unroll

unroll.cpp

#include <vector>
#include <iostream>
#include <algorithm>

template<int N>
struct unroll
{
  template<class InputIterator, class OutputIterator, class Op>
  inline static OutputIterator 
  apply( InputIterator& i, OutputIterator o, Op f)
  {
    *o++ = f(*i++);
    return unroll<N-1>::apply(i,o,f);
  }
};

template<>
struct unroll<1>
{
  template<class InputIterator, class OutputIterator, class Op>
  inline static OutputIterator
  apply( InputIterator& i, OutputIterator& o, Op f)
  {
    *o++ = f(*i++);
    return o;
  }
};

template<int N, class InputIterator, class OutputIterator, class Op>
OutputIterator unroll_transform( InputIterator  b, InputIterator e
                               , OutputIterator o, Op f)
{
  while(b != e)
  {
    o = unroll<N>::apply(b,o,f);
  }
  return o;
}

struct op_
{
  op_(){};
  int operator()(int i) { return i*i; }
};

int main()
{
  std::vector<int> first;
  std::vector<int> second;
  std::vector<int>::iterator it,out;
  op_ f;
  // set some values:
  for(int i=0; i<16; i++) first.push_back(i*10); //  first: 10 20 30 40 50

  second.resize(first.size());     // allocate space
  std::cout << "Unroll\n";
  out = unroll_transform<2>(first.begin(), first.end(), second.end(), f);
  
  //for(it=second.begin(); it!=second.end(); ++it)
  //  std::cout << " " << *it;
}