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prime_tables.h (4247B)

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// This provides interface PrimeTable that determines whether a number is a
// prime and determines a next prime number. This interface is used
// in Google Test samples demonstrating use of parameterized tests.

#ifndef GOOGLETEST_SAMPLES_PRIME_TABLES_H_
#define GOOGLETEST_SAMPLES_PRIME_TABLES_H_

#include <algorithm>

// The prime table interface.
class PrimeTable {
public:
 virtual ~PrimeTable() {}

 // Returns true if and only if n is a prime number.
 virtual bool IsPrime(int n) const = 0;

 // Returns the smallest prime number greater than p; or returns -1
 // if the next prime is beyond the capacity of the table.
 virtual int GetNextPrime(int p) const = 0;
};

// Implementation #1 calculates the primes on-the-fly.
class OnTheFlyPrimeTable : public PrimeTable {
public:
 bool IsPrime(int n) const override {
   if (n <= 1) return false;

   for (int i = 2; i*i <= n; i++) {
     // n is divisible by an integer other than 1 and itself.
     if ((n % i) == 0) return false;
   }

   return true;
 }

 int GetNextPrime(int p) const override {
   if (p < 0) return -1;

   for (int n = p + 1;; n++) {
     if (IsPrime(n)) return n;
   }
 }
};

// Implementation #2 pre-calculates the primes and stores the result
// in an array.
class PreCalculatedPrimeTable : public PrimeTable {
public:
 // 'max' specifies the maximum number the prime table holds.
 explicit PreCalculatedPrimeTable(int max)
     : is_prime_size_(max + 1), is_prime_(new bool[max + 1]) {
   CalculatePrimesUpTo(max);
 }
 ~PreCalculatedPrimeTable() override { delete[] is_prime_; }

 bool IsPrime(int n) const override {
   return 0 <= n && n < is_prime_size_ && is_prime_[n];
 }

 int GetNextPrime(int p) const override {
   for (int n = p + 1; n < is_prime_size_; n++) {
     if (is_prime_[n]) return n;
   }

   return -1;
 }

private:
 void CalculatePrimesUpTo(int max) {
   ::std::fill(is_prime_, is_prime_ + is_prime_size_, true);
   is_prime_[0] = is_prime_[1] = false;

   // Checks every candidate for prime number (we know that 2 is the only even
   // prime).
   for (int i = 2; i*i <= max; i += i%2+1) {
     if (!is_prime_[i]) continue;

     // Marks all multiples of i (except i itself) as non-prime.
     // We are starting here from i-th multiplier, because all smaller
     // complex numbers were already marked.
     for (int j = i*i; j <= max; j += i) {
       is_prime_[j] = false;
     }
   }
 }

 const int is_prime_size_;
 bool* const is_prime_;

 // Disables compiler warning "assignment operator could not be generated."
 void operator=(const PreCalculatedPrimeTable& rhs);
};

#endif  // GOOGLETEST_SAMPLES_PRIME_TABLES_H_