   } while (iStart <= iEnd);
   return 0xFFFF;
 }
-
-void FX_SwapByteOrder(wchar_t* pStr, int32_t iLength) {
-  ASSERT(pStr);
-
-  if (iLength < 0)
-    iLength = FXSYS_wcslen(pStr);
-
-  uint16_t wch;
-  if (sizeof(wchar_t) > 2) {
-    while (iLength-- > 0) {
-      wch = (uint16_t)*pStr;
-      wch = (wch >> 8) | (wch << 8);
-      wch &= 0x00FF;
-      *pStr++ = wch;
-    }
-    return;
-  }
-
-  while (iLength-- > 0) {
-    wch = (uint16_t)*pStr;
-    wch = (wch >> 8) | (wch << 8);
-    *pStr++ = wch;
-  }
-}
-
-void FX_UTF16ToWChar(void* pBuffer, int32_t iLength) {
-  ASSERT(pBuffer && iLength > 0);
-  if (sizeof(wchar_t) == 2)
-    return;
-
-  uint16_t* pSrc = static_cast<uint16_t*>(pBuffer);
-  wchar_t* pDst = static_cast<wchar_t*>(pBuffer);
-  while (--iLength >= 0)
-    pDst[iLength] = static_cast<wchar_t>(pSrc[iLength]);
-}
-
-int32_t FX_DecodeString(uint16_t wCodePage,
-                        const char* pSrc,
-                        int32_t* pSrcLen,
-                        wchar_t* pDst,
-                        int32_t* pDstLen,
-                        bool bErrBreak) {
-  if (wCodePage == FX_CODEPAGE_UTF8)
-    return FX_UTF8Decode(pSrc, pSrcLen, pDst, pDstLen);
-  return -1;
-}
-
-int32_t FX_UTF8Decode(const char* pSrc,
-                      int32_t* pSrcLen,
-                      wchar_t* pDst,
-                      int32_t* pDstLen) {
-  if (!pSrcLen || !pDstLen)
-    return -1;
-
-  int32_t iSrcLen = *pSrcLen;
-  if (iSrcLen < 1) {
-    *pSrcLen = *pDstLen = 0;
-    return 1;
-  }
-
-  int32_t iDstLen = *pDstLen;
-  bool bValidDst = (pDst && iDstLen > 0);
-  uint32_t dwCode = 0;
-  int32_t iPending = 0;
-  int32_t iSrcNum = 0;
-  int32_t iDstNum = 0;
-  int32_t iIndex = 0;
-  int32_t k = 1;
-  while (iIndex < iSrcLen) {
-    uint8_t byte = static_cast<uint8_t>(*(pSrc + iIndex));
-    if (byte < 0x80) {
-      iPending = 0;
-      k = 1;
-      iDstNum++;
-      iSrcNum += k;
-      if (bValidDst) {
-        *pDst++ = byte;
-        if (iDstNum >= iDstLen)
-          break;
-      }
-    } else if (byte < 0xc0) {
-      if (iPending < 1)
-        break;
-
-      iPending--;
-      dwCode |= (byte & 0x3f) << (iPending * 6);
-      if (iPending == 0) {
-        iDstNum++;
-        iSrcNum += k;
-        if (bValidDst) {
-          *pDst++ = dwCode;
-          if (iDstNum >= iDstLen)
-            break;
-        }
-      }
-    } else if (byte < 0xe0) {
-      iPending = 1;
-      k = 2;
-      dwCode = (byte & 0x1f) << 6;
-    } else if (byte < 0xf0) {
-      iPending = 2;
-      k = 3;
-      dwCode = (byte & 0x0f) << 12;
-    } else if (byte < 0xf8) {
-      iPending = 3;
-      k = 4;
-      dwCode = (byte & 0x07) << 18;
-    } else if (byte < 0xfc) {
-      iPending = 4;
-      k = 5;
-      dwCode = (byte & 0x03) << 24;
-    } else if (byte < 0xfe) {
-      iPending = 5;
-      k = 6;
-      dwCode = (byte & 0x01) << 30;
-    } else {
-      break;
-    }
-    iIndex++;
-  }
-  *pSrcLen = iSrcNum;
-  *pDstLen = iDstNum;
-  return 1;
-}
diff --git a/xfa/fgas/crt/fgas_codepage.h b/xfa/fgas/crt/fgas_codepage.h
index 3dfabe600e..8e29736149 100644
--- a/xfa/fgas/crt/fgas_codepage.h
+++ b/xfa/fgas/crt/fgas_codepage.h
@@ -134,18 +134,5 @@
 #define FX_CHARSET_OEM 255
 
 uint16_t FX_GetCodePageFromCharset(uint8_t charset);
-void FX_SwapByteOrder(wchar_t* pStr, int32_t iLength);
-
-void FX_UTF16ToWChar(void* pBuffer, int32_t iLength);
-int32_t FX_DecodeString(uint16_t wCodePage,
-                        const char* pSrc,
-                        int32_t* pSrcLen,
-                        wchar_t* pDst,
-                        int32_t* pDstLen,
-                        bool bErrBreak);
-int32_t FX_UTF8Decode(const char* pSrc,
-                      int32_t* pSrcLen,
-                      wchar_t* pDst,
-                      int32_t* pDstLen);
 
 #endif  // XFA_FGAS_CRT_FGAS_CODEPAGE_H_
diff --git a/xfa/fgas/crt/ifgas_stream.cpp b/xfa/fgas/crt/ifgas_stream.cpp
index 712243d701..b4d03cf810 100644
--- a/xfa/fgas/crt/ifgas_stream.cpp
+++ b/xfa/fgas/crt/ifgas_stream.cpp
@@ -27,16 +27,16 @@ class IFGAS_StreamImp {
  public:
   virtual ~IFGAS_StreamImp() {}
 
