99 lines
2.9 KiB
C++
99 lines
2.9 KiB
C++
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// Copyright (c) Microsoft Open Technologies, Inc. All rights reserved. See License.txt in the project root for license information.
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#if !defined(CPPLINQ_LINQ_TAKE_HPP)
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#define CPPLINQ_LINQ_TAKE_HPP
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#pragma once
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#include <cstddef>
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namespace cpplinq
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{
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template <class InnerCursor>
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struct linq_take_cursor
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{
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typedef typename InnerCursor::element_type element_type;
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typedef typename InnerCursor::reference_type reference_type;
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typedef typename InnerCursor::cursor_category cursor_category;
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linq_take_cursor(const InnerCursor& cur, std::size_t rem) : cur(cur), rem(rem) {}
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void forget() { cur.forget(); }
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bool empty() const { return cur.empty() || rem == 0; }
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void inc() { cur.inc(); --rem; }
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reference_type get() const { return cur.get(); }
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bool atbegin() const { return cur.atbegin(); }
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void dec() { cur.dec(); --rem; }
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void skip(std::size_t n) { cur.skip(n); rem -= n; }
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std::size_t position() const { return cur.position(); }
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std::size_t size() const { return cur.size(); }
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private:
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InnerCursor cur;
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std::size_t rem;
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};
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namespace detail {
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template <class Collection>
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linq_take_cursor<typename Collection::cursor>
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take_get_cursor_(
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const Collection& c,
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std::size_t n,
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onepass_cursor_tag
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)
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{
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return linq_take_cursor<typename Collection::cursor>(c.get_cursor(), n);
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}
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template <class Collection>
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typename Collection::cursor
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take_get_cursor_(
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const Collection& c,
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std::size_t n,
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random_access_cursor_tag
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)
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{
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auto cur = c.get_cursor();
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if (cur.size() > n) {
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cur.truncate(n);
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}
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return cur;
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}
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}
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template <class Collection>
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struct linq_take
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{
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typedef typename std::conditional<
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util::less_or_equal_cursor_category<
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random_access_cursor_tag,
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typename Collection::cursor::cursor_category>::value,
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typename Collection::cursor,
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linq_take_cursor<typename Collection::cursor>>::type
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cursor;
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linq_take(const Collection& c, std::size_t n) : c(c), n(n) {}
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cursor get_cursor() const {
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return detail::take_get_cursor_(c, n, typename Collection::cursor::cursor_category());
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}
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Collection c;
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std::size_t n;
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};
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template <class Collection>
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auto get_cursor(
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const linq_take<Collection>& take
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)
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-> decltype(get_cursor_(take, typename Collection::cursor::cursor_category()))
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{
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return get_cursor_(take, typename Collection::cursor::cursor_category());
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}
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}
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#endif // !defined(CPPLINQ_LINQ_TAKE_HPP)
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