beginning work on custom image cache

backend/imagecache.go

@@ -0,0 +1,118 @@
+package backend
+
+import (
+	"context"
+	"errors"
+	"image"
+	"sync"
+	"time"
+)
+
+type CacheItem struct {
+	val image.Image
+	ttl time.Duration
+
+	// unix time
+	expiresAt    int64
+	lastAccessed int64
+}
+
+// A custom cache for images with the following eviction strategy:
+//   - If there are fewer than MinSize items in the cache, none will be evicted
+//   - If a new addition would make the cache exceed MaxSize, an item will be immediately evicted
+//   - evicted item will be an expired item if possible, else the least recently used
+//   - If the size of the cache is between MaxSize and MinSize, expired items will be periodically evicted
+type ImageCache struct {
+	MinSize    int
+	MaxSize    int
+	DefaultTTL time.Duration
+
+	mu    sync.RWMutex
+	cache map[string]CacheItem
+}
+
+var (
+	ErrNotFound = errors.New("item not found")
+)
+
+func (i *ImageCache) Init(ctx context.Context) {
+	i.cache = make(map[string]CacheItem)
+}
+
+func (i *ImageCache) AddWithTTL(key string, val image.Image, ttl time.Duration) {
+	i.mu.Lock()
+	defer i.mu.Unlock()
+
+	if v, ok := i.cache[key]; ok {
+		v.val = val
+		v.ttl = ttl
+		v.expiresAt = time.Now().Add(v.ttl).Unix()
+		v.lastAccessed = time.Now().Unix()
+		return
+	}
+	if len(i.cache) == i.MaxSize {
+		i.evictOne()
+		i.cache[key] = CacheItem{
+			val:          val,
+			ttl:          ttl,
+			expiresAt:    time.Now().Add(ttl).Unix(),
+			lastAccessed: time.Now().Unix(),
+		}
+	}
+}
+
+func (i *ImageCache) Add(key string, val image.Image) {
+	i.AddWithTTL(key, val, i.DefaultTTL)
+}
+
+func (i *ImageCache) Has(key string) bool {
+	i.mu.RLock()
+	defer i.mu.RUnlock()
+
+	_, ok := i.cache[key]
+	return ok
+}
+
+func (i *ImageCache) Get(key string, resetTTL bool) (image.Image, error) {
+	i.mu.RLock()
+	defer i.mu.RUnlock()
+
+	if v, ok := i.cache[key]; ok {
+		v.lastAccessed = time.Now().Unix()
+		if resetTTL {
+			v.expiresAt = time.Now().Add(v.ttl).Unix()
+		}
+		return v.val, nil
+	}
+	return nil, ErrNotFound
+}
+
+// assuming max size is small enough that linear scan is fine
+func (i *ImageCache) evictOne() {
+	var lruKey string
+	now := time.Now().Unix()
+	lruTime := now
+	for k, v := range i.cache {
+		if v.expiresAt < now {
+			delete(i.cache, k)
+			return
+		}
+		if v.lastAccessed < lruTime {
+			lruTime = v.lastAccessed
+			lruKey = k
+		}
+	}
+	delete(i.cache, lruKey)
+}
+
+// TODO
+func (i *ImageCache) EvictExpired() {
+	i.mu.Lock()
+	defer i.mu.Unlock()
+
+	count := len(i.cache)
+	for count > i.MinSize {
+
+	}
+
+}