#ifndef BOOST_PYTHON_SLICE_JDB20040105_HPP

 #define BOOST_PYTHON_SLICE_JDB20040105_HPP

 // Copyright (c) 2004 Jonathan Brandmeyer

 //  Use, modification and distribution are subject to the

 //  Boost Software License, Version 1.0. (See accompanying file 

 //  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)

 #include <boost/python/detail/prefix.hpp>

 #include <boost/config.hpp>

 #include <boost/python/object.hpp>

 #include <boost/python/extract.hpp>

 #include <boost/python/converter/pytype_object_mgr_traits.hpp>

 #include <boost/iterator/iterator_traits.hpp>

 #include <iterator>

 #include <algorithm>

 namespace boost { namespace python {

 namespace detail

 {

   class BOOST_PYTHON_DECL slice_base : public object

   {

    public:

       // Get the Python objects associated with the slice.  In principle, these 

       // may be any arbitrary Python type, but in practice they are usually 

       // integers.  If one or more parameter is ommited in the Python expression 

       // that created this slice, than that parameter is None here, and compares 

       // equal to a default-constructed boost::python::object.

       // If a user-defined type wishes to support slicing, then support for the 

       // special meaning associated with negative indicies is up to the user.

       object start() const;

       object stop() const;

       object step() const;

    protected:

       explicit slice_base(PyObject*, PyObject*, PyObject*);

       BOOST_PYTHON_FORWARD_OBJECT_CONSTRUCTORS(slice_base, object)

   };

 }

 class slice : public detail::slice_base

 {

     typedef detail::slice_base base;

  public:

     // Equivalent to slice(::)

     slice() : base(0,0,0) {}

     // Each argument must be slice_nil, or implicitly convertable to object.

     // They should normally be integers.

     template<typename Integer1, typename Integer2>

     slice( Integer1 start, Integer2 stop)

         : base( object(start).ptr(), object(stop).ptr(), 0 )

     {}

     template<typename Integer1, typename Integer2, typename Integer3>

     slice( Integer1 start, Integer2 stop, Integer3 stride)

         : base( object(start).ptr(), object(stop).ptr(), object(stride).ptr() )

     {}

     // The following algorithm is intended to automate the process of 

     // determining a slice range when you want to fully support negative

     // indicies and non-singular step sizes.  Its functionallity is simmilar to 

     // PySlice_GetIndicesEx() in the Python/C API, but tailored for C++ users.

     // This template returns a slice::range struct that, when used in the 

     // following iterative loop, will traverse a slice of the function's

     // arguments.

     // while (start != end) { 

     //     do_foo(...); 

     //     std::advance( start, step); 

     // }

     // do_foo(...); // repeat exactly once more.

     // Arguments: a [begin, end) pair of STL-conforming random-access iterators.

     // Return: slice::range, where start and stop define a _closed_ interval

     // that covers at most [begin, end-1] of the provided arguments, and a step 

     // that is non-zero.

     // Throws: error_already_set() if any of the indices are neither None nor 

     //   integers, or the slice has a step value of zero.

     // std::invalid_argument if the resulting range would be empty.  Normally, 

     //   you should catch this exception and return an empty sequence of the

     //   appropriate type.

     // Performance: constant time for random-access iterators.

     // Rationale: 

     //   closed-interval: If an open interval were used, then for a non-singular

     //     value for step, the required state for the end iterator could be 

     //     beyond the one-past-the-end postion of the specified range.  While 

     //     probably harmless, the behavior of STL-conforming iterators is 

     //     undefined in this case.

     //   exceptions on zero-length range: It is impossible to define a closed 

     //     interval over an empty range, so some other form of error checking 

     //     would have to be used by the user to prevent undefined behavior.  In

     //     the case where the user fails to catch the exception, it will simply

     //     be translated to Python by the default exception handling mechanisms.

     template<typename RandomAccessIterator>

     struct range

     {

         RandomAccessIterator start;

         RandomAccessIterator stop;

         typename iterator_difference<RandomAccessIterator>::type step;

     };

     template<typename RandomAccessIterator>

     slice::range<RandomAccessIterator>

     get_indicies( const RandomAccessIterator& begin, 

         const RandomAccessIterator& end) const

     {

         // This is based loosely on PySlice_GetIndicesEx(), but it has been 

         // carefully crafted to ensure that these iterators never fall out of

         // the range of the container.

         slice::range<RandomAccessIterator> ret;

         typedef typename iterator_difference<RandomAccessIterator>::type difference_type;

         difference_type max_dist = boost::detail::distance(begin, end);

         object slice_start = this->start();

         object slice_stop = this->stop();

         object slice_step = this->step();

