Initial QFileBuffer implementation

scwx-qt/source/scwx/qt/util/q_file_buffer.cpp

@@ -0,0 +1,306 @@
+#include <scwx/qt/util/q_file_buffer.hpp>
+#include <scwx/util/logger.hpp>
+
+#include <QFile>
+
+namespace scwx
+{
+namespace qt
+{
+namespace util
+{
+
+static const std::string logPrefix_ = "scwx::qt::util::q_file_buffer";
+static const auto        logger_    = scwx::util::Logger::Create(logPrefix_);
+
+// Adapted from Microsoft filebuf reference implementation
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+class QFileBuffer::Impl
+{
+public:
+   explicit Impl(QFileBuffer* self) : self_ {self} {};
+   ~Impl() = default;
+
+   void ResetPutback();
+   void SetPutback();
+
+   QFileBuffer* self_;
+   QFile        file_ {};
+   char_type    putbackChar_ {};
+   char_type*   putbackEback_ {nullptr};
+   char_type*   putbackEgptr_ {nullptr};
+};
+
+void QFileBuffer::Impl::ResetPutback()
+{
+   if (self_->eback() == &putbackChar_)
+   {
+      // Restore get buffer after putback
+      self_->setg(putbackEback_, putbackEback_, putbackEgptr_);
+   }
+}
+
+void QFileBuffer::Impl::SetPutback()
+{
+   if (self_->eback() != &putbackChar_)
+   {
+      // Save current get buffer
+      putbackEback_ = self_->eback();
+      putbackEgptr_ = self_->egptr();
+   }
+   self_->setg(&putbackChar_, &putbackChar_, &putbackChar_ + 1);
+}
+
+QFileBuffer::QFileBuffer() : std::streambuf(), p(std::make_unique<Impl>(this))
+{
+   // Initialize read/write pointers
+   setg(0, 0, 0);
+   setp(0, 0);
+}
+
+QFileBuffer::QFileBuffer(const std::string&      filename,
+                         std::ios_base::openmode mode) :
+    QFileBuffer()
+{
+   open(filename, mode);
+}
+
+QFileBuffer::~QFileBuffer() = default;
+
+QFileBuffer::QFileBuffer(QFileBuffer&&) noexcept            = default;
+QFileBuffer& QFileBuffer::operator=(QFileBuffer&&) noexcept = default;
+
+bool QFileBuffer::is_open() const
+{
+   return p->file_.isOpen();
+}
+
+QFileBuffer* QFileBuffer::open(const std::string&      filename,
+                               std::ios_base::openmode mode)
+{
+   // If the associated file is already open, return a null pointer right away
+   if (is_open())
+   {
+      return nullptr;
+   }
+
+   // Validate supported modes
+   if (mode & std::ios_base::out || mode & std::ios_base::app ||
+       mode & std::ios_base::trunc)
+   {
+      logger_->error("open(): write mode not supported");
+      return nullptr;
+   }
+
+   // Convert std iostream flags to Qt flags
+   QIODeviceBase::OpenMode flags {};
+   if (mode & std::ios_base::in)
+   {
+      flags |= QIODeviceBase::OpenModeFlag::ReadOnly;
+   }
+   if ((mode & std::ios_base::binary) != std::ios_base::binary)
+   {
+      flags |= QIODeviceBase::OpenModeFlag::Text;
+   }
+
+   // Set the filename and open the file
+   p->file_.setFileName(QString::fromStdString(filename));
+   bool isOpen = p->file_.open(flags);
+
+   if (isOpen)
+   {
+      // Seek to end if requested
+      if (mode & std::ios_base::ate)
+      {
+         // Seek to end
+         p->file_.seek(p->file_.size());
+      }
+
+      // Initialize read/write pointers
+      setg(0, 0, 0);
+      setp(0, 0);
+   }
+
+   return isOpen ? this : nullptr;
+}
+
+QFileBuffer* QFileBuffer::close()
+{
+   // If the associated file is already closed, return a null pointer right away
+   if (!p->file_.isOpen())
+   {
+      return nullptr;
+   }
+
+   // Close the file
+   p->file_.close();
+
+   return this;
+}
+
+QFileBuffer::int_type QFileBuffer::pbackfail(int_type c)
+{
+   if (gptr() && eback() < gptr() &&
+       (traits_type::eq_int_type(traits_type::eof(), c) ||
+        traits_type::eq_int_type(traits_type::to_int_type(gptr()[-1]), c)))
+   {
+      // Just back up position
+      gbump(static_cast<int>(sizeof(char_type)) * -1);
+      return traits_type::not_eof(c);
+   }
+   else if (!is_open() || traits_type::eq_int_type(traits_type::eof(), c))
+   {
+      // No open QFile or EOF, fail
+      return traits_type::eof();
+   }
+   else if (gptr() != &p->putbackChar_)
+   {
+      // Put back to buffer
+      p->putbackChar_ = traits_type::to_char_type(c);
+      p->SetPutback();
+      return c;
+   }
+   else
+   {
+      // Nowhere to put back
+      return traits_type::eof();
+   }
+}
+
+QFileBuffer::pos_type QFileBuffer::seekoff(off_type               off,
+                                           std::ios_base::seekdir dir,
+                                           std::ios_base::openmode /* which */)
+{
