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  1. #include <iostream>
  2. #include <cmath>
  3. #include<limits>
  4.  
  5.  
  6. 	template<typename numeric_t>
  7. 	numeric_t ln_primitive(numeric_t const &n){
  8. 		numeric_t d = std::numeric_limits<numeric_t>::digits;
  9. 		while(d && !(n & (1<< (--d)))){}
  10. 		return d;
  11. 	}
  12.  
  13. 	template<typename numeric_t>
  14. 	constexpr numeric_t log2(numeric_t n, std::size_t approx_digits){
  15. 		if(approx_digits == 0) return 0;
  16. 		numeric_t _ln = 0;
  17. 		std::size_t lower_digit =0;
  18. 		while(approx_digits && n > std::numeric_limits<unsigned short>::max() ){
  19. 			lower_digit= approx_digits >> 1;
  20. 			numeric_t const upper = n>> lower_digit;
  21. 			if(upper){
  22. 				n = upper;
  23. 				_ln += lower_digit;
  24. 				approx_digits-= lower_digit;
  25. 			}else{
  26. 				approx_digits = lower_digit;
  27. 			}
  28. 		}
  29. 		if(approx_digits && n) _ln += ln_primitive((unsigned short)(n));
  30. 		return _ln;
  31. 	}
  32.  
  33. 	template<typename numeric_t, std::size_t N>
  34. 	constexpr numeric_t log(numeric_t const &n){
  35. 		if(N==2) return log2(n);
  36. 		return log2(n) / log2(N);
  37. 	}
  38.  
  39. void test(unsigned x){
  40. 	std::cout << "log10("<< x <<")="<< std::log10(x) << std::endl;
  41. 	std::cout << "(approx)log10("<< x <<")="<< log<unsigned,10>(x) << std::endl;
  42. }
  43.  
  44. int main() {
  45. 	test(10);
  46. 	test(100);
  47. 	test(1000);
  48. 	test(10000);
  49. 	test(100000);
  50. 	return 0;
  51. }
Success #stdin #stdout 0s 16056KB
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Standard input is empty
stdout
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log10(10)=1
(approx)log10(10)=1
log10(100)=2
(approx)log10(100)=2
log10(1000)=3
(approx)log10(1000)=3
log10(10000)=4
(approx)log10(10000)=4
log10(100000)=5
(approx)log10(100000)=5
https://ideone.com/PTc8u9
language:
C++14 (gcc 8.3)
created:
6 years ago
visibility:
public

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