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  1. #include <iostream>
  2.  
  3. struct nil {};
  4. template <int head, typename tail> struct cons;
  5.  
  6. template <int... list> struct variadic2list;
  7. template <int head, int... rest>
  8. struct variadic2list<head, rest...> {
  9. 	typedef cons<head, typename variadic2list<rest...>::type> type;
  10. };
  11. template <> struct variadic2list<> { typedef nil type; };
  12.  
  13. template <typename typelist>
  14. struct printlist {
  15. 	template <typename T>
  16. 	static void print(T& os) {}
  17. };
  18. template <int head, typename tail>
  19. struct printlist<cons<head, tail>> {
  20. 	template <typename T>
  21. 	static void print(T& os) {
  22. 		os << head;
  23. 		printlist<tail>::print(os);
  24. 	}
  25. };
  26.  
  27. template <int val, int count, typename rest> struct single_look_and_say;
  28. template <int val, int count, int next, typename rest>
  29. struct single_look_and_say<val, count, cons<next, rest>> {
  30. 	typedef cons<count, cons<val, typename single_look_and_say<next, 1, rest>::type>> type;
  31. };
  32. template <int val, int count, typename rest>
  33. struct single_look_and_say<val, count, cons<val, rest>> {
  34. 	typedef typename single_look_and_say<val, count + 1, rest>::type type;
  35. };
  36. template <int val, int count>
  37. struct single_look_and_say<val, count, nil> {
  38. 	typedef typename variadic2list<count, val>::type type;
  39. };
  40.  
  41. template <size_t iters, typename seq> struct look_and_say_impl;
  42. template <size_t iters, int head, typename tail>
  43. struct look_and_say_impl<iters, cons<head, tail>> {
  44. 	typedef typename look_and_say_impl<iters - 1,
  45. 		typename single_look_and_say<head, 1, tail>::type>::type type;
  46. };
  47. // I need to pull apart head and tail to tell the compiler that this is more specialized.
  48. template <int head, typename tail>
  49. struct look_and_say_impl<1, cons<head, tail>> {
  50. 	typedef cons<head, tail> type;
  51. };
  52.  
  53. template <size_t iters, int... seed>
  54. struct look_and_say {
  55. 	typedef typename look_and_say_impl<iters, typename variadic2list<seed...>::type>::type type;
  56. };
  57. // Seed defaults to 1
  58. template <size_t iters>
  59. struct look_and_say<iters> {
  60. 	typedef typename look_and_say<iters, 1>::type type;
  61. };
  62.  
  63. int main() {
  64. 	printlist<look_and_say<6>::type>::print(std::cout);       // 6th value
  65. 	std::cout << '\n';
  66. 	printlist<look_and_say<4, 2, 2>::type>::print(std::cout); // 4th value from 22
  67. 	return 0;
  68. }
Success #stdin #stdout 0s 3296KB
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Standard input is empty
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312211
22
https://ideone.com/LjfriJ
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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