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 #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 indices 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
     // indices 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_indices( 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 = std::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 = 
             std::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;
     }
 
     // Incorrect spelling. DO NOT USE. Only here for backward compatibility.
     // Corrected 2011-06-14.
     template<typename RandomAccessIterator>
     slice::range<RandomAccessIterator>
     get_indicies( const RandomAccessIterator& begin, 
         const RandomAccessIterator& end) const
     {
         return get_indices(begin, end);
     }
         
  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

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