Consolidate more work into CFX_WideTextBuf::ExpandWideBuf().

- Let ExpandWideBuf() adjust |m_DataSize|, instead of the caller. Since
  ExpandWideBuf() already knows how many bytes |m_DataSize| will
  increase by.
- Let ExpandWideBuf() return a span for the caller to fill, instead of
  having the caller manually do pointer math and casting.

Change-Id: I99e9d16fbb24d844c6bc7f00fb6e11b1d13ce850
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/68271
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>

core/fxcrt/cfx_widetextbuf.cpp

diff --git a/core/fxcrt/cfx_widetextbuf.cpp b/core/fxcrt/cfx_widetextbuf.cpp
index 90e4de5..c52fc6c 100644
--- a/core/fxcrt/cfx_widetextbuf.cpp
+++ b/core/fxcrt/cfx_widetextbuf.cpp
@@ -31,9 +31,8 @@
 }
 
 void CFX_WideTextBuf::AppendChar(wchar_t ch) {
-  ExpandWideBuf(1);
-  *reinterpret_cast<wchar_t*>(m_pBuffer.get() + m_DataSize) = ch;
-  m_DataSize += sizeof(wchar_t);
+  pdfium::span<wchar_t> new_span = ExpandWideBuf(1);
+  new_span[0] = ch;
 }
 
 void CFX_WideTextBuf::Delete(int start_index, int count) {
@@ -41,11 +40,9 @@
 }
 
 CFX_WideTextBuf& CFX_WideTextBuf::operator<<(ByteStringView ascii) {
-  ExpandWideBuf(ascii.GetLength());
-  for (uint8_t ch : ascii) {
-    *reinterpret_cast<wchar_t*>(m_pBuffer.get() + m_DataSize) = ch;
-    m_DataSize += sizeof(wchar_t);
-  }
+  pdfium::span<wchar_t> new_span = ExpandWideBuf(ascii.GetLength());
+  for (size_t i = 0; i < ascii.GetLength(); ++i)
+    new_span[i] = ascii[i];
   return *this;
 }
 
@@ -63,24 +60,18 @@
   char buf[32];
   FXSYS_itoa(i, buf, 10);
   size_t len = strlen(buf);
-  ExpandWideBuf(len);
-  wchar_t* str = reinterpret_cast<wchar_t*>(m_pBuffer.get() + m_DataSize);
-  for (size_t j = 0; j < len; j++) {
-    *str++ = buf[j];
-  }
-  m_DataSize += len * sizeof(wchar_t);
+  pdfium::span<wchar_t> new_span = ExpandWideBuf(len);
+  for (size_t j = 0; j < len; j++)
+    new_span[j] = buf[j];
   return *this;
 }
 
 CFX_WideTextBuf& CFX_WideTextBuf::operator<<(double f) {
   char buf[32];
   size_t len = FloatToString((float)f, buf);
-  ExpandWideBuf(len);
-  wchar_t* str = reinterpret_cast<wchar_t*>(m_pBuffer.get() + m_DataSize);
-  for (size_t i = 0; i < len; i++) {
-    *str++ = buf[i];
-  }
-  m_DataSize += len * sizeof(wchar_t);
+  pdfium::span<wchar_t> new_span = ExpandWideBuf(len);
+  for (size_t i = 0; i < len; i++)
+    new_span[i] = buf[i];
   return *this;
 }
 
@@ -94,8 +85,12 @@
   return *this;
 }
 
-void CFX_WideTextBuf::ExpandWideBuf(size_t char_count) {
-  FX_SAFE_SIZE_T safe_count = char_count;
-  safe_count *= sizeof(wchar_t);
-  ExpandBuf(safe_count.ValueOrDie());
+pdfium::span<wchar_t> CFX_WideTextBuf::ExpandWideBuf(size_t char_count) {
+  size_t original_count = GetLength();
+  FX_SAFE_SIZE_T safe_bytes = char_count;
+  safe_bytes *= sizeof(wchar_t);
+  size_t bytes = safe_bytes.ValueOrDie();
+  ExpandBuf(bytes);
+  m_DataSize += bytes;
+  return GetWideSpan().subspan(original_count);
 }

core/fxcrt/cfx_widetextbuf.h

diff --git a/core/fxcrt/cfx_widetextbuf.h b/core/fxcrt/cfx_widetextbuf.h
index 90daa48..fb1be8a 100644
--- a/core/fxcrt/cfx_widetextbuf.h
+++ b/core/fxcrt/cfx_widetextbuf.h
@@ -34,7 +34,8 @@
   CFX_WideTextBuf& operator<<(const CFX_WideTextBuf& buf);
 
  private:
-  void ExpandWideBuf(size_t char_count);
+  // Returned span is the newly-expanded space.
+  pdfium::span<wchar_t> ExpandWideBuf(size_t char_count);
 };
 
 #endif  // CORE_FXCRT_CFX_WIDETEXTBUF_H_