-  virtual int32_t GetLength() const = 0;
-  virtual int32_t Seek(FX_STREAMSEEK eSeek, int32_t iOffset) = 0;
-  virtual int32_t GetPosition() = 0;
+  virtual FX_FILESIZE GetLength() const = 0;
+  virtual FX_FILESIZE GetPosition() = 0;
+  virtual void Seek(FX_STREAMSEEK eSeek, FX_FILESIZE iOffset) = 0;
   virtual bool IsEOF() const = 0;
-  virtual int32_t ReadData(uint8_t* pBuffer, int32_t iBufferSize) = 0;
-  virtual int32_t ReadString(wchar_t* pStr, int32_t iMaxLength, bool& bEOS) = 0;
-  virtual int32_t WriteData(const uint8_t* pBuffer, int32_t iBufferSize) = 0;
-  virtual int32_t WriteString(const wchar_t* pStr, int32_t iLength) = 0;
-  virtual void Flush() = 0;
-  virtual bool SetLength(int32_t iLength) = 0;
+  virtual FX_STRSIZE ReadData(uint8_t* pBuffer, FX_STRSIZE iBufferSize) = 0;
+  virtual FX_STRSIZE ReadString(wchar_t* pStr,
+                                FX_STRSIZE iMaxLength,
+                                bool* bEOS) = 0;
+  virtual void WriteData(const uint8_t* pBuffer, FX_STRSIZE iBufferSize) = 0;
+  virtual void WriteString(const wchar_t* pStr, FX_STRSIZE iLength) = 0;
 
  protected:
   IFGAS_StreamImp();
@@ -44,57 +44,52 @@ class IFGAS_StreamImp {
 
 class CFGAS_FileReadStreamImp : public IFGAS_StreamImp {
  public:
-  CFGAS_FileReadStreamImp();
+  explicit CFGAS_FileReadStreamImp(
+      const CFX_RetainPtr<IFX_SeekableReadStream>& pFileRead);
   ~CFGAS_FileReadStreamImp() override {}
 
-  bool LoadFileRead(const CFX_RetainPtr<IFX_SeekableReadStream>& pFileRead);
-
   // IFGAS_StreamImp:
-  int32_t GetLength() const override;
-  int32_t Seek(FX_STREAMSEEK eSeek, int32_t iOffset) override;
-  int32_t GetPosition() override { return m_iPosition; }
+  FX_FILESIZE GetLength() const override;
+  FX_FILESIZE GetPosition() override { return m_iPosition; }
+  void Seek(FX_STREAMSEEK eSeek, FX_FILESIZE iOffset) override;
   bool IsEOF() const override;
-  int32_t ReadData(uint8_t* pBuffer, int32_t iBufferSize) override;
-  int32_t ReadString(wchar_t* pStr, int32_t iMaxLength, bool& bEOS) override;
-  int32_t WriteData(const uint8_t* pBuffer, int32_t iBufferSize) override {
-    return 0;
-  }
-  int32_t WriteString(const wchar_t* pStr, int32_t iLength) override {
-    return 0;
-  }
-  void Flush() override {}
-  bool SetLength(int32_t iLength) override { return false; }
+  FX_STRSIZE ReadData(uint8_t* pBuffer, FX_STRSIZE iBufferSize) override;
+  FX_STRSIZE ReadString(wchar_t* pStr,
+                        FX_STRSIZE iMaxLength,
+                        bool* bEOS) override;
+  void WriteData(const uint8_t* pBuffer, FX_STRSIZE iBufferSize) override {}
+  void WriteString(const wchar_t* pStr, FX_STRSIZE iLength) override {}
 
  private:
   CFX_RetainPtr<IFX_SeekableReadStream> m_pFileRead;
-  int32_t m_iPosition;
-  int32_t m_iLength;
+  FX_FILESIZE m_iPosition;
 };
 
 class CFGAS_FileWriteStreamImp : public IFGAS_StreamImp {
  public:
-  CFGAS_FileWriteStreamImp();
+  CFGAS_FileWriteStreamImp(
+      const CFX_RetainPtr<IFX_SeekableWriteStream>& pFileWrite);
   ~CFGAS_FileWriteStreamImp() override {}
 
-  bool LoadFileWrite(const CFX_RetainPtr<IFX_SeekableWriteStream>& pFileWrite);
-
   // IFGAS_StreamImp:
-  int32_t GetLength() const override;
-  int32_t Seek(FX_STREAMSEEK eSeek, int32_t iOffset) override;
-  int32_t GetPosition() override { return m_iPosition; }
+  FX_FILESIZE GetLength() const override;
+  FX_FILESIZE GetPosition() override { return m_iPosition; }
+  void Seek(FX_STREAMSEEK eSeek, FX_FILESIZE iOffset) override;
   bool IsEOF() const override;
-  int32_t ReadData(uint8_t* pBuffer, int32_t iBufferSize) override { return 0; }
-  int32_t ReadString(wchar_t* pStr, int32_t iMaxLength, bool& bEOS) override {
+  FX_STRSIZE ReadData(uint8_t* pBuffer, FX_STRSIZE iBufferSize) override {
     return 0;
   }
-  int32_t WriteData(const uint8_t* pBuffer, int32_t iBufferSize) override;
-  int32_t WriteString(const wchar_t* pStr, int32_t iLength) override;
-  void Flush() override;
-  bool SetLength(int32_t iLength) override { return false; }
+  FX_STRSIZE ReadString(wchar_t* pStr,
+                        FX_STRSIZE iMaxLength,
+                        bool* bEOS) override {
+    return 0;
+  }
+  void WriteData(const uint8_t* pBuffer, FX_STRSIZE iBufferSize) override;
+  void WriteString(const wchar_t* pStr, FX_STRSIZE iLength) override;
 
  private:
   CFX_RetainPtr<IFX_SeekableWriteStream> m_pFileWrite;
-  int32_t m_iPosition;
+  FX_FILESIZE m_iPosition;
 };
 
 class CFGAS_TextStream : public IFGAS_Stream {
@@ -103,19 +98,19 @@ class CFGAS_TextStream : public IFGAS_Stream {
   friend CFX_RetainPtr<T> pdfium::MakeRetain(Args&&... args);
 
