Task: Successive prime differences


C++: #include <iostream>
#include <cstdint>
#include <vector>
#include "prime_sieve.hpp"

using integer = uint32_t;
using vector = std::vector<integer>;

void print_vector(const vector& vec) {
    if (!vec.empty()) {
        auto i = vec.begin();
        std::cout << '(' << *i;
        for (++i; i != vec.end(); ++i)
            std::cout << ", " << *i;
        std::cout << ')';
    }
}

class diffs {
public:
    diffs(std::initializer_list<integer> list) : diffs_(list) {}
    diffs(const vector& vec) : diffs_(vec) {}
    void test_group(const vector&);
    size_t size() const {
        return diffs_.size();
    }
    void print(std::ostream&);
private:
    vector diffs_;
    vector first_;
    vector last_;
    integer count_ = 0;
};

void diffs::test_group(const vector& vec) {
    if (vec.size() < size() + 1)
        return;
    size_t start = vec.size() - size() - 1;
    for (size_t i = 0, j = start + 1; i < size(); ++i, ++j) {
        if (vec[j] - vec[j - 1] != diffs_[i])
            return;
    }
    vector group(vec.begin() + start, vec.end());
    if (count_ == 0)
        first_ = group;
    last_ = group;
    ++count_;
}

void diffs::print(std::ostream& out) {
    print_vector(diffs_);
    out << ": first group = ";
    print_vector(first_);
    out << ", last group = ";
    print_vector(last_);
    out << ", count = " << count_ << '\n';
}

int main() {
    const integer limit = 1000000;
    const size_t max_group_size = 4;
    prime_sieve sieve(limit);
    diffs d[] = { {2}, {1}, {2, 2}, {2, 4}, {4, 2}, {6, 4, 2} };
    vector group;
    for (integer p = 0; p < limit; ++p) {
        if (!sieve.is_prime(p))
            continue;
        if (group.size() >= max_group_size)
            group.erase(group.begin());
        group.push_back(p);
        for (auto&& diff : d) {
            diff.test_group(group);
        }
    }
    for (auto&& diff : d) {
        diff.print(std::cout);
    }
    return 0;
}






Python: 
from sympy import Sieve

def nsuccprimes(count, mx):
    "return tuple of <count> successive primes <= mx (generator)"
    sieve = Sieve()
    sieve.extend(mx)
    primes = sieve._list
    return zip(*(primes[n:] for n in range(count)))

def check_value_diffs(diffs, values):
    "Differences between successive values given by successive items in diffs?"
    return all(v[1] - v[0] == d 
               for d, v in zip(diffs, zip(values, values[1:])))

def successive_primes(offsets=(2, ), primes_max=1_000_000):
    return (sp for sp in nsuccprimes(len(offsets) + 1, primes_max) 
            if check_value_diffs(offsets, sp))

if __name__ == '__main__':
    for offsets, mx in [((2,),      1_000_000), 
                        ((1,),      1_000_000),
                        ((2, 2),    1_000_000),
                        ((2, 4),    1_000_000),
                        ((4, 2),    1_000_000),
                        ((6, 4, 2), 1_000_000),
                       ]:
        print(f"
              f"SUCCESSIVE DIFFERENCES OF {str(list(offsets))[1:-1]}")
        for count, last in enumerate(successive_primes(offsets, mx), 1):
            if count == 1:
                first = last
        print("  First group:", str(first)[1:-1])
        print("   Last group:", str(last)[1:-1])
        print("        Count:", count)