277 lines
11 KiB
C++
277 lines
11 KiB
C++
#ifndef BOOST_PYTHON_SLICE_JDB20040105_HPP
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#define BOOST_PYTHON_SLICE_JDB20040105_HPP
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// Copyright (c) 2004 Jonathan Brandmeyer
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// Use, modification and distribution are subject to the
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// Boost Software License, Version 1.0. (See accompanying file
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// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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#include <boost/python/detail/prefix.hpp>
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#include <boost/config.hpp>
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#include <boost/python/object.hpp>
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#include <boost/python/extract.hpp>
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#include <boost/python/converter/pytype_object_mgr_traits.hpp>
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#include <boost/iterator/iterator_traits.hpp>
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#include <iterator>
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#include <algorithm>
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namespace boost { namespace python {
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namespace detail
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{
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class BOOST_PYTHON_DECL slice_base : public object
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{
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public:
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// Get the Python objects associated with the slice. In principle, these
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// may be any arbitrary Python type, but in practice they are usually
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// integers. If one or more parameter is ommited in the Python expression
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// that created this slice, than that parameter is None here, and compares
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// equal to a default-constructed boost::python::object.
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// If a user-defined type wishes to support slicing, then support for the
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// special meaning associated with negative indices is up to the user.
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object start() const;
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object stop() const;
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object step() const;
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protected:
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explicit slice_base(PyObject*, PyObject*, PyObject*);
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BOOST_PYTHON_FORWARD_OBJECT_CONSTRUCTORS(slice_base, object)
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};
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}
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class slice : public detail::slice_base
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{
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typedef detail::slice_base base;
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public:
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// Equivalent to slice(::)
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slice() : base(0,0,0) {}
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// Each argument must be slice_nil, or implicitly convertable to object.
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// They should normally be integers.
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template<typename Integer1, typename Integer2>
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slice( Integer1 start, Integer2 stop)
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: base( object(start).ptr(), object(stop).ptr(), 0 )
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{}
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template<typename Integer1, typename Integer2, typename Integer3>
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slice( Integer1 start, Integer2 stop, Integer3 stride)
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: base( object(start).ptr(), object(stop).ptr(), object(stride).ptr() )
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{}
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// The following algorithm is intended to automate the process of
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// determining a slice range when you want to fully support negative
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// indices and non-singular step sizes. Its functionallity is simmilar to
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// PySlice_GetIndicesEx() in the Python/C API, but tailored for C++ users.
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// This template returns a slice::range struct that, when used in the
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// following iterative loop, will traverse a slice of the function's
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// arguments.
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// while (start != end) {
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// do_foo(...);
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// std::advance( start, step);
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// }
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// do_foo(...); // repeat exactly once more.
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// Arguments: a [begin, end) pair of STL-conforming random-access iterators.
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// Return: slice::range, where start and stop define a _closed_ interval
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// that covers at most [begin, end-1] of the provided arguments, and a step
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// that is non-zero.
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// Throws: error_already_set() if any of the indices are neither None nor
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// integers, or the slice has a step value of zero.
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// std::invalid_argument if the resulting range would be empty. Normally,
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// you should catch this exception and return an empty sequence of the
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// appropriate type.
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// Performance: constant time for random-access iterators.
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// Rationale:
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// closed-interval: If an open interval were used, then for a non-singular
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// value for step, the required state for the end iterator could be
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// beyond the one-past-the-end postion of the specified range. While
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// probably harmless, the behavior of STL-conforming iterators is
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// undefined in this case.
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// exceptions on zero-length range: It is impossible to define a closed
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// interval over an empty range, so some other form of error checking
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// would have to be used by the user to prevent undefined behavior. In
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// the case where the user fails to catch the exception, it will simply
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// be translated to Python by the default exception handling mechanisms.
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template<typename RandomAccessIterator>
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struct range
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{
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RandomAccessIterator start;
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RandomAccessIterator stop;
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typename iterator_difference<RandomAccessIterator>::type step;
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};
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template<typename RandomAccessIterator>
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slice::range<RandomAccessIterator>
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get_indices( const RandomAccessIterator& begin,
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const RandomAccessIterator& end) const
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{
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// This is based loosely on PySlice_GetIndicesEx(), but it has been
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// carefully crafted to ensure that these iterators never fall out of
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// the range of the container.
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slice::range<RandomAccessIterator> ret;
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typedef typename iterator_difference<RandomAccessIterator>::type difference_type;
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difference_type max_dist = std::distance(begin, end);
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object slice_start = this->start();
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object slice_stop = this->stop();
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object slice_step = this->step();
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// Extract the step.