   // IFGAS_Stream
-  int32_t GetLength() const override;
-  int32_t Seek(FX_STREAMSEEK eSeek, int32_t iOffset) override;
-  int32_t GetPosition() override;
+  FX_FILESIZE GetLength() const override;
+  FX_FILESIZE GetPosition() override;
+  FX_STRSIZE GetBOMLength() const override;
+  void Seek(FX_STREAMSEEK eSeek, FX_FILESIZE iOffset) override;
   bool IsEOF() const override;
-  int32_t ReadData(uint8_t* pBuffer, int32_t iBufferSize) override;
-  int32_t ReadString(wchar_t* pStr, int32_t iMaxLength, bool& bEOS) override;
-  int32_t WriteData(const uint8_t* pBuffer, int32_t iBufferSize) override;
-  int32_t WriteString(const wchar_t* pStr, int32_t iLength) override;
-  void Flush() override;
-  bool SetLength(int32_t iLength) override;
-  int32_t GetBOMLength() const override;
+  FX_STRSIZE ReadData(uint8_t* pBuffer, FX_STRSIZE iBufferSize) override;
+  FX_STRSIZE ReadString(wchar_t* pStr,
+                        FX_STRSIZE iMaxLength,
+                        bool* bEOS) override;
+  void WriteData(const uint8_t* pBuffer, FX_STRSIZE iBufferSize) override;
+  void WriteString(const wchar_t* pStr, FX_STRSIZE iLength) override;
   uint16_t GetCodePage() const override;
-  uint16_t SetCodePage(uint16_t wCodePage) override;
+  void SetCodePage(uint16_t wCodePage) override;
 
  private:
   CFGAS_TextStream(std::unique_ptr<IFGAS_StreamImp> imp, bool isWriteSteam);
@@ -124,7 +119,7 @@ class CFGAS_TextStream : public IFGAS_Stream {
   void InitStream();
 
   uint16_t m_wCodePage;
-  int32_t m_wBOMLength;
+  FX_STRSIZE m_wBOMLength;
   bool m_IsWriteStream;
   std::unique_ptr<IFGAS_StreamImp> m_pStreamImp;
 };
@@ -135,47 +130,148 @@ class CFGAS_WideStringReadStream : public IFGAS_Stream {
   friend CFX_RetainPtr<T> pdfium::MakeRetain(Args&&... args);
 
   // IFGAS_Stream
-  int32_t GetLength() const override;
-  int32_t Seek(FX_STREAMSEEK eSeek, int32_t iOffset) override;
-  int32_t GetPosition() override;
+  FX_FILESIZE GetLength() const override;
+  FX_FILESIZE GetPosition() override;
+  FX_STRSIZE GetBOMLength() const override { return 0; }
+  void Seek(FX_STREAMSEEK eSeek, FX_FILESIZE iOffset) override;
   bool IsEOF() const override;
-  int32_t ReadData(uint8_t* pBuffer, int32_t iBufferSize) override;
-  int32_t ReadString(wchar_t* pStr, int32_t iMaxLength, bool& bEOS) override;
-  int32_t WriteData(const uint8_t* pBuffer, int32_t iBufferSize) override;
-  int32_t WriteString(const wchar_t* pStr, int32_t iLength) override;
-  void Flush() override {}
-  bool SetLength(int32_t iLength) override;
-  int32_t GetBOMLength() const override;
+  FX_STRSIZE ReadData(uint8_t* pBuffer, FX_STRSIZE iBufferSize) override {
+    return 0;
+  }
+  FX_STRSIZE ReadString(wchar_t* pStr,
+                        FX_STRSIZE iMaxLength,
+                        bool* bEOS) override;
+  void WriteData(const uint8_t* pBuffer, FX_STRSIZE iBufferSize) override {}
+  void WriteString(const wchar_t* pStr, FX_STRSIZE iLength) override {}
   uint16_t GetCodePage() const override;
-  uint16_t SetCodePage(uint16_t wCodePage) override;
+  void SetCodePage(uint16_t wCodePage) override {}
 
  private:
   explicit CFGAS_WideStringReadStream(const CFX_WideString& wsBuffer);
   ~CFGAS_WideStringReadStream() override;
 
   CFX_WideString m_wsBuffer;
-  int32_t m_iPosition;
+  FX_FILESIZE m_iPosition;
 };
 
