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