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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 GTEST_SAMPLES_PRIME_TABLES_H_
#define GTEST_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 // GTEST_SAMPLES_PRIME_TABLES_H_