-IFGAS_StreamImp::IFGAS_StreamImp() {}
+// Returns {src bytes consumed, dst bytes produced}.
+std::pair<FX_STRSIZE, FX_STRSIZE> UTF8Decode(const char* pSrc,
+                                             FX_STRSIZE srcLen,
+                                             wchar_t* pDst,
+                                             FX_STRSIZE dstLen) {
+  ASSERT(pDst && dstLen > 0);
+
+  if (srcLen < 1)
+    return {0, 0};
+
+  uint32_t dwCode = 0;
+  int32_t iPending = 0;
+  FX_STRSIZE iSrcNum = 0;
+  FX_STRSIZE iDstNum = 0;
+  FX_STRSIZE iIndex = 0;
+  int32_t k = 1;
+  while (iIndex < srcLen) {
+    uint8_t byte = static_cast<uint8_t>(*(pSrc + iIndex));
+    if (byte < 0x80) {
+      iPending = 0;
+      k = 1;
+      iDstNum++;
+      iSrcNum += k;
+      *pDst++ = byte;
+      if (iDstNum >= dstLen)
+        break;
+    } else if (byte < 0xc0) {
+      if (iPending < 1)
+        break;
+
+      iPending--;
+      dwCode |= (byte & 0x3f) << (iPending * 6);
+      if (iPending == 0) {
+        iDstNum++;
+        iSrcNum += k;
+        *pDst++ = dwCode;
+        if (iDstNum >= dstLen)
+          break;
+      }
+    } else if (byte < 0xe0) {
+      iPending = 1;
+      k = 2;
+      dwCode = (byte & 0x1f) << 6;
+    } else if (byte < 0xf0) {
+      iPending = 2;
+      k = 3;
+      dwCode = (byte & 0x0f) << 12;
+    } else if (byte < 0xf8) {
+      iPending = 3;
+      k = 4;
+      dwCode = (byte & 0x07) << 18;
+    } else if (byte < 0xfc) {
+      iPending = 4;
+      k = 5;
+      dwCode = (byte & 0x03) << 24;
+    } else if (byte < 0xfe) {
+      iPending = 5;
+      k = 6;
+      dwCode = (byte & 0x01) << 30;
+    } else {
+      break;
+    }
+    iIndex++;
+  }
+  return {iSrcNum, iDstNum};
+}
 
-CFGAS_FileReadStreamImp::CFGAS_FileReadStreamImp()
-    : m_pFileRead(nullptr), m_iPosition(0), m_iLength(0) {}
+void UTF16ToWChar(void* pBuffer, FX_STRSIZE iLength) {
+  ASSERT(pBuffer && iLength > 0);
 
-bool CFGAS_FileReadStreamImp::LoadFileRead(
-    const CFX_RetainPtr<IFX_SeekableReadStream>& pFileRead) {
-  ASSERT(!m_pFileRead && pFileRead);
+  if (sizeof(wchar_t) == 2)
+    return;
+
+  uint16_t* pSrc = static_cast<uint16_t*>(pBuffer);
+  wchar_t* pDst = static_cast<wchar_t*>(pBuffer);
+  while (--iLength >= 0)
+    pDst[iLength] = static_cast<wchar_t>(pSrc[iLength]);
+}
+
+void SwapByteOrder(wchar_t* pStr, FX_STRSIZE iLength) {
+  ASSERT(pStr);
+
+  if (iLength < 0)
+    iLength = FXSYS_wcslen(pStr);
+
+  uint16_t wch;
+  if (sizeof(wchar_t) > 2) {
+    while (iLength-- > 0) {
+      wch = static_cast<uint16_t>(*pStr);
+      wch = (wch >> 8) | (wch << 8);
+      wch &= 0x00FF;
+      *pStr = wch;
+      pStr++;
+    }
+    return;
+  }
 
-  m_pFileRead = pFileRead;
-  m_iLength = m_pFileRead->GetSize();
-  return true;
+  while (iLength-- > 0) {
+    wch = static_cast<uint16_t>(*pStr);
+    wch = (wch >> 8) | (wch << 8);
+    *pStr = wch;
+    pStr++;
+  }
 }
 
-int32_t CFGAS_FileReadStreamImp::GetLength() const {
-  return m_iLength;
+IFGAS_StreamImp::IFGAS_StreamImp() {}
+
+CFGAS_FileReadStreamImp::CFGAS_FileReadStreamImp(
+    const CFX_RetainPtr<IFX_SeekableReadStream>& pFileRead)
+    : m_pFileRead(pFileRead), m_iPosition(0) {
+  ASSERT(m_pFileRead);
 }
 
-int32_t CFGAS_FileReadStreamImp::Seek(FX_STREAMSEEK eSeek, int32_t iOffset) {
+FX_FILESIZE CFGAS_FileReadStreamImp::GetLength() const {
+  return m_pFileRead->GetSize();
+}
+
+void CFGAS_FileReadStreamImp::Seek(FX_STREAMSEEK eSeek, FX_FILESIZE iOffset) {
   switch (eSeek) {
     case FX_STREAMSEEK_Begin:
       m_iPosition = iOffset;
@@ -183,71 +279,63 @@ int32_t CFGAS_FileReadStreamImp::Seek(FX_STREAMSEEK eSeek, int32_t iOffset) {
     case FX_STREAMSEEK_Current:
       m_iPosition += iOffset;
       break;
-    case FX_STREAMSEEK_End:
-      m_iPosition = m_iLength + iOffset;
-      break;
   }
-  if (m_iPosition < 0) {
-    m_iPosition = 0;
-  } else if (m_iPosition >= m_iLength) {
-    m_iPosition = m_iLength;
-  }
-  return m_iPosition;
+  m_iPosition = pdfium::clamp(m_iPosition, static_cast<FX_FILESIZE>(0),
+                              m_pFileRead->GetSize());
 }
 
