# Spoiler for C++ prime challenge

• Posted intentionally in a separate area from the challenge itself, for what it's worth.  This is my solution to the Prime Challenge twist (C++ challenge):

<font style="color: rgb(0, 0, 0); background-color: rgb(0, 0, 0);" size="1">// =============================================================

// Summary:

// =============================================================

// Divides          - Test for divisibility

// Prime            - Test for primality

// NextPrimeInRange - Looks for next prime number

// PrimeList        - Collects the primes for reporting, and

//
generates the error triggering output

// SearchForPrimes  - Starts the prime list

// =============================================================

// =============================================================

// Divides<a,b>::result is true if a divides b

// =============================================================

template <int a, int b, int isLess=false>

struct Divides

{

static const bool result = Divides< a, b-a, (b-a<a) >::result;

};

// Termination case

template <int a, int b>

struct Divides<a, b, true> { static const bool result=b==0;};

// =============================================================

// Prime<test>::result is true if test is prime

// =============================================================

template <int test,

int curtest=1+(test>>1),

bool knownComposite=false,

bool knownPrime=false>

struct Prime

{

static const bool result

= Prime< test, curtest-1,

Divides<curtest, test>::result,

curtest<2 >::result;

};

// Terminating case--if we searched everything, this is prime

template<int test, int curtest, bool knownComposite>

struct Prime<test, curtest, knownComposite, true>

{

static const bool result = true;

};

// Terminating case--if we found something that divides this,

// this is composite

template<int test, int curtest>

struct Prime<test, curtest, true>

{

static const bool result=false;

};

// =============================================================

// NextPrimeInRange<start,end>::result is the first prime

// between start and end, if there is one, or is zero.

// =============================================================

template<int start, int end, bool found=false,bool eos=false>

struct NextPrimeInRange

{

static const int result

= NextPrimeInRange<start+1, end,

Prime<start>::result,

(start>=end)

>::result;

};

template<int start,int end,bool found>

struct NextPrimeInRange<start,end,found,true>

{

static const int result=0;

};

template<int start,int end>

struct NextPrimeInRange<start, end, true, false>

{

static const int result=start-1;

};

// =============================================================

// Range is just a readability template for PrimeList

// (only serves to make errors readable in certain compilers,

//  such as MSVC)

// =============================================================

template<int rStart, int rEnd>

struct Range

{

static const int end=rEnd;

};

// =============================================================

// PrimeList<RESULT, Range<,end> > is a list of primes.

// Each RESULT walks a prime, until the end of the range is

// reached, at which case PrimeList generates an error

// (by trying to access a private member).  The error displays

// the resulting list of primes.

// =============================================================

template<int RESULT, typename InRange>

struct PrimeList

{

static const int result

= PrimeList<NextPrimeInRange<RESULT+1,InRange::end>::result,

InRange>::result;

};

template<typename EndOfSearch>

class PrimeList<0,EndOfSearch>

{

static const int result = 0;

};

// =============================================================

// SearchForPrimes<start,end> primes (pun intended) the search

// by finding the next prime after start, and starting a

// PrimeList.

// =============================================================

template<int start,int end>

struct SearchForPrimes

{

static const int result

= PrimeList<NextPrimeInRange<start,end>::result,

Range<start,end> >::result;

};

int Result = SearchForPrimes<100,200>::result;

int main() {return 0;}

</font>

• Did you black it out on purpose or are the sinister (or kinky?) forum gods at it again?

• Well, if you highlight the blacked-out text, you can see the code....

• @Ulvhamne said:

Did you black it out on purpose or are the sinister (or kinky?) forum gods at it again?

On purpose--just believe it to generally be good form to make spoilers
unreadable via casual viewing. In my mind, it's easy to accidentally
click on a thread and glance at the spoiler, but it's pretty hard to
accidentally click on the thread and highlight the spoiler.

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