         // Extract the step.

         if (slice_step == object()) {

             ret.step = 1;

         }

         else {

             ret.step = extract<long>( slice_step);

             if (ret.step == 0) {

                 PyErr_SetString( PyExc_IndexError, "step size cannot be zero.");

                 throw_error_already_set();

             }

         }

         // Setup the start iterator.

         if (slice_start == object()) {

             if (ret.step < 0) {

                 ret.start = end;

                 --ret.start;

             }

             else

                 ret.start = begin;

         }

         else {

             difference_type i = extract<long>( slice_start);

             if (i >= max_dist && ret.step > 0)

                     throw std::invalid_argument( "Zero-length slice");

             if (i >= 0) {

                 ret.start = begin;

                 BOOST_USING_STD_MIN();

                 std::advance( ret.start, min BOOST_PREVENT_MACRO_SUBSTITUTION(i, max_dist-1));

             }

             else {

                 if (i < -max_dist && ret.step < 0)

                     throw std::invalid_argument( "Zero-length slice");

                 ret.start = end;

                 // Advance start (towards begin) not farther than begin.

                 std::advance( ret.start, (-i < max_dist) ? i : -max_dist );

             }

         }

         // Set up the stop iterator.  This one is a little trickier since slices

         // define a [) range, and we are returning a [] range.

         if (slice_stop == object()) {

             if (ret.step < 0) {

                 ret.stop = begin;

             }

             else {

                 ret.stop = end;

                 std::advance( ret.stop, -1);

             }

         }

         else {

             difference_type i = extract<long>(slice_stop);

             // First, branch on which direction we are going with this.

             if (ret.step < 0) {

                 if (i+1 >= max_dist || i == -1)

                     throw std::invalid_argument( "Zero-length slice");

                 if (i >= 0) {

                     ret.stop = begin;

                     std::advance( ret.stop, i+1);

                 }

                 else { // i is negative, but more negative than -1.

                     ret.stop = end;

                     std::advance( ret.stop, (-i < max_dist) ? i : -max_dist);

                 }

             }

             else { // stepping forward

                 if (i == 0 || -i >= max_dist)

                     throw std::invalid_argument( "Zero-length slice");

                 if (i > 0) {

                     ret.stop = begin;

                     std::advance( ret.stop, (std::min)( i-1, max_dist-1));

                 }

                 else { // i is negative, but not more negative than -max_dist

                     ret.stop = end;

                     std::advance( ret.stop, i-1);

                 }

             }

         }

         // Now the fun part, handling the possibilites surrounding step.

         // At this point, step has been initialized, ret.stop, and ret.step

         // represent the widest possible range that could be traveled

         // (inclusive), and final_dist is the maximum distance covered by the

         // slice.

         typename iterator_difference<RandomAccessIterator>::type final_dist = 

             boost::detail::distance( ret.start, ret.stop);

         // First case, if both ret.start and ret.stop are equal, then step

         // is irrelevant and we can return here.

         if (final_dist == 0)

             return ret;

         // Second, if there is a sign mismatch, than the resulting range and 

         // step size conflict: std::advance( ret.start, ret.step) goes away from

         // ret.stop.

         if ((final_dist > 0) != (ret.step > 0))

             throw std::invalid_argument( "Zero-length slice.");

         // Finally, if the last step puts us past the end, we move ret.stop

         // towards ret.start in the amount of the remainder.

         // I don't remember all of the oolies surrounding negative modulii,

         // so I am handling each of these cases separately.

         if (final_dist < 0) {

             difference_type remainder = -final_dist % -ret.step;

             std::advance( ret.stop, remainder);

         }

         else {

             difference_type remainder = final_dist % ret.step;

             std::advance( ret.stop, -remainder);

         }

         return ret;

     }

  public:

     // This declaration, in conjunction with the specialization of 

     // object_manager_traits<> below, allows C++ functions accepting slice 

     // arguments to be called from from Python.  These constructors should never

     // be used in client code.

     BOOST_PYTHON_FORWARD_OBJECT_CONSTRUCTORS(slice, detail::slice_base)

 };

 namespace converter {

 template<>

 struct object_manager_traits<slice>

     : pytype_object_manager_traits<&PySlice_Type, slice>

 {

 };

 } // !namesapce converter

 } } // !namespace ::boost::python

 #endif // !defined BOOST_PYTHON_SLICE_JDB20040105_HPP