 bool CFGAS_FileReadStreamImp::IsEOF() const {
-  return m_iPosition >= m_iLength;
+  return m_iPosition >= m_pFileRead->GetSize();
 }
-int32_t CFGAS_FileReadStreamImp::ReadData(uint8_t* pBuffer,
-                                          int32_t iBufferSize) {
-  ASSERT(m_pFileRead);
+
+FX_STRSIZE CFGAS_FileReadStreamImp::ReadData(uint8_t* pBuffer,
+                                             FX_STRSIZE iBufferSize) {
   ASSERT(pBuffer && iBufferSize > 0);
-  if (iBufferSize > m_iLength - m_iPosition) {
-    iBufferSize = m_iLength - m_iPosition;
-  }
+
+  iBufferSize =
+      std::min(iBufferSize,
+               static_cast<FX_STRSIZE>(m_pFileRead->GetSize() - m_iPosition));
   if (m_pFileRead->ReadBlock(pBuffer, m_iPosition, iBufferSize)) {
-    m_iPosition += iBufferSize;
+    pdfium::base::CheckedNumeric<FX_FILESIZE> new_pos = m_iPosition;
+    new_pos += iBufferSize;
+    if (!new_pos.IsValid())
+      return 0;
+
+    m_iPosition = new_pos.ValueOrDie();
     return iBufferSize;
   }
   return 0;
 }
-int32_t CFGAS_FileReadStreamImp::ReadString(wchar_t* pStr,
-                                            int32_t iMaxLength,
-                                            bool& bEOS) {
-  ASSERT(m_pFileRead);
+
+FX_STRSIZE CFGAS_FileReadStreamImp::ReadString(wchar_t* pStr,
+                                               FX_STRSIZE iMaxLength,
+                                               bool* bEOS) {
   ASSERT(pStr && iMaxLength > 0);
-  iMaxLength = ReadData((uint8_t*)pStr, iMaxLength * 2) / 2;
-  if (iMaxLength <= 0) {
+
+  iMaxLength = ReadData(reinterpret_cast<uint8_t*>(pStr), iMaxLength * 2) / 2;
+  if (iMaxLength <= 0)
     return 0;
-  }
-  int32_t i = 0;
-  while (i < iMaxLength && pStr[i] != L'\0') {
+
+  FX_STRSIZE i = 0;
+  while (i < iMaxLength && pStr[i] != L'\0')
     ++i;
-  }
-  bEOS = (m_iPosition >= m_iLength) || pStr[i] == L'\0';
+
+  *bEOS = m_iPosition >= m_pFileRead->GetSize() || pStr[i] == L'\0';
   return i;
 }
 
-CFGAS_FileWriteStreamImp::CFGAS_FileWriteStreamImp()
-    : m_pFileWrite(nullptr), m_iPosition(0) {}
-
-bool CFGAS_FileWriteStreamImp::LoadFileWrite(
-    const CFX_RetainPtr<IFX_SeekableWriteStream>& pFileWrite) {
-  ASSERT(!m_pFileWrite && pFileWrite);
-
-  m_iPosition = pFileWrite->GetSize();
-  m_pFileWrite = pFileWrite;
-  return true;
+CFGAS_FileWriteStreamImp::CFGAS_FileWriteStreamImp(
+    const CFX_RetainPtr<IFX_SeekableWriteStream>& pFileWrite)
+    : m_pFileWrite(pFileWrite), m_iPosition(m_pFileWrite->GetSize()) {
+  ASSERT(m_pFileWrite);
 }
 
-int32_t CFGAS_FileWriteStreamImp::GetLength() const {
-  if (!m_pFileWrite)
-    return 0;
-
-  return (int32_t)m_pFileWrite->GetSize();
+FX_FILESIZE CFGAS_FileWriteStreamImp::GetLength() const {
+  return m_pFileWrite->GetSize();
 }
-int32_t CFGAS_FileWriteStreamImp::Seek(FX_STREAMSEEK eSeek, int32_t iOffset) {
-  int32_t iLength = GetLength();
+
+void CFGAS_FileWriteStreamImp::Seek(FX_STREAMSEEK eSeek, FX_FILESIZE iOffset) {
+  FX_FILESIZE iLength = GetLength();
   switch (eSeek) {
     case FX_STREAMSEEK_Begin:
       m_iPosition = iOffset;
@@ -255,38 +343,31 @@ int32_t CFGAS_FileWriteStreamImp::Seek(FX_STREAMSEEK eSeek, int32_t iOffset) {
     case FX_STREAMSEEK_Current:
       m_iPosition += iOffset;
       break;
-    case FX_STREAMSEEK_End:
-      m_iPosition = iLength + iOffset;
-      break;
-  }
-  if (m_iPosition < 0) {
-    m_iPosition = 0;
-  } else if (m_iPosition >= iLength) {
-    m_iPosition = iLength;
   }
-  return m_iPosition;
+  m_iPosition =
+      pdfium::clamp(m_iPosition, static_cast<FX_FILESIZE>(0), iLength);
 }
+
 bool CFGAS_FileWriteStreamImp::IsEOF() const {
   return m_iPosition >= GetLength();
 }
-int32_t CFGAS_FileWriteStreamImp::WriteData(const uint8_t* pBuffer,
-                                            int32_t iBufferSize) {
-  if (!m_pFileWrite) {
-    return 0;
-  }
+
+void CFGAS_FileWriteStreamImp::WriteData(const uint8_t* pBuffer,
+                                         FX_STRSIZE iBufferSize) {
   if (m_pFileWrite->WriteBlock(pBuffer, m_iPosition, iBufferSize)) {
-    m_iPosition += iBufferSize;
+    pdfium::base::CheckedNumeric<FX_STRSIZE> new_pos = m_iPosition;
+    new_pos += iBufferSize;
+    // TODO(dsinclair): Not sure what to do if we over flow ....
+    if (!new_pos.IsValid())
+      return;
+
+    m_iPosition = new_pos.ValueOrDie();
   }
-  return iBufferSize;
 }
-int32_t CFGAS_FileWriteStreamImp::WriteString(const wchar_t* pStr,
-                                              int32_t iLength) {
-  return WriteData((const uint8_t*)pStr, iLength * sizeof(wchar_t));
-}
-void CFGAS_FileWriteStreamImp::Flush() {
-  if (m_pFileWrite) {
-    m_pFileWrite->Flush();
-  }
+
+void CFGAS_FileWriteStreamImp::WriteString(const wchar_t* pStr,
+                                           FX_STRSIZE iLength) {
+  WriteData(reinterpret_cast<const uint8_t*>(pStr), iLength * sizeof(wchar_t));
 }
 
