1 /* 2 * Copyright 2019 Google Inc. 3 * 4 * Use of this source code is governed by a BSD-style license that can be 5 * found in the LICENSE file. 6 */ 7 8 #ifndef SkEnumerate_DEFINED 9 #define SkEnumerate_DEFINED 10 11 #include <cstddef> 12 #include <iterator> 13 #include <tuple> 14 #include <variant> 15 16 template <typename Iter, typename C = std::monostate> 17 class SkEnumerate { 18 using Captured = decltype(*std::declval<Iter>()); 19 template <typename> struct is_tuple : std::false_type {}; 20 template <typename... T> struct is_tuple<std::tuple<T...>> : std::true_type {}; 21 22 // v must be a r-value to bind to temporary non-const references. 23 static constexpr auto MakeResult(size_t i, Captured&& v) { 24 if constexpr (is_tuple<Captured>::value) { 25 return std::tuple_cat(std::tuple<size_t>{i}, v); 26 } else { 27 // Capture v by reference instead of by value by using std::tie. 28 return std::tuple_cat(std::tuple<size_t>{i}, std::tie(v)); 29 } 30 } 31 32 using Result = decltype(MakeResult(0, std::declval<Captured>())); 33 34 class Iterator { 35 public: 36 using value_type = Result; 37 using difference_type = ptrdiff_t; 38 using pointer = value_type*; 39 using reference = value_type; 40 using iterator_category = std::input_iterator_tag; 41 constexpr Iterator(ptrdiff_t index, Iter it) : fIndex{index}, fIt{it} { } 42 constexpr Iterator(const Iterator&) = default; 43 constexpr Iterator operator++() { ++fIndex; ++fIt; return *this; } 44 constexpr Iterator operator++(int) { Iterator tmp(*this); operator++(); return tmp; } 45 constexpr bool operator==(const Iterator& rhs) const { return fIt == rhs.fIt; } 46 constexpr bool operator!=(const Iterator& rhs) const { return fIt != rhs.fIt; } 47 constexpr reference operator*() { return MakeResult(fIndex, *fIt); } 48 49 private: 50 ptrdiff_t fIndex; 51 Iter fIt; 52 }; 53 54 public: 55 constexpr SkEnumerate(Iter begin, Iter end) : SkEnumerate{0, begin, end} {} 56 explicit constexpr SkEnumerate(C&& c) 57 : fCollection{std::move(c)} 58 , fBeginIndex{0} 59 , fBegin{std::begin(fCollection)} 60 , fEnd{std::end(fCollection)} { } 61 constexpr SkEnumerate(const SkEnumerate& that) = default; 62 constexpr SkEnumerate& operator=(const SkEnumerate& that) { 63 fBegin = that.fBegin; 64 fEnd = that.fEnd; 65 return *this; 66 } 67 constexpr Iterator begin() const { return Iterator{fBeginIndex, fBegin}; } 68 constexpr Iterator end() const { return Iterator{fBeginIndex + this->ssize(), fEnd}; } 69 constexpr bool empty() const { return fBegin == fEnd; } 70 constexpr size_t size() const { return std::distance(fBegin, fEnd); } 71 constexpr ptrdiff_t ssize() const { return std::distance(fBegin, fEnd); } 72 constexpr SkEnumerate first(size_t n) { 73 SkASSERT(n <= this->size()); 74 ptrdiff_t deltaEnd = this->ssize() - n; 75 return SkEnumerate{fBeginIndex, fBegin, std::prev(fEnd, deltaEnd)}; 76 } 77 constexpr SkEnumerate last(size_t n) { 78 SkASSERT(n <= this->size()); 79 ptrdiff_t deltaBegin = this->ssize() - n; 80 return SkEnumerate{fBeginIndex + deltaBegin, std::next(fBegin, deltaBegin), fEnd}; 81 } 82 constexpr SkEnumerate subspan(size_t offset, size_t count) { 83 SkASSERT(offset < this->size()); 84 SkASSERT(count <= this->size() - offset); 85 auto newBegin = std::next(fBegin, offset); 86 return SkEnumerate(fBeginIndex + offset, newBegin, std::next(newBegin, count)); 87 } 88 89 private: 90 constexpr SkEnumerate(ptrdiff_t beginIndex, Iter begin, Iter end) 91 : fBeginIndex{beginIndex} 92 , fBegin(begin) 93 , fEnd(end) {} 94 95 C fCollection; 96 const ptrdiff_t fBeginIndex; 97 Iter fBegin; 98 Iter fEnd; 99 }; 100 101 template <typename C, typename Iter = decltype(std::begin(std::declval<C>()))> 102 inline constexpr SkEnumerate<Iter> SkMakeEnumerate(C& c) { 103 return SkEnumerate<Iter>{std::begin(c), std::end(c)}; 104 } 105 template <typename C, typename Iter = decltype(std::begin(std::declval<C>()))> 106 inline constexpr SkEnumerate<Iter, C> SkMakeEnumerate(C&& c) { 107 return SkEnumerate<Iter, C>{std::forward<C>(c)}; 108 } 109 110 template <class T, std::size_t N, typename Iter = decltype(std::begin(std::declval<T(&)[N]>()))> 111 inline constexpr SkEnumerate<Iter> SkMakeEnumerate(T (&a)[N]) { 112 return SkEnumerate<Iter>{std::begin(a), std::end(a)}; 113 } 114 #endif // SkEnumerate_DEFINED 115