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      1 // Copyright 2008 Google Inc.
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     29 //
     30 // Author: wan (at) google.com (Zhanyong Wan)
     31 // Author: vladl (at) google.com (Vlad Losev)
     32 
     33 // This provides interface PrimeTable that determines whether a number is a
     34 // prime and determines a next prime number. This interface is used
     35 // in Google Test samples demonstrating use of parameterized tests.
     36 
     37 #ifndef GTEST_SAMPLES_PRIME_TABLES_H_
     38 #define GTEST_SAMPLES_PRIME_TABLES_H_
     39 
     40 #include <algorithm>
     41 
     42 // The prime table interface.
     43 class PrimeTable {
     44  public:
     45   virtual ~PrimeTable() {}
     46 
     47   // Returns true iff n is a prime number.
     48   virtual bool IsPrime(int n) const = 0;
     49 
     50   // Returns the smallest prime number greater than p; or returns -1
     51   // if the next prime is beyond the capacity of the table.
     52   virtual int GetNextPrime(int p) const = 0;
     53 };
     54 
     55 // Implementation #1 calculates the primes on-the-fly.
     56 class OnTheFlyPrimeTable : public PrimeTable {
     57  public:
     58   virtual bool IsPrime(int n) const {
     59     if (n <= 1) return false;
     60 
     61     for (int i = 2; i*i <= n; i++) {
     62       // n is divisible by an integer other than 1 and itself.
     63       if ((n % i) == 0) return false;
     64     }
     65 
     66     return true;
     67   }
     68 
     69   virtual int GetNextPrime(int p) const {
     70     for (int n = p + 1; n > 0; n++) {
     71       if (IsPrime(n)) return n;
     72     }
     73 
     74     return -1;
     75   }
     76 };
     77 
     78 // Implementation #2 pre-calculates the primes and stores the result
     79 // in an array.
     80 class PreCalculatedPrimeTable : public PrimeTable {
     81  public:
     82   // 'max' specifies the maximum number the prime table holds.
     83   explicit PreCalculatedPrimeTable(int max)
     84       : is_prime_size_(max + 1), is_prime_(new bool[max + 1]) {
     85     CalculatePrimesUpTo(max);
     86   }
     87   virtual ~PreCalculatedPrimeTable() { delete[] is_prime_; }
     88 
     89   virtual bool IsPrime(int n) const {
     90     return 0 <= n && n < is_prime_size_ && is_prime_[n];
     91   }
     92 
     93   virtual int GetNextPrime(int p) const {
     94     for (int n = p + 1; n < is_prime_size_; n++) {
     95       if (is_prime_[n]) return n;
     96     }
     97 
     98     return -1;
     99   }
    100 
    101  private:
    102   void CalculatePrimesUpTo(int max) {
    103     ::std::fill(is_prime_, is_prime_ + is_prime_size_, true);
    104     is_prime_[0] = is_prime_[1] = false;
    105 
    106     for (int i = 2; i <= max; i++) {
    107       if (!is_prime_[i]) continue;
    108 
    109       // Marks all multiples of i (except i itself) as non-prime.
    110       for (int j = 2*i; j <= max; j += i) {
    111         is_prime_[j] = false;
    112       }
    113     }
    114   }
    115 
    116   const int is_prime_size_;
    117   bool* const is_prime_;
    118 };
    119 
    120 #endif  // GTEST_SAMPLES_PRIME_TABLES_H_
    121