 CFGAS_TextStream::CFGAS_TextStream(std::unique_ptr<IFGAS_StreamImp> imp,
@@ -316,7 +397,7 @@ CFGAS_TextStream::~CFGAS_TextStream() {}
 #endif  // _FX_ENDIAN_ == _FX_LITTLE_ENDIAN_
 
 void CFGAS_TextStream::InitStream() {
-  int32_t iPosition = m_pStreamImp->GetPosition();
+  FX_FILESIZE iPosition = m_pStreamImp->GetPosition();
   m_pStreamImp->Seek(FX_STREAMSEEK_Begin, 0);
 
   uint32_t bom;
@@ -340,18 +421,20 @@ void CFGAS_TextStream::InitStream() {
     }
   }
 
-  m_pStreamImp->Seek(FX_STREAMSEEK_Begin, std::max(m_wBOMLength, iPosition));
+  m_pStreamImp->Seek(
+      FX_STREAMSEEK_Begin,
+      std::max(static_cast<FX_FILESIZE>(m_wBOMLength), iPosition));
 }
 
-int32_t CFGAS_TextStream::GetLength() const {
+FX_FILESIZE CFGAS_TextStream::GetLength() const {
   return m_pStreamImp->GetLength();
 }
 
-int32_t CFGAS_TextStream::Seek(FX_STREAMSEEK eSeek, int32_t iOffset) {
-  return m_pStreamImp->Seek(eSeek, iOffset);
+void CFGAS_TextStream::Seek(FX_STREAMSEEK eSeek, FX_FILESIZE iOffset) {
+  m_pStreamImp->Seek(eSeek, iOffset);
 }
 
-int32_t CFGAS_TextStream::GetPosition() {
+FX_FILESIZE CFGAS_TextStream::GetPosition() {
   return m_pStreamImp->GetPosition();
 }
 
@@ -359,42 +442,33 @@ bool CFGAS_TextStream::IsEOF() const {
   return m_pStreamImp->IsEOF();
 }
 
-int32_t CFGAS_TextStream::ReadData(uint8_t* pBuffer, int32_t iBufferSize) {
+FX_STRSIZE CFGAS_TextStream::ReadData(uint8_t* pBuffer,
+                                      FX_STRSIZE iBufferSize) {
   ASSERT(pBuffer && iBufferSize > 0);
 
   if (m_IsWriteStream)
     return -1;
 
-  int32_t iLen = std::min(
-      m_pStreamImp->GetLength() - m_pStreamImp->GetPosition(), iBufferSize);
+  FX_STRSIZE iLen =
+      std::min(static_cast<FX_STRSIZE>(m_pStreamImp->GetLength() -
+                                       m_pStreamImp->GetPosition()),
+               iBufferSize);
   if (iLen <= 0)
     return 0;
 
   return m_pStreamImp->ReadData(pBuffer, iLen);
 }
 
-int32_t CFGAS_TextStream::WriteData(const uint8_t* pBuffer,
-                                    int32_t iBufferSize) {
+void CFGAS_TextStream::WriteData(const uint8_t* pBuffer,
+                                 FX_STRSIZE iBufferSize) {
   ASSERT(pBuffer && iBufferSize > 0);
 
   if (!m_IsWriteStream)
-    return -1;
-  return m_pStreamImp->WriteData(pBuffer, iBufferSize);
-}
-
-void CFGAS_TextStream::Flush() {
-  if (!m_IsWriteStream)
     return;
-  m_pStreamImp->Flush();
-}
-
-bool CFGAS_TextStream::SetLength(int32_t iLength) {
-  if (!m_IsWriteStream)
-    return false;
-  return m_pStreamImp->SetLength(iLength);
+  m_pStreamImp->WriteData(pBuffer, iBufferSize);
 }
 
-int32_t CFGAS_TextStream::GetBOMLength() const {
+FX_STRSIZE CFGAS_TextStream::GetBOMLength() const {
   if (m_wBOMLength < 1)
     return 0;
   return m_wBOMLength;
@@ -404,76 +478,78 @@ uint16_t CFGAS_TextStream::GetCodePage() const {
   return m_wCodePage;
 }
 
-uint16_t CFGAS_TextStream::SetCodePage(uint16_t wCodePage) {
+void CFGAS_TextStream::SetCodePage(uint16_t wCodePage) {
   if (m_wBOMLength > 0)
-    return m_wCodePage;
+    return;
 
-  uint16_t v = m_wCodePage;
   m_wCodePage = wCodePage;
-  return v;
 }
 
-int32_t CFGAS_TextStream::ReadString(wchar_t* pStr,
-                                     int32_t iMaxLength,
-                                     bool& bEOS) {
+FX_STRSIZE CFGAS_TextStream::ReadString(wchar_t* pStr,
+                                        FX_STRSIZE iMaxLength,
+                                        bool* bEOS) {
   ASSERT(pStr && iMaxLength > 0);
+
   if (m_IsWriteStream)
     return -1;
 
   if (m_wCodePage == FX_CODEPAGE_UTF16LE ||
       m_wCodePage == FX_CODEPAGE_UTF16BE) {
-    int32_t iBytes = iMaxLength * 2;
-    int32_t iLen = m_pStreamImp->ReadData((uint8_t*)pStr, iBytes);
+    FX_FILESIZE iBytes = iMaxLength * 2;
+    FX_STRSIZE iLen = m_pStreamImp->ReadData((uint8_t*)pStr, iBytes);
     iMaxLength = iLen / 2;
     if (sizeof(wchar_t) > 2)
-      FX_UTF16ToWChar(pStr, iMaxLength);
+      UTF16ToWChar(pStr, iMaxLength);
 
 #if _FX_ENDIAN_ == _FX_BIG_ENDIAN_
     if (m_wCodePage == FX_CODEPAGE_UTF16LE)
-      FX_SwapByteOrder(pStr, iMaxLength);
+      SwapByteOrder(pStr, iMaxLength);
 #else
     if (m_wCodePage == FX_CODEPAGE_UTF16BE)
-      FX_SwapByteOrder(pStr, iMaxLength);
+      SwapByteOrder(pStr, iMaxLength);
 #endif
 
