add basic caching support

includes two implementations, a no-op NopCache and an in-memory
MemoryCache.

cache/cache.go

@@ -0,0 +1,17 @@
+// Package cache implements a image cache.
+package cache
+
+import "github.com/willnorris/go-imageproxy/data"
+
+// Cache provides a cache for image metadata and transformed variants of the
+// image.
+type Cache interface {
+	// Get retrieves the cached Image for the provided image URL.
+	Get(string) (image *data.Image, ok bool)
+
+	// Put caches the provided Image.
+	Save(*data.Image)
+
+	// Delete deletes the cached Image and all variants for the image at the specified URL.
+	Delete(string)
+}

cache/memory.go

@@ -0,0 +1,27 @@
+package cache
+
+import "github.com/willnorris/go-imageproxy/data"
+
+// MemoryCache provides an in-memory Cache implementation.
+type MemoryCache struct {
+	images map[string]*data.Image
+}
+
+func NewMemoryCache() *MemoryCache {
+	return &MemoryCache{
+		make(map[string]*data.Image),
+	}
+}
+
+func (c MemoryCache) Get(u string) (*data.Image, bool) {
+	image, ok := c.images[u]
+	return image, ok
+}
+
+func (c MemoryCache) Save(image *data.Image) {
+	c.images[image.URL] = image
+}
+
+func (c MemoryCache) Delete(u string) {
+	delete(c.images, u)
+}

cache/nop.go

@@ -0,0 +1,12 @@
+package cache
+
+import "github.com/willnorris/go-imageproxy/data"
+
+// NopCache provides a no-op cache implementation that doesn't actually cache anything.
+var NopCache = new(nopCache)
+
+type nopCache struct{}
+
+func (c nopCache) Get(u string) (*data.Image, bool) { return nil, false }
+func (c nopCache) Save(image *data.Image)           {}
+func (c nopCache) Delete(u string)                  {}