+   pos_type newPos {pos_type(off_type(-1))};
+
+   switch (dir)
+   {
+   case std::ios_base::beg:
+      // Seek using absolute position
+      newPos = seekpos(off, std::ios_base::in);
+      break;
+
+   case std::ios_base::cur:
+   {
+      const pos_type currentPos {p->file_.pos() - (egptr() - gptr())};
+      pos_type       updatePos {currentPos + off};
+
+      // If the putback buffer is not empty, decrement the offset
+      if (gptr() == &p->putbackChar_)
+      {
+         updatePos -= static_cast<off_type>(sizeof(char_type));
+      }
+
+      // Seek the file
+      if (p->file_.seek(updatePos))
+      {
+         // Record updated position
+         newPos = updatePos;
+      }
+
+      break;
+   }
+
+   case std::ios_base::end:
+   {
+      const pos_type endPos {p->file_.size()};
+      const pos_type updatePos {endPos + off};
+
+      // Seek the file
+      if (p->file_.seek(updatePos))
+      {
+         // Record updated position
+         newPos = updatePos;
+      }
+
+      break;
+   }
+   }
+
+   if (newPos != static_cast<off_type>(-1))
+   {
+      p->ResetPutback();
+   }
+
+   return newPos;
+}
+
+QFileBuffer::pos_type QFileBuffer::seekpos(pos_type pos,
+                                           std::ios_base::openmode /* which */)
+{
+   pos_type newPos {pos_type(off_type(-1))};
+
+   // Seek the file
+   if (p->file_.seek(pos))
+   {
+      // Record updated position
+      newPos = pos;
+
+      p->ResetPutback();
+   }
+
+   return newPos;
+}
+
+QFileBuffer::int_type QFileBuffer::underflow()
+{
+   // This function is only called if gptr() == nullptr or gptr() > egptr()
+   // (i.e., all buffer data has been read)
+   int_type c;
+
+   if (gptr() && gptr() < egptr())
+   {
+      // Return buffered
+      return traits_type::to_int_type(*gptr());
+   }
+   else if (traits_type::eq_int_type(traits_type::eof(), c = uflow()))
+   {
+      // uflow failed, return EOF
+      return c;
+   }
+   else
+   {
+      // Get a character, don't point past it
+      pbackfail(c);
+      return c;
+   }
+}
+
+QFileBuffer::int_type QFileBuffer::uflow()
+{
+   if (gptr() && gptr() < egptr())
+   {
+      // Return buffered
+      int_type c = traits_type::to_int_type(*gptr());
+      gbump(sizeof(char_type));
+      return c;
+   }
+
+   if (!p->file_.isOpen())
+   {
+      // No open QFile, fail
+      return traits_type::eof();
+   }
+
+   p->ResetPutback();
+
+   // Read the next character
+   char_type c;
+   return p->file_.read(reinterpret_cast<char*>(&c), sizeof(char_type)) > 0 ?
+             traits_type::to_int_type(c) :
+             traits_type::eof();
+}
+
+std::streamsize QFileBuffer::xsgetn(char_type* s, std::streamsize count)
+{
+   // Read up to count bytes, forwarding the return value from QFile::read
+   // (return negative values as zero)
+   return std::max<std::streamsize>(
+      p->file_.read(reinterpret_cast<char*>(s), count * sizeof(char_type)), 0);
+}
+
+} // namespace util
+} // namespace qt
+} // namespace scwx

scwx-qt/source/scwx/qt/util/q_file_buffer.hpp

@@ -0,0 +1,191 @@
+#pragma once
+
+#include <streambuf>
+
+namespace scwx
+{
+namespace qt
+{
+namespace util
+{
+
+/**
+ * The QFileBuffer class is a std::streambuf interface to a QFile, allowing use
+ * with C++ stream-based I/O.
+ *
+ * QFileBuffer has read-only support. Locales are ignored, and no conversions
+ * are performed.
+ *
+ * Documentation for functions derived from
+ * https://en.cppreference.com/ SPDX-License-Identifier: CC BY-SA 3.0
+ */
+class QFileBuffer : public std::streambuf
+{
+public:
+   /**
+    * Constructs a new QFileBuffer object. The created object is not associated
+    * with a file, and is_open() returns false.
+    */
+   explicit QFileBuffer();
+
+   /**
+    * Constructs a new QFileBuffer object, then associated the object with a
+    * file by calling open(filename, mode). If the open call is successful,
+    * is_open() returns true.
+    */
+   explicit QFileBuffer(const std::string&      filename,
+                        std::ios_base::openmode mode = std::ios_base::in);
+   ~QFileBuffer();
+
+   QFileBuffer(const QFileBuffer&)            = delete;
+   QFileBuffer& operator=(const QFileBuffer&) = delete;
+
+   QFileBuffer(QFileBuffer&&) noexcept;
+   QFileBuffer& operator=(QFileBuffer&&) noexcept;
+
+   /**
+    * @brief Checks if the associated file is open
+    *
+    * Returns true if the most recent call to open() succeeded and there has
+    * been no call to close() since then.
