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 // -*- mode: c++ -*-

 // Copyright (c) 2010, Google Inc.

 // All rights reserved.

 //

 // Redistribution and use in source and binary forms, with or without

 // modification, are permitted provided that the following conditions are

 // met:

 //

 //     * Redistributions of source code must retain the above copyright

 // notice, this list of conditions and the following disclaimer.

 //     * Redistributions in binary form must reproduce the above

 // copyright notice, this list of conditions and the following disclaimer

 // in the documentation and/or other materials provided with the

 // distribution.

 //     * Neither the name of Google Inc. nor the names of its

 // contributors may be used to endorse or promote products derived from

 // this software without specific prior written permission.

 //

 // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS

 // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT

 // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR

 // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT

 // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,

 // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT

 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,

 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY

 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT

 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE

 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

 // Original author: Jim Blandy <jimb@mozilla.com> <jimb@red-bean.com>

 // byte_cursor.h: Classes for parsing values from a buffer of bytes.

 // The ByteCursor class provides a convenient interface for reading

 // fixed-size integers of arbitrary endianness, being thorough about

 // checking for buffer overruns.

 #ifndef COMMON_BYTE_CURSOR_H_

 #define COMMON_BYTE_CURSOR_H_

 #include <assert.h>

 #include <stdint.h>

 #include <stdlib.h>

 #include <string.h>

 #include <string>

 #include "common/using_std_string.h"

 namespace google_breakpad {

 // A buffer holding a series of bytes.

 struct ByteBuffer {

   ByteBuffer() : start(0), end(0) { }

   ByteBuffer(const uint8_t *set_start, size_t set_size)

       : start(set_start), end(set_start + set_size) { }

   ~ByteBuffer() { };

   // Equality operators. Useful in unit tests, and when we're using

   // ByteBuffers to refer to regions of a larger buffer.

   bool operator==(const ByteBuffer &that) const {

     return start == that.start && end == that.end;

   }

   bool operator!=(const ByteBuffer &that) const {

     return start != that.start || end != that.end;

   }

   // Not C++ style guide compliant, but this definitely belongs here.

   size_t Size() const {

     assert(start <= end);

     return end - start;

   }

   const uint8_t *start, *end;

 };

 // A cursor pointing into a ByteBuffer that can parse numbers of various

 // widths and representations, strings, and data blocks, advancing through

 // the buffer as it goes. All ByteCursor operations check that accesses

 // haven't gone beyond the end of the enclosing ByteBuffer.

 class ByteCursor {

  public:

   // Create a cursor reading bytes from the start of BUFFER. By default, the

   // cursor reads multi-byte values in little-endian form.

   ByteCursor(const ByteBuffer *buffer, bool big_endian = false)

       : buffer_(buffer), here_(buffer->start), 

         big_endian_(big_endian), complete_(true) { }

   // Accessor and setter for this cursor's endianness flag.

   bool big_endian() const { return big_endian_; }

   void set_big_endian(bool big_endian) { big_endian_ = big_endian; }

   // Accessor and setter for this cursor's current position. The setter

   // returns a reference to this cursor.

   const uint8_t *here() const { return here_; }

   ByteCursor &set_here(const uint8_t *here) {

     assert(buffer_->start <= here && here <= buffer_->end);

     here_ = here;

     return *this;

   }

   // Return the number of bytes available to read at the cursor.

   size_t Available() const { return size_t(buffer_->end - here_); }

   // Return true if this cursor is at the end of its buffer.

   bool AtEnd() const { return Available() == 0; }

   // When used as a boolean value this cursor converts to true if all

   // prior reads have been completed, or false if we ran off the end

   // of the buffer.

   operator bool() const { return complete_; }

   // Read a SIZE-byte integer at this cursor, signed if IS_SIGNED is true,

   // unsigned otherwise, using the cursor's established endianness, and set

   // *RESULT to the number. If we read off the end of our buffer, clear

   // this cursor's complete_ flag, and store a dummy value in *RESULT.

   // Return a reference to this cursor.

   template<typename T>

   ByteCursor &Read(size_t size, bool is_signed, T *result) {

     if (CheckAvailable(size)) {

       T v = 0;

       if (big_endian_) {

         for (size_t i = 0; i < size; i++)

           v = (v << 8) + here_[i];

       } else {

         // This loop condition looks weird, but size_t is unsigned, so

         // decrementing i after it is zero yields the largest size_t value.

         for (size_t i = size - 1; i < size; i--)

           v = (v << 8) + here_[i];

       }

       if (is_signed && size < sizeof(T)) {

         size_t sign_bit = (T)1 << (size * 8 - 1);

         v = (v ^ sign_bit) - sign_bit;

       }

       here_ += size;

       *result = v;

     } else {

       *result = (T) 0xdeadbeef;

     }

     return *this;

   }

   // Read an integer, using the cursor's established endianness and

   // *RESULT's size and signedness, and set *RESULT to the number. If we

   // read off the end of our buffer, clear this cursor's complete_ flag.

