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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