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29
30// This provides interface PrimeTable that determines whether a number is a
31// prime and determines a next prime number. This interface is used
32// in Google Test samples demonstrating use of parameterized tests.
33
34#ifndef GOOGLETEST_SAMPLES_PRIME_TABLES_H_
35#define GOOGLETEST_SAMPLES_PRIME_TABLES_H_
36
37#include <algorithm>
38
39// The prime table interface.
40class PrimeTable {
41 public:
42  virtual ~PrimeTable() = default;
43
44  // Returns true if and only if n is a prime number.
45  virtual bool IsPrime(int n) const = 0;
46
47  // Returns the smallest prime number greater than p; or returns -1
48  // if the next prime is beyond the capacity of the table.
49  virtual int GetNextPrime(int p) const = 0;
50};
51
52// Implementation #1 calculates the primes on-the-fly.
53class OnTheFlyPrimeTable : public PrimeTable {
54 public:
55  bool IsPrime(int n) const override {
56    if (n <= 1) return false;
57
58    for (int i = 2; i * i <= n; i++) {
59      // n is divisible by an integer other than 1 and itself.
60      if ((n % i) == 0) return false;
61    }
62
63    return true;
64  }
65
66  int GetNextPrime(int p) const override {
67    if (p < 0) return -1;
68
69    for (int n = p + 1;; n++) {
70      if (IsPrime(n)) return n;
71    }
72  }
73};
74
75// Implementation #2 pre-calculates the primes and stores the result
76// in an array.
77class PreCalculatedPrimeTable : public PrimeTable {
78 public:
79  // 'max' specifies the maximum number the prime table holds.
80  explicit PreCalculatedPrimeTable(int max)
81      : is_prime_size_(std::max(1, max + 1)),
82        is_prime_(new bool[static_cast<size_t>(is_prime_size_)]) {
83    CalculatePrimesUpTo(is_prime_size_ - 1);
84  }
85  ~PreCalculatedPrimeTable() override { delete[] is_prime_; }
86
87  bool IsPrime(int n) const override {
88    return 0 <= n && n < is_prime_size_ && is_prime_[n];
89  }
90
91  int GetNextPrime(int p) const override {
92    for (int n = p + 1; n < is_prime_size_; n++) {
93      if (is_prime_[n]) return n;
94    }
95
96    return -1;
97  }
98
99 private:
100  void CalculatePrimesUpTo(int max) {
101    ::std::fill(is_prime_, is_prime_ + is_prime_size_, true);
102    is_prime_[0] = is_prime_[1] = false;
103
104    // Checks every candidate for prime number (we know that 2 is the only even
105    // prime).
106    for (int i = 2; i * i <= max; i += i % 2 + 1) {
107      if (!is_prime_[i]) continue;
108
109      // Marks all multiples of i (except i itself) as non-prime.
110      // We are starting here from i-th multiplier, because all smaller
111      // complex numbers were already marked.
112      for (int j = i * i; j <= max; j += i) {
113        is_prime_[j] = false;
114      }
115    }
116  }
117
118  const int is_prime_size_;
119  bool* const is_prime_;
120
121  // Disables compiler warning "assignment operator could not be generated."
122  void operator=(const PreCalculatedPrimeTable& rhs);
123};
124
125#endif  // GOOGLETEST_SAMPLES_PRIME_TABLES_H_
126