LRU cache: lookup and recency form one invariant

// Requires container/list. This cache has a single owner; it is not concurrent.
type lruEntry struct{ key, value int }
type LRU struct {
	capacity int
	order    *list.List
	entries  map[int]*list.Element
}

func NewLRU(capacity int) *LRU {
	if capacity < 0 {
		capacity = 0
	}
	return &LRU{capacity: capacity, order: list.New(), entries: make(map[int]*list.Element)}
}
func (c *LRU) Get(key int) (int, bool) {
	element, ok := c.entries[key]
	if !ok {
		return 0, false
	}
	c.order.MoveToFront(element)
	return element.Value.(lruEntry).value, true
}
func (c *LRU) Put(key, value int) {
	if c.capacity == 0 {
		return
	}
	if element, ok := c.entries[key]; ok {
		element.Value = lruEntry{key, value}
		c.order.MoveToFront(element)
		return
	}
	c.entries[key] = c.order.PushFront(lruEntry{key, value})
	if c.order.Len() > c.capacity {
		oldest := c.order.Back()
		delete(c.entries, oldest.Value.(lruEntry).key)
		c.order.Remove(oldest)
	}
}