1 | #ifndef BOOST_PYTHON_SLICE_JDB20040105_HPP |
2 | #define BOOST_PYTHON_SLICE_JDB20040105_HPP |
3 | |
4 | // Copyright (c) 2004 Jonathan Brandmeyer |
5 | // Use, modification and distribution are subject to the |
6 | // Boost Software License, Version 1.0. (See accompanying file |
7 | // LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) |
8 | |
9 | #include <boost/python/detail/prefix.hpp> |
10 | #include <boost/config.hpp> |
11 | #include <boost/python/object.hpp> |
12 | #include <boost/python/extract.hpp> |
13 | #include <boost/python/converter/pytype_object_mgr_traits.hpp> |
14 | |
15 | #include <boost/iterator/iterator_traits.hpp> |
16 | |
17 | #include <iterator> |
18 | #include <algorithm> |
19 | |
20 | namespace boost { namespace python { |
21 | |
22 | namespace detail |
23 | { |
24 | class BOOST_PYTHON_DECL slice_base : public object |
25 | { |
26 | public: |
27 | // Get the Python objects associated with the slice. In principle, these |
28 | // may be any arbitrary Python type, but in practice they are usually |
29 | // integers. If one or more parameter is ommited in the Python expression |
30 | // that created this slice, than that parameter is None here, and compares |
31 | // equal to a default-constructed boost::python::object. |
32 | // If a user-defined type wishes to support slicing, then support for the |
33 | // special meaning associated with negative indices is up to the user. |
34 | object start() const; |
35 | object stop() const; |
36 | object step() const; |
37 | |
38 | protected: |
39 | explicit slice_base(PyObject*, PyObject*, PyObject*); |
40 | |
41 | BOOST_PYTHON_FORWARD_OBJECT_CONSTRUCTORS(slice_base, object) |
42 | }; |
43 | } |
44 | |
45 | class slice : public detail::slice_base |
46 | { |
47 | typedef detail::slice_base base; |
48 | public: |
49 | // Equivalent to slice(::) |
50 | slice() : base(0,0,0) {} |
51 | |
52 | // Each argument must be slice_nil, or implicitly convertable to object. |
53 | // They should normally be integers. |
54 | template<typename Integer1, typename Integer2> |
55 | slice( Integer1 start, Integer2 stop) |
56 | : base( object(start).ptr(), object(stop).ptr(), 0 ) |
57 | {} |
58 | |
59 | template<typename Integer1, typename Integer2, typename Integer3> |
60 | slice( Integer1 start, Integer2 stop, Integer3 stride) |
61 | : base( object(start).ptr(), object(stop).ptr(), object(stride).ptr() ) |
62 | {} |
63 | |
64 | // The following algorithm is intended to automate the process of |
65 | // determining a slice range when you want to fully support negative |
66 | // indices and non-singular step sizes. Its functionallity is simmilar to |
67 | // PySlice_GetIndicesEx() in the Python/C API, but tailored for C++ users. |
68 | // This template returns a slice::range struct that, when used in the |
69 | // following iterative loop, will traverse a slice of the function's |
70 | // arguments. |
71 | // while (start != end) { |
72 | // do_foo(...); |
73 | // std::advance( start, step); |
74 | // } |
75 | // do_foo(...); // repeat exactly once more. |
76 | |
77 | // Arguments: a [begin, end) pair of STL-conforming random-access iterators. |
78 | |
79 | // Return: slice::range, where start and stop define a _closed_ interval |
80 | // that covers at most [begin, end-1] of the provided arguments, and a step |
81 | // that is non-zero. |
82 | |
83 | // Throws: error_already_set() if any of the indices are neither None nor |
84 | // integers, or the slice has a step value of zero. |
85 | // std::invalid_argument if the resulting range would be empty. Normally, |
86 | // you should catch this exception and return an empty sequence of the |
87 | // appropriate type. |
88 | |
89 | // Performance: constant time for random-access iterators. |
90 | |
91 | // Rationale: |
92 | // closed-interval: If an open interval were used, then for a non-singular |
93 | // value for step, the required state for the end iterator could be |
94 | // beyond the one-past-the-end postion of the specified range. While |
95 | // probably harmless, the behavior of STL-conforming iterators is |
96 | // undefined in this case. |
97 | // exceptions on zero-length range: It is impossible to define a closed |
98 | // interval over an empty range, so some other form of error checking |
99 | // would have to be used by the user to prevent undefined behavior. In |
100 | // the case where the user fails to catch the exception, it will simply |
101 | // be translated to Python by the default exception handling mechanisms. |
102 | |
103 | template<typename RandomAccessIterator> |
104 | struct range |
105 | { |
106 | RandomAccessIterator start; |
107 | RandomAccessIterator stop; |
108 | typename iterator_difference<RandomAccessIterator>::type step; |
109 | }; |
110 | |
111 | template<typename RandomAccessIterator> |
112 | slice::range<RandomAccessIterator> |
113 | get_indices( const RandomAccessIterator& begin, |
114 | const RandomAccessIterator& end) const |
115 | { |
116 | // This is based loosely on PySlice_GetIndicesEx(), but it has been |
117 | // carefully crafted to ensure that these iterators never fall out of |
118 | // the range of the container. |
119 | slice::range<RandomAccessIterator> ret; |
120 | |
121 | typedef typename iterator_difference<RandomAccessIterator>::type difference_type; |
