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I am trying to implement a PRNG I found online yet I am having compile time issues (seen below):

1>c:program files (x86)microsoft visual studio2017communityvctoolsmsvc14.15.26726includexutility(4010): error C2061: syntax error: identifier 'result_type'
1>c:program files (x86)microsoft visual studio2017communityvctoolsmsvc14.15.26726includexutility(4012): error C2065: '_Ty1': undeclared identifier
1>c:program files (x86)microsoft visual studio2017communityvctoolsmsvc14.15.26726includexutility(4012): error C2065: '_Ty1': undeclared identifier
1>c:program files (x86)microsoft visual studio2017communityvctoolsmsvc14.15.26726includexutility(4012): error C2923: 'std::conditional_t': '_Ty1' is not a valid template type argument for parameter '_Ty2'

This is my code:

std::random_device rd;
small_prng engine;
std::uniform_int_distribution< int > ud( 0, 50 );

for ( auto i = 0; i < 100; i++ ) 
 printf( "%in", ud( engine ) );

And this is the code I got online...

class small_prng 
 uint32_t a;
 uint32_t b;
 uint32_t c;
 uint32_t d;

 static inline uint32_t rot( uint32_t x, uint32_t k ) noexcept ( ( x ) >> ( 32 - ( k ) ) ) );
 
public:
 using value_type = uint32_t;

 explicit small_prng( uint32_t seed = 0xdeadbeef ) noexcept 
 a = 0xf1ea5eed;
 b = c = d = seed;
 for ( size_t i = 0; i < 20; ++i )
 ( *this )( );
 

 inline uint32_t operator()( ) noexcept 
 uint32_t e = a - rot( b, 27 );
 a = b ^ rot( c, 17 );
 b = c + d;
 c = d + e;
 d = e + a;
 return d;
 
;

Is there any reason this isn't working? How come? Compiling using Visual Studio 2017.

The requirements for a random engine are outlined at [rand.req.eng]; you can find here a summary of them; in particular you are missing (highlighted through the code with a MISSING comment):

class small_prng 
 uint32_t a;
 uint32_t b;
 uint32_t c;
 uint32_t d;

 static inline uint32_t rot( uint32_t x, uint32_t k ) noexcept ( ( x ) >> ( 32 - ( k ) ) ) );
 
public:
 // MISSING: the result type must indeed be called `result_type`
 using result_type = uint32_t;

 // MISSING: you must provide min and max, with the
 // minimum/maximum value your RNG can return
 result_type min() const noexcept return 0; 
 result_type max() const noexcept return 0xffffffff; 

 explicit small_prng( result_type seed = 0xdeadbeef ) noexcept 
 a = 0xf1ea5eed;
 b = c = d = seed;
 for ( size_t i = 0; i < 20; ++i )
 ( *this )( );
 

 // MISSING: constructor from a SeedSequence (see https://en.cppreference.com/w/cpp/named_req/SeedSequence)
 template<typename S>
 explicit small_prng(S &seq) 
 uint32_t nseed[1];
 seq.generate(nseed, nseed+1);
 seed(nseed[0]);
 

 // MISSING: seed() overloads
 void seed() *this = small_prng(); 
 void seed(result_type seed) *this = small_prng(seed); 
 template<typename S> void seed(S &seq) *this = small_prng(seq); 

 inline result_type operator()( ) noexcept 
 uint32_t e = a - rot( b, 27 );
 a = b ^ rot( c, 17 );
 b = c + d;
 c = d + e;
 d = e + a;
 return d;
 

 // MISSING: discard n extractions
 void discard(unsigned long long z) 
 // does this engine implement more efficient jump-ahead?
 while(z--) (*this)();
 

 // MISSING: dump generator state operator
 friend std::ostream &operator<<(std::ostream &os, const small_prng &r) 
 return os<<r.a<<" "<<r.b<<" "<<r.c<<" "<<r.d<<" ";
 

 // MISSING: read from generator state dump operator
 friend std::istream &operator>>(std::istream &is, small_prng &r) 
 return is>>r.a>>r.b>>r.c>>r.d;
 
;