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  1. #include <iostream>
  2. using namespace std;
  3.  
  4. template<int N>
  5. struct lookup_table {
  6.     int table[N];
  7.  
  8.     constexpr int& operator[](size_t i) {
  9.         return table[i];
  10.     }
  11.  
  12.     constexpr const int& operator[](size_t i) const {
  13.         return table[i];
  14.     }
  15. };
  16.  
  17. constexpr int bit_count(int i) { 
  18. 	int bits = 0; 
  19. 	while (i) { i &= i-1; ++bits; } 
  20. 	return bits;
  21. }
  22.  
  23. template<int N> 
  24. constexpr lookup_table<N> generate() {
  25.     lookup_table<N> table = {};
  26.  
  27.     for (int i = 0; i < N; ++i)
  28.         table[i] = bit_count(i);
  29.  
  30.     return table;
  31. }
  32.  
  33. template<int I> struct Check {
  34.     Check() {
  35.         std::cout <<  I << "\n";
  36.     }
  37. };
  38.  
  39. constexpr auto table = generate<65536>();
  40.  
  41. int main() {
  42. 	// checks that they are evaluated at compile-time 
  43. 	Check<table[5]>();
  44. 	Check<table[65535]>();
  45.  
  46. 	// test
  47. 	int i;
  48. 	while (cin >> i) {
  49. 		cout << table[i] << ' ';
  50. 	}
  51.  
  52. 	return 0;
  53. }
Success #stdin #stdout 0s 15496KB
comments (?)
stdin 
1 2 3 4 5 6 7 8 9 10 65535
stdout 
2
16
1 1 2 1 2 2 3 1 2 2 16
https://ideone.com/zQB86O
language:
C++ (gcc 8.3)
created:
7 years ago
visibility:
public

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