   } else {
-    int32_t pos = m_pStreamImp->GetPosition();
-    int32_t iBytes = std::min(iMaxLength, m_pStreamImp->GetLength() - pos);
+    FX_FILESIZE pos = m_pStreamImp->GetPosition();
+    FX_STRSIZE iBytes = std::min(
+        iMaxLength, static_cast<FX_STRSIZE>(m_pStreamImp->GetLength() - pos));
+
     if (iBytes > 0) {
       std::vector<uint8_t> buf(iBytes);
 
-      int32_t iLen = m_pStreamImp->ReadData(buf.data(), iBytes);
-      int32_t iSrc = iLen;
-      int32_t iDecode = FX_DecodeString(
-          m_wCodePage, reinterpret_cast<const char*>(buf.data()), &iSrc, pStr,
-          &iMaxLength, true);
-      m_pStreamImp->Seek(FX_STREAMSEEK_Current, iSrc - iLen);
-      if (iDecode < 1)
+      FX_STRSIZE iLen = m_pStreamImp->ReadData(buf.data(), iBytes);
+      if (m_wCodePage != FX_CODEPAGE_UTF8)
         return -1;
+
+      FX_STRSIZE iSrc = 0;
+      std::tie(iSrc, iMaxLength) = UTF8Decode(
+          reinterpret_cast<const char*>(buf.data()), iLen, pStr, iMaxLength);
+      m_pStreamImp->Seek(FX_STREAMSEEK_Current, iSrc - iLen);
     } else {
       iMaxLength = 0;
     }
   }
-  bEOS = m_pStreamImp->IsEOF();
+
+  *bEOS = m_pStreamImp->IsEOF();
   return iMaxLength;
 }
 
-int32_t CFGAS_TextStream::WriteString(const wchar_t* pStr, int32_t iLength) {
+void CFGAS_TextStream::WriteString(const wchar_t* pStr, FX_STRSIZE iLength) {
   ASSERT(pStr && iLength > 0);
   if (!m_IsWriteStream)
-    return -1;
+    return;
 
-  if (m_wCodePage == FX_CODEPAGE_UTF8) {
-    int32_t len = iLength;
-    CFX_UTF8Encoder encoder;
-    while (len-- > 0) {
-      encoder.Input(*pStr++);
-    }
-    CFX_ByteStringC bsResult = encoder.GetResult();
-    m_pStreamImp->WriteData((const uint8_t*)bsResult.c_str(),
-                            bsResult.GetLength());
-  }
-  return iLength;
+  if (m_wCodePage != FX_CODEPAGE_UTF8)
+    return;
+
+  FX_STRSIZE len = iLength;
+  CFX_UTF8Encoder encoder;
+  while (len-- > 0)
+    encoder.Input(*pStr++);
+
+  CFX_ByteStringC bsResult = encoder.GetResult();
+  m_pStreamImp->WriteData(reinterpret_cast<const uint8_t*>(bsResult.c_str()),
+                          bsResult.GetLength());
 }
 
 CFGAS_WideStringReadStream::CFGAS_WideStringReadStream(
@@ -482,11 +558,12 @@ CFGAS_WideStringReadStream::CFGAS_WideStringReadStream(
 
 CFGAS_WideStringReadStream::~CFGAS_WideStringReadStream() {}
 
-int32_t CFGAS_WideStringReadStream::GetLength() const {
+FX_FILESIZE CFGAS_WideStringReadStream::GetLength() const {
   return m_wsBuffer.GetLength() * sizeof(wchar_t);
 }
 
-int32_t CFGAS_WideStringReadStream::Seek(FX_STREAMSEEK eSeek, int32_t iOffset) {
+void CFGAS_WideStringReadStream::Seek(FX_STREAMSEEK eSeek,
+                                      FX_FILESIZE iOffset) {
   switch (eSeek) {
     case FX_STREAMSEEK_Begin:
       m_iPosition = iOffset;
@@ -494,15 +571,12 @@ int32_t CFGAS_WideStringReadStream::Seek(FX_STREAMSEEK eSeek, int32_t iOffset) {
     case FX_STREAMSEEK_Current:
       m_iPosition += iOffset;
       break;
-    case FX_STREAMSEEK_End:
-      m_iPosition = m_wsBuffer.GetLength() + iOffset;
-      break;
   }
-  m_iPosition = pdfium::clamp(0, m_iPosition, m_wsBuffer.GetLength());
-  return GetPosition();
+  m_iPosition = pdfium::clamp(m_iPosition, static_cast<FX_FILESIZE>(0),
+                              static_cast<FX_FILESIZE>(m_wsBuffer.GetLength()));
 }
 
-int32_t CFGAS_WideStringReadStream::GetPosition() {
+FX_FILESIZE CFGAS_WideStringReadStream::GetPosition() {
   return m_iPosition * sizeof(wchar_t);
 }
 
@@ -510,50 +584,25 @@ bool CFGAS_WideStringReadStream::IsEOF() const {
   return m_iPosition >= m_wsBuffer.GetLength();
 }
 
-int32_t CFGAS_WideStringReadStream::ReadData(uint8_t* pBuffer,
-                                             int32_t iBufferSize) {
-  return 0;
-}
-
-int32_t CFGAS_WideStringReadStream::ReadString(wchar_t* pStr,
-                                               int32_t iMaxLength,
-                                               bool& bEOS) {
-  iMaxLength = std::min(iMaxLength, m_wsBuffer.GetLength() - m_iPosition);
+FX_STRSIZE CFGAS_WideStringReadStream::ReadString(wchar_t* pStr,
+                                                  FX_STRSIZE iMaxLength,
+                                                  bool* bEOS) {
+  iMaxLength =
+      std::min(iMaxLength,
+               static_cast<FX_STRSIZE>(m_wsBuffer.GetLength() - m_iPosition));
   if (iMaxLength == 0)
     return 0;
 