   // Return a reference to this cursor.

   template<typename T>

   ByteCursor &operator>>(T &result) {

     bool T_is_signed = (T)-1 < 0;

     return Read(sizeof(T), T_is_signed, &result); 

   }

   // Copy the SIZE bytes at the cursor to BUFFER, and advance this

   // cursor to the end of them. If we read off the end of our buffer,

   // clear this cursor's complete_ flag, and set *POINTER to NULL.

   // Return a reference to this cursor.

   ByteCursor &Read(uint8_t *buffer, size_t size) {

     if (CheckAvailable(size)) {

       memcpy(buffer, here_, size);

       here_ += size;

     }

     return *this;

   }

   // Set STR to a copy of the '\0'-terminated string at the cursor. If the

   // byte buffer does not contain a terminating zero, clear this cursor's

   // complete_ flag, and set STR to the empty string. Return a reference to

   // this cursor.

   ByteCursor &CString(string *str) {

     const uint8_t *end

       = static_cast<const uint8_t *>(memchr(here_, '\0', Available()));

     if (end) {

       str->assign(reinterpret_cast<const char *>(here_), end - here_);

       here_ = end + 1;

     } else {

       str->clear();

       here_ = buffer_->end;

       complete_ = false;

     }

     return *this;

   }

   // Like CString(STR), but extract the string from a fixed-width buffer

   // LIMIT bytes long, which may or may not contain a terminating '\0'

   // byte. Specifically:

   //

   // - If there are not LIMIT bytes available at the cursor, clear the

   //   cursor's complete_ flag and set STR to the empty string.

   //

   // - Otherwise, if the LIMIT bytes at the cursor contain any '\0'

   //   characters, set *STR to a copy of the bytes before the first '\0',

   //   and advance the cursor by LIMIT bytes.

   //   

   // - Otherwise, set *STR to a copy of those LIMIT bytes, and advance the

   //   cursor by LIMIT bytes.

   ByteCursor &CString(string *str, size_t limit) {

     if (CheckAvailable(limit)) {

       const uint8_t *end

         = static_cast<const uint8_t *>(memchr(here_, '\0', limit));

       if (end)

         str->assign(reinterpret_cast<const char *>(here_), end - here_);

       else

         str->assign(reinterpret_cast<const char *>(here_), limit);

       here_ += limit;

     } else {

       str->clear();

     }

     return *this;

   }

   // Set *POINTER to point to the SIZE bytes at the cursor, and advance

   // this cursor to the end of them. If SIZE is omitted, don't move the

   // cursor. If we read off the end of our buffer, clear this cursor's

   // complete_ flag, and set *POINTER to NULL. Return a reference to this

   // cursor.

   ByteCursor &PointTo(const uint8_t **pointer, size_t size = 0) {

     if (CheckAvailable(size)) {

       *pointer = here_;

       here_ += size;

     } else {

       *pointer = NULL;

     }

     return *this;

   }

   // Skip SIZE bytes at the cursor. If doing so would advance us off

   // the end of our buffer, clear this cursor's complete_ flag, and

   // set *POINTER to NULL. Return a reference to this cursor.

   ByteCursor &Skip(size_t size) {

     if (CheckAvailable(size))

       here_ += size;

     return *this;

   }

  private:

   // If there are at least SIZE bytes available to read from the buffer,

   // return true. Otherwise, set here_ to the end of the buffer, set

   // complete_ to false, and return false.

   bool CheckAvailable(size_t size) {

     if (Available() >= size) {

       return true;

     } else {

       here_ = buffer_->end;

       complete_ = false;

       return false;

     }

   }

   // The buffer we're reading bytes from.

   const ByteBuffer *buffer_;

   // The next byte within buffer_ that we'll read.

   const uint8_t *here_;

   // True if we should read numbers in big-endian form; false if we

   // should read in little-endian form.

   bool big_endian_;

   // True if we've been able to read all we've been asked to.

   bool complete_;

 };

 }  // namespace google_breakpad

 #endif  // COMMON_BYTE_CURSOR_H_