+    *
+    * @return true if the associated file is open, false otherwise
+    */
+   bool is_open() const;
+
+   /**
+    * @brief Opens a file and configures it as the associated character sequence
+    *
+    * Opens the file with the given name. If the associated file was already
+    * open, returns a null pointer right away.
+    *
+    * @param filename The file name to open
+    * @param openmode The file opening mode, a binary OR of the std::ios_base
+    * modes
+    *
+    * @return this on success, a null pointer on failure
+    */
+   QFileBuffer* open(const std::string&      filename,
+                     std::ios_base::openmode mode = std::ios_base::in);
+
+   /**
+    * @brief Flushes the put area buffer and closes the associated file
+    *
+    * Closes the file, regardless of whether any of the preceding calls
+    * succeeded or failed.
+    *
+    * If any of the function calls made fails, returns a null pointer. If any of
+    * the function calls made throws an exception, the exception is caught and
+    * rethrown after closing. If the file is already closed, returns a null
+    * pointer right away.
+    *
+    * @return this on success, a null pointer on failure
+    */
+   QFileBuffer* close();
+
+protected:
+   /**
+    * @brief Backs out the input sequence to unget a character, not affecting
+    * the associated file
+    *
+    * @param c The character to put back, or Traits::eof() to indicate that
+    * backing up of the get area is requested
+    *
+    * @return c on success except if c was Traits::eof(), in which case
+    * Traits::not_eof(c) is returned. Traits::eof() on failure.
+    */
+   virtual int_type pbackfail(int_type c = traits_type::eof()) override;
+
+   /**
+    * @brief Repositions the file position, using relative addressing
+    *
+    * Sets the position indicator of the input and/or output sequence relative
+    * to some other position.
+    *
+    * @param off Relative position to set the position indicator to
+    * @param dir Defines base position to apply the relative offset to
+    * @param which Defines which of the input and/or output sequences to affect
+    *
+    * @return The resulting absolute position as defined by the position
+    * indicator.
+    */
+   virtual pos_type
+   seekoff(off_type                off,
+           std::ios_base::seekdir  dir,
+           std::ios_base::openmode which = std::ios_base::in |
+                                           std::ios_base::out) override;
+
+   /**
+    * @brief Repositions the file position, using absolute addressing
+    *
+    * Sets the position indicator of the input and/or output sequence to an
+    * absolute position.
+    *
+    * @param pos Absolute position to set the position indicator to
+    * @param which Defines which of the input and/or output sequences to affect
+    *
+    * @return The resulting absolute position as defined by the position
+    * indicator.
+    */
+   virtual pos_type
+   seekpos(pos_type                pos,
+           std::ios_base::openmode which = std::ios_base::in |
+                                           std::ios_base::out) override;
+
+   /**
+    * @brief Reads from the associated file
+    *
+    * Ensures that at least one character is available in the input area by
+    * updating the pointers to the input area (if needed) and reading more data
+    * in from the input sequence (if applicable). Returns the value of that
+    * character (converted to int_type with Traits::to_int_type(c)) on success
+    * or Traits::eof() on failure.
+    *
+    * The function may update gptr, egptr and eback pointers to define the
+    * location of newly loaded data (if any). On failure, the function ensures
+    * that either gptr() == nullptr or gptr() == egptr.
+    *
+    * @return The value of the character pointed to by the get pointer after the
+    * call on success, or Traits::eof() otherwise.
+    */
+   virtual int_type underflow() override;
+
+   /**
+    * @brief Reads from the associated file and advances the next pointer in the
+    * get area
+    *
+    * Behaves like the underflow(), except that if underflow() succeeds (does
+    * not return Traits::eof()), then advances the next pointer for the get
+    * area. In orther words, consumes one of the characters obtained by
+    * underflow().
+    *
+    * @return The value of the character that was read and consumed in case of
+    * success, or Traits::eof() in case of failure.
+    */
+   virtual int_type uflow() override;
+
+   /**
+    * @brief Reads multiple characters from the input sequence
+    *
+    * Reads count characters from the input sequence and stores them into a
+    * character array pointed to by s. The characters are read as if by repeated
+    * calls to sbumpc(). That is, if less than count characters are immediately
+    * available, the function calls uflow() to provide more until Traits::eof()
+    * is returned.
+    *
+    * @param s Pointer to the beginning of a char_type array
+    * @param count Maximum number of characters to read
+    *
+    * @return The number of characters successfully read. If it is less than
+    * count the input sequence has reached the end.
+    */
+   virtual std::streamsize xsgetn(char_type* s, std::streamsize count) override;
+
+private:
+   class Impl;
+   std::unique_ptr<Impl> p;
+};
+
+} // namespace util
+} // namespace qt
+} // namespace scwx