package sieve

type node struct {
	value      string
	visited    bool
	prev, next *node
}

type SieveCache struct {
	cap, size        int
	cache            map[string]*node
	head, tail, hand *node
}

func NewCache(cap int) *SieveCache {
	return &SieveCache{
		cap:   cap,
		size:  0,
		cache: make(map[string]*node, cap),
	}
}

// Access touches the item `v` in the cache
func (sc *SieveCache) Access(v string) {
	if n, ok := sc.cache[v]; ok {
		n.visited = true
	} else {
		if sc.size == sc.cap {
			sc.evict()
		}

		newNode := &node{value: v}
		sc.addToHead(newNode)
		sc.cache[v] = newNode
		sc.size += 1
		newNode.visited = false
	}
}

// Range iterates over the values in the cache, stops the iteration if the
// closure returns false.
func (sc *SieveCache) Range(f func(v string) bool) {
	current := sc.head
	keepGoing := true
	for current != nil && keepGoing {
		keepGoing = f(current.value)
		current = current.next
	}
}

func (sc *SieveCache) evict() {
	var obj *node
	if sc.hand != nil {
		obj = sc.hand
	} else {
		obj = sc.tail
	}

	for obj != nil && obj.visited {
		obj.visited = false
		if obj.prev != nil {
			obj = obj.prev
		} else {
			obj = sc.tail
		}
	}

	sc.hand = obj.prev
	delete(sc.cache, obj.value)
	sc.removeNode(obj)
	sc.size -= 1
}

func (sc *SieveCache) removeNode(n *node) {
	if n.prev != nil {
		n.prev.next = n.next
	} else {
		sc.head = n.next
	}

	if n.next != nil {
		n.next.prev = n.prev
	} else {
		sc.tail = n.prev
	}
}

func (sc *SieveCache) addToHead(n *node) {
	n.next = sc.head
	n.prev = nil
	if sc.head != nil {
		sc.head.prev = n
	}
	sc.head = n
	if sc.tail == nil {
		sc.tail = n
	}
}