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if (slice_step == object()) {
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ret.step = 1;
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}
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else {
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ret.step = extract<long>( slice_step);
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if (ret.step == 0) {
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PyErr_SetString( PyExc_IndexError, "step size cannot be zero.");
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throw_error_already_set();
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}
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}
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// Setup the start iterator.
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if (slice_start == object()) {
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if (ret.step < 0) {
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ret.start = end;
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--ret.start;
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}
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else
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ret.start = begin;
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}
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else {
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difference_type i = extract<long>( slice_start);
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if (i >= max_dist && ret.step > 0)
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throw std::invalid_argument( "Zero-length slice");
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if (i >= 0) {
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ret.start = begin;
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BOOST_USING_STD_MIN();
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std::advance( ret.start, min BOOST_PREVENT_MACRO_SUBSTITUTION(i, max_dist-1));
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}
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else {
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if (i < -max_dist && ret.step < 0)
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throw std::invalid_argument( "Zero-length slice");
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ret.start = end;
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// Advance start (towards begin) not farther than begin.
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std::advance( ret.start, (-i < max_dist) ? i : -max_dist );
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}
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}
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// Set up the stop iterator. This one is a little trickier since slices
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// define a [) range, and we are returning a [] range.
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if (slice_stop == object()) {
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if (ret.step < 0) {
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ret.stop = begin;
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}
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else {
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ret.stop = end;
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std::advance( ret.stop, -1);
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}
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}
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else {
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difference_type i = extract<long>(slice_stop);
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// First, branch on which direction we are going with this.
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if (ret.step < 0) {
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if (i+1 >= max_dist || i == -1)
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throw std::invalid_argument( "Zero-length slice");
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if (i >= 0) {
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ret.stop = begin;
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std::advance( ret.stop, i+1);
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}
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else { // i is negative, but more negative than -1.
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ret.stop = end;
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std::advance( ret.stop, (-i < max_dist) ? i : -max_dist);
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}
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}
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else { // stepping forward
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if (i == 0 || -i >= max_dist)
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throw std::invalid_argument( "Zero-length slice");
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if (i > 0) {
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ret.stop = begin;
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std::advance( ret.stop, (std::min)( i-1, max_dist-1));
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}
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else { // i is negative, but not more negative than -max_dist
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ret.stop = end;
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std::advance( ret.stop, i-1);
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}
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}
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}
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// Now the fun part, handling the possibilites surrounding step.
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// At this point, step has been initialized, ret.stop, and ret.step
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// represent the widest possible range that could be traveled
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// (inclusive), and final_dist is the maximum distance covered by the
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// slice.
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typename iterator_difference<RandomAccessIterator>::type final_dist =
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std::distance( ret.start, ret.stop);
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// First case, if both ret.start and ret.stop are equal, then step
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// is irrelevant and we can return here.
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if (final_dist == 0)
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return ret;
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// Second, if there is a sign mismatch, than the resulting range and
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// step size conflict: std::advance( ret.start, ret.step) goes away from
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// ret.stop.
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if ((final_dist > 0) != (ret.step > 0))
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throw std::invalid_argument( "Zero-length slice.");
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// Finally, if the last step puts us past the end, we move ret.stop
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// towards ret.start in the amount of the remainder.
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// I don't remember all of the oolies surrounding negative modulii,
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// so I am handling each of these cases separately.
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if (final_dist < 0) {
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difference_type remainder = -final_dist % -ret.step;
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std::advance( ret.stop, remainder);
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}
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else {
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difference_type remainder = final_dist % ret.step;
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std::advance( ret.stop, -remainder);
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}
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return ret;
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}
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// Incorrect spelling. DO NOT USE. Only here for backward compatibility.
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// Corrected 2011-06-14.
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template<typename RandomAccessIterator>
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slice::range<RandomAccessIterator>
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get_indicies( const RandomAccessIterator& begin,
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const RandomAccessIterator& end) const
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{
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return get_indices(begin, end);
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}
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public:
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// This declaration, in conjunction with the specialization of
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// object_manager_traits<> below, allows C++ functions accepting slice
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// arguments to be called from from Python. These constructors should never
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// be used in client code.
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BOOST_PYTHON_FORWARD_OBJECT_CONSTRUCTORS(slice, detail::slice_base)
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};
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namespace converter {
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template<>
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struct object_manager_traits<slice>
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: pytype_object_manager_traits<&PySlice_Type, slice>
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{
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};
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} // !namesapce converter
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} } // !namespace ::boost::python
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#endif // !defined BOOST_PYTHON_SLICE_JDB20040105_HPP
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