122 | difference_type max_dist = boost::detail::distance(begin, end); |
123 | |
124 | object slice_start = this->start(); |
125 | object slice_stop = this->stop(); |
126 | object slice_step = this->step(); |
127 | |
128 | // Extract the step. |
129 | if (slice_step == object()) { |
130 | ret.step = 1; |
131 | } |
132 | else { |
133 | ret.step = extract<long>( slice_step); |
134 | if (ret.step == 0) { |
135 | PyErr_SetString( PyExc_IndexError, "step size cannot be zero." ); |
136 | throw_error_already_set(); |
137 | } |
138 | } |
139 | |
140 | // Setup the start iterator. |
141 | if (slice_start == object()) { |
142 | if (ret.step < 0) { |
143 | ret.start = end; |
144 | --ret.start; |
145 | } |
146 | else |
147 | ret.start = begin; |
148 | } |
149 | else { |
150 | difference_type i = extract<long>( slice_start); |
151 | if (i >= max_dist && ret.step > 0) |
152 | throw std::invalid_argument( "Zero-length slice" ); |
153 | if (i >= 0) { |
154 | ret.start = begin; |
155 | BOOST_USING_STD_MIN(); |
156 | std::advance( ret.start, min BOOST_PREVENT_MACRO_SUBSTITUTION(i, max_dist-1)); |
157 | } |
158 | else { |
159 | if (i < -max_dist && ret.step < 0) |
160 | throw std::invalid_argument( "Zero-length slice" ); |
161 | ret.start = end; |
162 | // Advance start (towards begin) not farther than begin. |
163 | std::advance( ret.start, (-i < max_dist) ? i : -max_dist ); |
164 | } |
165 | } |
166 | |
167 | // Set up the stop iterator. This one is a little trickier since slices |
168 | // define a [) range, and we are returning a [] range. |
169 | if (slice_stop == object()) { |
170 | if (ret.step < 0) { |
171 | ret.stop = begin; |
172 | } |
173 | else { |
174 | ret.stop = end; |
175 | std::advance( ret.stop, -1); |
176 | } |
177 | } |
178 | else { |
179 | difference_type i = extract<long>(slice_stop); |
180 | // First, branch on which direction we are going with this. |
181 | if (ret.step < 0) { |
182 | if (i+1 >= max_dist || i == -1) |
183 | throw std::invalid_argument( "Zero-length slice" ); |
184 | |
185 | if (i >= 0) { |
186 | ret.stop = begin; |
187 | std::advance( ret.stop, i+1); |
188 | } |
189 | else { // i is negative, but more negative than -1. |
190 | ret.stop = end; |
191 | std::advance( ret.stop, (-i < max_dist) ? i : -max_dist); |
192 | } |
193 | } |
194 | else { // stepping forward |
195 | if (i == 0 || -i >= max_dist) |
196 | throw std::invalid_argument( "Zero-length slice" ); |
197 | |
198 | if (i > 0) { |
199 | ret.stop = begin; |
200 | std::advance( ret.stop, (std::min)( i-1, max_dist-1)); |
201 | } |
202 | else { // i is negative, but not more negative than -max_dist |
203 | ret.stop = end; |
204 | std::advance( ret.stop, i-1); |
205 | } |
206 | } |
207 | } |
208 | |
209 | // Now the fun part, handling the possibilites surrounding step. |
210 | // At this point, step has been initialized, ret.stop, and ret.step |
211 | // represent the widest possible range that could be traveled |
212 | // (inclusive), and final_dist is the maximum distance covered by the |
213 | // slice. |
214 | typename iterator_difference<RandomAccessIterator>::type final_dist = |
215 | boost::detail::distance( ret.start, ret.stop); |
216 | |
217 | // First case, if both ret.start and ret.stop are equal, then step |
218 | // is irrelevant and we can return here. |
219 | if (final_dist == 0) |
220 | return ret; |
221 | |
222 | // Second, if there is a sign mismatch, than the resulting range and |
223 | // step size conflict: std::advance( ret.start, ret.step) goes away from |
224 | // ret.stop. |
225 | if ((final_dist > 0) != (ret.step > 0)) |
226 | throw std::invalid_argument( "Zero-length slice." ); |
227 | |
228 | // Finally, if the last step puts us past the end, we move ret.stop |
229 | // towards ret.start in the amount of the remainder. |
230 | // I don't remember all of the oolies surrounding negative modulii, |
231 | // so I am handling each of these cases separately. |
232 | if (final_dist < 0) { |
233 | difference_type remainder = -final_dist % -ret.step; |
234 | std::advance( ret.stop, remainder); |
235 | } |
236 | else { |
237 | difference_type remainder = final_dist % ret.step; |
238 | std::advance( ret.stop, -remainder); |
239 | } |
240 | |
241 | return ret; |
242 | } |
243 | |
244 | // Incorrect spelling. DO NOT USE. Only here for backward compatibility. |
245 | // Corrected 2011-06-14. |
246 | template<typename RandomAccessIterator> |
247 | slice::range<RandomAccessIterator> |
248 | get_indicies( const RandomAccessIterator& begin, |
249 | const RandomAccessIterator& end) const |
250 | { |
251 | return get_indices(begin, end); |
252 | } |
253 | |
254 | public: |
255 | // This declaration, in conjunction with the specialization of |
256 | // object_manager_traits<> below, allows C++ functions accepting slice |
257 | // arguments to be called from from Python. These constructors should never |
258 | // be used in client code. |
259 | BOOST_PYTHON_FORWARD_OBJECT_CONSTRUCTORS(slice, detail::slice_base) |
260 | }; |
261 | |
262 | |
263 | namespace converter { |
264 | |
265 | template<> |
266 | struct object_manager_traits<slice> |
267 | : pytype_object_manager_traits<&PySlice_Type, slice> |
268 | { |
269 | }; |
270 | |
271 | } // !namesapce converter |
272 | |
273 | } } // !namespace ::boost::python |
274 | |
275 | |
276 | #endif // !defined BOOST_PYTHON_SLICE_JDB20040105_HPP |
277 | |