   FXSYS_wcsncpy(pStr, m_wsBuffer.c_str() + m_iPosition, iMaxLength);
   m_iPosition += iMaxLength;
-  bEOS = IsEOF();
+  *bEOS = IsEOF();
   return iMaxLength;
 }
 
-int32_t CFGAS_WideStringReadStream::WriteData(const uint8_t* pBuffer,
-                                              int32_t iBufferSize) {
-  return 0;
-}
-
-int32_t CFGAS_WideStringReadStream::WriteString(const wchar_t* pStr,
-                                                int32_t iLength) {
-  return 0;
-}
-
-bool CFGAS_WideStringReadStream::SetLength(int32_t iLength) {
-  return false;
-}
-
-int32_t CFGAS_WideStringReadStream::GetBOMLength() const {
-  return 0;
-}
-
 uint16_t CFGAS_WideStringReadStream::GetCodePage() const {
   return (sizeof(wchar_t) == 2) ? FX_CODEPAGE_UTF16LE : FX_CODEPAGE_UTF32LE;
 }
 
-uint16_t CFGAS_WideStringReadStream::SetCodePage(uint16_t wCodePage) {
-  return GetCodePage();
-}
-
 }  // namespace
 
 // static
@@ -562,13 +611,8 @@ CFX_RetainPtr<IFGAS_Stream> IFGAS_Stream::CreateReadStream(
   if (!pFileRead)
     return nullptr;
 
-  std::unique_ptr<IFGAS_StreamImp> pImp =
-      pdfium::MakeUnique<CFGAS_FileReadStreamImp>();
-  if (!static_cast<CFGAS_FileReadStreamImp*>(pImp.get())
-           ->LoadFileRead(pFileRead)) {
-    return nullptr;
-  }
-  return pdfium::MakeRetain<CFGAS_TextStream>(std::move(pImp), false);
+  return pdfium::MakeRetain<CFGAS_TextStream>(
+      pdfium::MakeUnique<CFGAS_FileReadStreamImp>(pFileRead), false);
 }
 
 // static
@@ -577,13 +621,8 @@ CFX_RetainPtr<IFGAS_Stream> IFGAS_Stream::CreateWriteStream(
   if (!pFileWrite)
     return nullptr;
 
-  std::unique_ptr<IFGAS_StreamImp> pImp =
-      pdfium::MakeUnique<CFGAS_FileWriteStreamImp>();
-  if (!static_cast<CFGAS_FileWriteStreamImp*>(pImp.get())
-           ->LoadFileWrite(pFileWrite)) {
-    return nullptr;
-  }
-  return pdfium::MakeRetain<CFGAS_TextStream>(std::move(pImp), true);
+  return pdfium::MakeRetain<CFGAS_TextStream>(
+      pdfium::MakeUnique<CFGAS_FileWriteStreamImp>(pFileWrite), true);
 }
 
 // static
diff --git a/xfa/fgas/crt/ifgas_stream.h b/xfa/fgas/crt/ifgas_stream.h
index b3f8a928d8..658afe73bf 100644
--- a/xfa/fgas/crt/ifgas_stream.h
+++ b/xfa/fgas/crt/ifgas_stream.h
@@ -14,7 +14,6 @@
 enum FX_STREAMSEEK {
   FX_STREAMSEEK_Begin = 0,
   FX_STREAMSEEK_Current,
-  FX_STREAMSEEK_End,
 };
 
 class IFGAS_Stream : public CFX_Retainable {
@@ -26,19 +25,24 @@ class IFGAS_Stream : public CFX_Retainable {
   static CFX_RetainPtr<IFGAS_Stream> CreateWideStringReadStream(
       const CFX_WideString& buffer);
 
-  virtual int32_t GetLength() const = 0;
-  virtual int32_t Seek(FX_STREAMSEEK eSeek, int32_t iOffset) = 0;
-  virtual int32_t GetPosition() = 0;
-  virtual bool IsEOF() const = 0;
-  virtual int32_t ReadData(uint8_t* pBuffer, int32_t iBufferSize) = 0;
-  virtual int32_t ReadString(wchar_t* pStr, int32_t iMaxLength, bool& bEOS) = 0;
-  virtual int32_t WriteData(const uint8_t* pBuffer, int32_t iBufferSize) = 0;
-  virtual int32_t WriteString(const wchar_t* pStr, int32_t iLength) = 0;
-  virtual void Flush() = 0;
-  virtual bool SetLength(int32_t iLength) = 0;
-  virtual int32_t GetBOMLength() const = 0;
+  virtual FX_FILESIZE GetLength() const = 0;
+  virtual FX_FILESIZE GetPosition() = 0;
+  virtual FX_STRSIZE GetBOMLength() const = 0;
+
+  virtual void Seek(FX_STREAMSEEK eSeek, FX_FILESIZE iOffset) = 0;
+
+  virtual FX_STRSIZE ReadString(wchar_t* pStr,
+                                FX_STRSIZE iMaxLength,
+                                bool* bEOS) = 0;
+  virtual void WriteData(const uint8_t* pBuffer, FX_STRSIZE iBufferSize) = 0;
+  virtual void WriteString(const wchar_t* pStr, FX_STRSIZE iLength) = 0;
+
   virtual uint16_t GetCodePage() const = 0;
-  virtual uint16_t SetCodePage(uint16_t wCodePage) = 0;
+  virtual void SetCodePage(uint16_t wCodePage) = 0;
+
+ protected:
+  virtual bool IsEOF() const = 0;
+  virtual FX_STRSIZE ReadData(uint8_t* pBuffer, FX_STRSIZE iBufferSize) = 0;
 };
 
 #endif  // XFA_FGAS_CRT_IFGAS_STREAM_H_