package primes

type wheel struct {
	cache    Cache
	elements []int64
	primes   []int64
	size     int64
	offset   int
	n        int64
}

func newWheel6(cache Cache) *wheel {
	return &wheel{cache, []int64{2, 4}, []int64{2, 3}, 6, 0, 5}
}

func (s *wheel) resetTo(n int64) {
	nn := n + 1
	s.n = (nn/s.size)*s.size - 1
	s.offset = 0
	for s.n+s.elements[s.offset] <= n {
		s.next()
	}
}

func (s *wheel) next() int64 {
	s.n += s.elements[s.offset]
	s.offset = (s.offset + 1) % len(s.elements)
	return s.n
}

func (s *wheel) expand() *wheel {
	prime := s.primes[len(s.primes)-1]
	it := NewIteratorFromCache(s.cache)
	for it.Next() && it.Get() <= prime {
	}
	prime = it.Get()
	if s.n <= prime {
		return nil
	}
	primes := append([]int64(nil), s.primes...)
	primes = append(primes, prime)
	var elements []int64
	var v int64 = -1
	var inc int64
	for i := int64(0); i < prime; i++ {
		for _, el := range s.elements {
			v += el
			inc += el
			if 0 != v%prime {
				elements = append(elements, inc)
				inc = 0
			}
		}
	}
	exp := &wheel{s.cache, elements, primes, s.size * prime, 0, s.n}
	exp.resetTo(s.n)
	return exp
}