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  1. #include <cstdlib>
  2. #include <cmath>
  3. #include <iostream>
  4.  
  5. using namespace std;
  6.  
  7. uint64_t isqrt( uint64_t const n )
  8. {
  9.     double x = n >> (uint64_t)log2(n)/2; // x ist höchstens 2^32
  10.     double old_x;
  11.     for(;;)
  12.     {
  13.         old_x = x;
  14.         x = (x + n/x)/2;
  15.  
  16.         if( std::abs(x - old_x) < 1 )
  17.             break;
  18.     }
  19.  
  20.     return x;
  21. }
  22.  
  23. int main()
  24. {
  25.     for ( uint64_t i = 2; i < 3000000; ++i )
  26.     {
  27.         if ( ( i & ( i - 1 ) ) == 0 )
  28.             cout << i << '\n';
  29.         uint64_t square = i * i;
  30.         if ( isqrt( square - 1 ) != i - 1 )
  31.             cout << "error: " << i << " ^2 - 1\n";
  32.         if ( isqrt( square ) != i )
  33.             cout << "error: " << i << " ^2\n";
  34.         if ( isqrt( square + 1 ) != i )
  35.             cout << "error: " << i << " ^2 + 1\n";
  36.     }
  37. }
Success #stdin #stdout 1.94s 3340KB
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stdin
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Standard input is empty
stdout
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2
4
8
16
32
64
128
256
512
1024
2048
4096
8192
16384
32768
65536
131072
262144
524288
1048576
error: 1435790 ^2 - 1
2097152
https://ideone.com/c5gyaR
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
secret

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