gmock/gtest/samples/prime_tables.h

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// Author: wan@google.com (Zhanyong Wan)
// Author: vladl@google.com (Vlad Losev)

// 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 GTEST_SAMPLES_PRIME_TABLES_H_
#define GTEST_SAMPLES_PRIME_TABLES_H_

#include <algorithm>

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

  // Returns true iff 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:
  virtual bool IsPrime(int n) const
  {
    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;
  }

  virtual int GetNextPrime(int p) const
  {
    for (int n = p + 1; n > 0; n++)
      {
        if (IsPrime(n)) { return n; }
      }

    return -1;
  }
};

// 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);
  }
  virtual ~PreCalculatedPrimeTable() { delete[] is_prime_; }

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

  virtual int GetNextPrime(int p) const
  {
    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;

    for (int i = 2; i <= max; i++)
      {
        if (!is_prime_[i]) { continue; }

        // Marks all multiples of i (except i itself) as non-prime.
        for (int j = 2 * 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  // GTEST_SAMPLES_PRIME_TABLES_H_