endian/example/use_cases.cpp

//  endian/example/uses_cases.cpp  -----------------------------------------------------//

//  Copyright Beman Dawes 2014

//  Distributed under the Boost Software License, Version 1.0.
//  See http://www.boost.org/LICENSE_1_0.txt

//--------------------------------------------------------------------------------------//

#define _SCL_SECURE_NO_WARNINGS

#include <boost/endian/conversion.hpp>
#include <boost/endian/buffers.hpp>
#include <boost/endian/arithmetic.hpp>
#include <iostream>

//  Maximize chance of name clashes for testing purposes
using namespace boost::endian;

using std::cout;
using std::endl;


void read(void* data, std::size_t sz);         // for exposition
void write(const void* data, std::size_t sz);  // for exposition
const int32_t fee(100);                        // for exposition

int main(int, char *[])
{
  {
    // Q: Should endian_buffer supply "value_type operator value_type() const noexcept"?
    // A: No. The whole point of the endian_buffers is to prevent high-cost hidden
    //    conversions. If an implicit conversion operator is supplied, hidden conversions
    //    can occur.

    big_buf32_t v(5);
    int32_t x;
    x  = v * v;    // error: operator not defined & no conversion available
    x = v.value() * v.value();  // OK, conversion visable. "cvt" or "native" better name?
 
  }

  { // Use case 1 - Conversion functions

    struct Record
    {
      uint32_t count;  // big endian
      int32_t  value;  // big endian
    };

    Record rec;

    read(&rec, sizeof(Record));

    uint32_t count = big(rec.count);
    int32_t value = big(rec.value);

    ++count;
    value += fee;

    rec.count = big(count);
    rec.value = big(value);

    write(&rec, sizeof(Record));
  }

  { // Use case 2 - Endian buffer types

    struct Record
    {
      big_ubuf32_t count;  // big endian
      big_buf32_t  value;  // big endian
    };

    Record rec;

    read(&rec, sizeof(Record));

    uint32_t count = rec.count.value();
    int32_t value = rec.value.value();

    ++count;
    value += fee;

    rec.count = count;
    rec.value = value;

    write(&rec, sizeof(Record));
  }

  { // Use case 3a - Endian arithmetic types

    struct Record
    {
      big_uint32_t count;  // big endian
      big_int32_t  value;  // big endian
    };

    Record rec;

    read(&rec, sizeof(Record));

    ++rec.count;
    rec.value += fee;

    write(&rec, sizeof(Record));
  }

  { // Use case 3b - Endian arithmetic types

    struct Record
    {
      big_uint32_t count;  // big endian
      big_int32_t  value;  // big endian
    };

    Record rec;

    read(&rec, sizeof(Record));

    uint32_t count = rec.count;
    int32_t value = rec.value;

    ++count;
    value += fee;

    rec.count = count;
    rec.value = value;

    write(&rec, sizeof(Record));
  }

  //  Recommended approach when conversion time is not a concern
  //
  //  Conversion time is not a concert with this application because the minimum
  //  possible number of conversions is performed and because I/O time will be
  //  much greater than conversion time.

  {
    struct Record
    {
      big_uint32_t count;  // big endian
      big_int32_t  value;  // big endian
    };

    Record rec;

    read(&rec, sizeof(Record));

    ++rec.count;
    rec.value += fee;

    write(&rec, sizeof(Record));
  }

//  Recommended approach when conversion time is a concern
  //
  //  Conversion time is a concert with this application because (1) any conversions
  //  performed in the loop will consume a great deal of time and because (2) 
  //  computation time will be much greater than I/O time.

  {
    struct Record
    {
      big_uint32_t count;  // big endian
      big_int32_t  value;  // big endian
    };

    Record rec;

    read(&rec, sizeof(Record));

    uint32_t count = rec.count;
    int32_t value = rec.value;

    for (long long i = 0; i < several_gazillion; ++i)  // (1)
    {
      ... immensely complex computation using rec variables many times // (2)
    }

    rec.count = count;
    rec.value = value;

    write(&rec, sizeof(Record));
  }

}