package lru

import (
	"container/list"
	"testing"
)

type entry[K comparable, V any] struct {
	key   K
	value V
}

// Cache is a fixed-capacity LRU cache.
// Get and Put are O(1).
type Cache[K comparable, V any] struct {
	capacity int
	items    map[K]*list.Element
	order    *list.List
}

// New creates an LRU cache that holds at most capacity entries.
func New[K comparable, V any](capacity int) *Cache[K, V] {
	if capacity < 0 {
		capacity = 0
	}

	return &Cache[K, V]{
		capacity: capacity,
		items:    make(map[K]*list.Element),
		order:    list.New(),
	}
}

// Get returns a cached value and marks it as recently used.
func (c *Cache[K, V]) Get(key K) (V, bool) {
	if elem, ok := c.items[key]; ok {
		c.order.MoveToFront(elem)
		return elem.Value.(entry[K, V]).value, true
	}

	var zero V
	return zero, false
}

// Put inserts or updates a value and evicts the least recently used entry
// when the cache exceeds capacity.
func (c *Cache[K, V]) Put(key K, value V) {
	if c.capacity == 0 {
		return
	}

	if elem, ok := c.items[key]; ok {
		elem.Value = entry[K, V]{key: key, value: value}
		c.order.MoveToFront(elem)
		return
	}

	elem := c.order.PushFront(entry[K, V]{key: key, value: value})
	c.items[key] = elem

	if len(c.items) > c.capacity {
		c.evictOldest()
	}
}

// Len returns the number of entries currently stored.
func (c *Cache[K, V]) Len() int {
	return len(c.items)
}

func (c *Cache[K, V]) evictOldest() {
	elem := c.order.Back()
	if elem == nil {
		return
	}

	c.order.Remove(elem)
	delete(c.items, elem.Value.(entry[K, V]).key)
}

func TestCacheEvictsLeastRecentlyUsed(t *testing.T) {
	cache := New[string, int](2)

	cache.Put("a", 1)
	cache.Put("b", 2)

	if value, ok := cache.Get("a"); !ok || value != 1 {
		t.Fatalf("expected a=1, got value=%d ok=%v", value, ok)
	}

	cache.Put("c", 3)

	if _, ok := cache.Get("b"); ok {
		t.Fatal("expected b to be evicted")
	}
	if value, ok := cache.Get("a"); !ok || value != 1 {
		t.Fatalf("expected a=1 to remain, got value=%d ok=%v", value, ok)
	}
	if value, ok := cache.Get("c"); !ok || value != 3 {
		t.Fatalf("expected c=3, got value=%d ok=%v", value, ok)
	}
}

func TestCacheUpdatesExistingEntry(t *testing.T) {
	cache := New[string, int](2)

	cache.Put("a", 1)
	cache.Put("b", 2)
	cache.Put("a", 10)
	cache.Put("c", 3)

	if value, ok := cache.Get("a"); !ok || value != 10 {
		t.Fatalf("expected updated a=10, got value=%d ok=%v", value, ok)
	}
	if _, ok := cache.Get("b"); ok {
		t.Fatal("expected b to be evicted")
	}
	if cache.Len() != 2 {
		t.Fatalf("expected len=2, got %d", cache.Len())
	}
}