[libc++] Implement [P0769] "Add shift to algorithm" (shift_left, shift_right)
I believe this is a complete implementation of std::shift_left and std::shift_right from
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0769r2.pdf
Some test cases copied-with-modification from D60027.
Differential Revision: https://reviews.llvm.org/D93819
GitOrigin-RevId: 3fbd3eaf28c1e6f2bb9519022611829dfe3b0464
diff --git a/include/algorithm b/include/algorithm
index da55e5e..77711d2 100644
--- a/include/algorithm
+++ b/include/algorithm
@@ -301,6 +301,16 @@
void shuffle(RandomAccessIterator first, RandomAccessIterator last,
UniformRandomNumberGenerator&& g);
+template<class ForwardIterator>
+ constexpr ForwardIterator
+ shift_left(ForwardIterator first, ForwardIterator last,
+ typename iterator_traits<ForwardIterator>::difference_type n); // C++20
+
+template<class ForwardIterator>
+ constexpr ForwardIterator
+ shift_right(ForwardIterator first, ForwardIterator last,
+ typename iterator_traits<ForwardIterator>::difference_type n); // C++20
+
template <class InputIterator, class Predicate>
constexpr bool // constexpr in C++20
is_partitioned(InputIterator first, InputIterator last, Predicate pred);
@@ -3259,6 +3269,111 @@
}
}
+#if _LIBCPP_STD_VER > 17
+
+// shift_left, shift_right
+
+template <class _ForwardIterator>
+inline _LIBCPP_INLINE_VISIBILITY constexpr
+_ForwardIterator
+shift_left(_ForwardIterator __first, _ForwardIterator __last,
+ typename iterator_traits<_ForwardIterator>::difference_type __n)
+{
+ if (__n == 0) {
+ return __last;
+ }
+
+ _ForwardIterator __m = __first;
+ if constexpr (__is_cpp17_random_access_iterator<_ForwardIterator>::value) {
+ if (__n >= __last - __first) {
+ return __first;
+ }
+ __m += __n;
+ } else {
+ for (; __n > 0; --__n) {
+ if (__m == __last) {
+ return __first;
+ }
+ ++__m;
+ }
+ }
+ return _VSTD::move(__m, __last, __first);
+}
+
+template <class _ForwardIterator>
+inline _LIBCPP_INLINE_VISIBILITY constexpr
+_ForwardIterator
+shift_right(_ForwardIterator __first, _ForwardIterator __last,
+ typename iterator_traits<_ForwardIterator>::difference_type __n)
+{
+ if (__n == 0) {
+ return __first;
+ }
+
+ if constexpr (__is_cpp17_random_access_iterator<_ForwardIterator>::value) {
+ decltype(__n) __d = __last - __first;
+ if (__n >= __d) {
+ return __last;
+ }
+ _ForwardIterator __m = __first + (__d - __n);
+ return _VSTD::move_backward(__first, __m, __last);
+ } else if constexpr (__is_cpp17_bidirectional_iterator<_ForwardIterator>::value) {
+ _ForwardIterator __m = __last;
+ for (; __n > 0; --__n) {
+ if (__m == __first) {
+ return __last;
+ }
+ --__m;
+ }
+ return _VSTD::move_backward(__first, __m, __last);
+ } else {
+ _ForwardIterator __ret = __first;
+ for (; __n > 0; --__n) {
+ if (__ret == __last) {
+ return __last;
+ }
+ ++__ret;
+ }
+
+ // We have an __n-element scratch space from __first to __ret.
+ // Slide an __n-element window [__trail, __lead) from left to right.
+ // We're essentially doing swap_ranges(__first, __ret, __trail, __lead)
+ // over and over; but once __lead reaches __last we needn't bother
+ // to save the values of elements [__trail, __last).
+
+ auto __trail = __first;
+ auto __lead = __ret;
+ while (__trail != __ret) {
+ if (__lead == __last) {
+ _VSTD::move(__first, __trail, __ret);
+ return __ret;
+ }
+ ++__trail;
+ ++__lead;
+ }
+
+ _ForwardIterator __mid = __first;
+ while (true) {
+ if (__lead == __last) {
+ __trail = _VSTD::move(__mid, __ret, __trail);
+ _VSTD::move(__first, __mid, __trail);
+ return __ret;
+ }
+ swap(*__mid, *__trail);
+ ++__mid;
+ ++__trail;
+ ++__lead;
+ if (__mid == __ret) {
+ __mid = __first;
+ }
+ }
+ }
+}
+
+#endif // _LIBCPP_STD_VER > 17
+
+// is_partitioned
+
template <class _InputIterator, class _Predicate>
_LIBCPP_NODISCARD_EXT _LIBCPP_CONSTEXPR_AFTER_CXX17 bool
is_partitioned(_InputIterator __first, _InputIterator __last, _Predicate __pred)