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+:e4d67989cc36
+#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