prime_tables.h 4.1 KB

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  1. // Copyright 2008 Google Inc.
  2. // All Rights Reserved.
  3. //
  4. // Redistribution and use in source and binary forms, with or without
  5. // modification, are permitted provided that the following conditions are
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  10. // * Redistributions in binary form must reproduce the above
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  15. // contributors may be used to endorse or promote products derived from
  16. // this software without specific prior written permission.
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  18. // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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  21. // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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  28. // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  29. // This provides interface PrimeTable that determines whether a number is a
  30. // prime and determines a next prime number. This interface is used
  31. // in Google Test samples demonstrating use of parameterized tests.
  32. #ifndef GOOGLETEST_SAMPLES_PRIME_TABLES_H_
  33. #define GOOGLETEST_SAMPLES_PRIME_TABLES_H_
  34. #include <algorithm>
  35. // The prime table interface.
  36. class PrimeTable {
  37. public:
  38. virtual ~PrimeTable() {}
  39. // Returns true if and only if n is a prime number.
  40. virtual bool IsPrime(int n) const = 0;
  41. // Returns the smallest prime number greater than p; or returns -1
  42. // if the next prime is beyond the capacity of the table.
  43. virtual int GetNextPrime(int p) const = 0;
  44. };
  45. // Implementation #1 calculates the primes on-the-fly.
  46. class OnTheFlyPrimeTable : public PrimeTable {
  47. public:
  48. bool IsPrime(int n) const override {
  49. if (n <= 1) return false;
  50. for (int i = 2; i*i <= n; i++) {
  51. // n is divisible by an integer other than 1 and itself.
  52. if ((n % i) == 0) return false;
  53. }
  54. return true;
  55. }
  56. int GetNextPrime(int p) const override {
  57. if (p < 0) return -1;
  58. for (int n = p + 1;; n++) {
  59. if (IsPrime(n)) return n;
  60. }
  61. }
  62. };
  63. // Implementation #2 pre-calculates the primes and stores the result
  64. // in an array.
  65. class PreCalculatedPrimeTable : public PrimeTable {
  66. public:
  67. // 'max' specifies the maximum number the prime table holds.
  68. explicit PreCalculatedPrimeTable(int max)
  69. : is_prime_size_(max + 1), is_prime_(new bool[max + 1]) {
  70. CalculatePrimesUpTo(max);
  71. }
  72. ~PreCalculatedPrimeTable() override { delete[] is_prime_; }
  73. bool IsPrime(int n) const override {
  74. return 0 <= n && n < is_prime_size_ && is_prime_[n];
  75. }
  76. int GetNextPrime(int p) const override {
  77. for (int n = p + 1; n < is_prime_size_; n++) {
  78. if (is_prime_[n]) return n;
  79. }
  80. return -1;
  81. }
  82. private:
  83. void CalculatePrimesUpTo(int max) {
  84. ::std::fill(is_prime_, is_prime_ + is_prime_size_, true);
  85. is_prime_[0] = is_prime_[1] = false;
  86. // Checks every candidate for prime number (we know that 2 is the only even
  87. // prime).
  88. for (int i = 2; i*i <= max; i += i%2+1) {
  89. if (!is_prime_[i]) continue;
  90. // Marks all multiples of i (except i itself) as non-prime.
  91. // We are starting here from i-th multiplier, because all smaller
  92. // complex numbers were already marked.
  93. for (int j = i*i; j <= max; j += i) {
  94. is_prime_[j] = false;
  95. }
  96. }
  97. }
  98. const int is_prime_size_;
  99. bool* const is_prime_;
  100. // Disables compiler warning "assignment operator could not be generated."
  101. void operator=(const PreCalculatedPrimeTable& rhs);
  102. };
  103. #endif // GOOGLETEST_SAMPLES_PRIME_TABLES_H_