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  1. #include <iostream>
  2.  
  3. #pragma mark - Tape
  4.  
  5. constexpr int Blank = -1;
  6.  
  7. template<int... xs>
  8. class Tape {
  9. public:
  10.     using type = Tape<xs...>;
  11.     constexpr static int length = sizeof...(xs);
  12. };
  13.  
  14. #pragma mark - Print
  15.  
  16. template<class T>
  17. void print(T);
  18.  
  19. template<>
  20. void print(Tape<>) {
  21.     std::cout << std::endl;
  22. }
  23.  
  24. template<int x, int... xs>
  25. void print(Tape<x, xs...>) {
  26.     if (x == Blank) {
  27.         std::cout << "_ ";
  28.     } else {
  29.         std::cout << x << " ";
  30.     }
  31.     print(Tape<xs...>());
  32. }
  33.  
  34. #pragma mark - Concatenate
  35.  
  36. template<class, class>
  37. class Concatenate;
  38.  
  39. template<int... xs, int... ys>
  40. class Concatenate<Tape<xs...>, Tape<ys...>> {
  41. public:
  42.     using type = Tape<xs..., ys...>;
  43. };
  44.  
  45. #pragma mark - Invert
  46.  
  47. template<class>
  48. class Invert;
  49.  
  50. template<>
  51. class Invert<Tape<>> {
  52. public:
  53.     using type = Tape<>;
  54. };
  55.  
  56. template<int x, int... xs>
  57. class Invert<Tape<x, xs...>> {
  58. public:
  59.     using type = typename Concatenate<
  60.         typename Invert<Tape<xs...>>::type,
  61.         Tape<x>
  62.     >::type;
  63. };
  64.  
  65. #pragma mark - Read
  66.  
  67. template<int, class>
  68. class Read;
  69.  
  70. template<int n, int x, int... xs>
  71. class Read<n, Tape<x, xs...>> {
  72. public:
  73.     using type = typename std::conditional<
  74.         (n == 0),
  75.         std::integral_constant<int, x>,
  76.         Read<n - 1, Tape<xs...>>
  77.     >::type::type;
  78. };
  79.  
  80. #pragma mark - N first and N last
  81.  
  82. template<int, class>
  83. class NLast;
  84.  
  85. template<int n, int x, int... xs>
  86. class NLast<n, Tape<x, xs...>> {
  87. public:
  88.     using type = typename std::conditional<
  89.         (n == sizeof...(xs)),
  90.         Tape<xs...>,
  91.         NLast<n, Tape<xs...>>
  92.     >::type::type;
  93. };
  94.  
  95. template<int, class>
  96. class NFirst;
  97.  
  98. template<int n, int... xs>
  99. class NFirst<n, Tape<xs...>> {
  100. public:
  101.     using type = typename Invert<
  102.         typename NLast<
  103.             n, typename Invert<Tape<xs...>>::type
  104.         >::type
  105.     >::type;
  106. };
  107.  
  108. int main() {
  109. 	using tape = Tape<1, 2, 3, 4, 5>;
  110. 	print(NFirst<3, tape>::type());
  111. 	print(NLast<2, tape>::type());
  112. 	return 0;
  113. }
Success #stdin #stdout 0s 3412KB
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https://ideone.com/igYF3W
language:
C++14 (gcc 8.3)
created:
8 years ago
visibility:
public

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