trjhtr

Here's a simple LCG that I've made to learn a bit more about pseudo random number generation.

Is this implementation correct, and if so, how can I improve it further?

The following code is self contained and it should run without problems.

#include <chrono>
#include <iostream>
#include <map>

namespace Random 
 class LCG 0 ;


int main() 

 Random::LCG lcg(0);
 std::map<int64_t, uint64_t> buckets;
 for (uint64_t i = 0; i != 1000000; ++i) 
 ++buckets[lcg.get(10)];
 
 for (auto const [a, b] : buckets) 
 std::cout << a << 't' << b << 'n';
 

 return 0;

Your mask value M is gaining you nothing, since the types for this_seed and M are the same and you have every bit in M set to 1. Your comment for next is wrong; it can return 2⁶⁴-1 (i.e., all bits set). The correct range can be stated as either [0, 2 ^ 64 - 1] or [0, 2 ^ 64). This in turn can cause your get functions to return a value larger than expected (1.0, x, or b).

Where did you get the values for the A and B constants? I've not looked to see if they are appropriate.

this_seed = (this_seed * A + C) & M;

should just read

this_seed = this_seed * A + C;

The modulo is done free for you - unsigned arithmetic in a 64-bit word.
You do not need M

For the double function [0.0,1.0) this gives a uniform distribution:

return static_cast<double>(next() >> 11) * (1.0 / (UINT64_C(1) << 53));

The less random low order bits are discarded.
The multiplier compiles to 0x1p-53

The now function doesn’t have anything to do with the class; it is just a helper. So make it static.

void seed(uint64_t const value)

The const is OK, but you are not doing that consistently. The constructor with the same kind of argument does not have const.