generate animated thumbnails for animated gifs

thumbnails/pkg/thumbnail/encoding.go

@@ -1,17 +1,33 @@
 package thumbnail
 
 import (
+	"errors"
 	"image"
+	"image/gif"
 	"image/jpeg"
 	"image/png"
 	"io"
 	"strings"
 )
 
+const (
+	typePng  = "png"
+	typeJpg  = "jpg"
+	typeJpeg = "jpeg"
+	typeGif  = "gif"
+)
+
+var (
+	// ErrInvalidType represents the error when a type can't be encoded.
+	ErrInvalidType = errors.New("can't encode this type")
+	// ErrNoEncoderForType represents the error when an encoder couldn't be found for a type.
+	ErrNoEncoderForType = errors.New("no encoder for this type found")
+)
+
 // Encoder encodes the thumbnail to a specific format.
 type Encoder interface {
 	// Encode encodes the image to a format.
-	Encode(io.Writer, image.Image) error
+	Encode(io.Writer, interface{}) error
 	// Types returns the formats suffixes.
 	Types() []string
 	// MimeType returns the mimetype used by the encoder.
@@ -22,13 +38,17 @@ type Encoder interface {
 type PngEncoder struct{}
 
 // Encode encodes to png format
-func (e PngEncoder) Encode(w io.Writer, i image.Image) error {
-	return png.Encode(w, i)
+func (e PngEncoder) Encode(w io.Writer, img interface{}) error {
+	m, ok := img.(image.Image)
+	if !ok {
+		return ErrInvalidType
+	}
+	return png.Encode(w, m)
 }
 
 // Types returns the png suffix
 func (e PngEncoder) Types() []string {
-	return []string{"png"}
+	return []string{typePng}
 }
 
 // MimeType returns the mimetype for png files.
@@ -40,13 +60,17 @@ func (e PngEncoder) MimeType() string {
 type JpegEncoder struct{}
 
 // Encode encodes to jpg
-func (e JpegEncoder) Encode(w io.Writer, i image.Image) error {
-	return jpeg.Encode(w, i, nil)
+func (e JpegEncoder) Encode(w io.Writer, img interface{}) error {
+	m, ok := img.(image.Image)
+	if !ok {
+		return ErrInvalidType
+	}
+	return jpeg.Encode(w, m, nil)
 }
 
 // Types returns the jpg suffixes.
 func (e JpegEncoder) Types() []string {
-	return []string{"jpeg", "jpg"}
+	return []string{typeJpeg, typeJpg}
 }
 
 // MimeType returns the mimetype for jpg files.
@@ -54,15 +78,35 @@ func (e JpegEncoder) MimeType() string {
 	return "image/jpeg"
 }
 
+type GifEncoder struct{}
+
+func (e GifEncoder) Encode(w io.Writer, img interface{}) error {
+	g, ok := img.(*gif.GIF)
+	if !ok {
+		return ErrInvalidType
+	}
+	return gif.EncodeAll(w, g)
+}
+
+func (e GifEncoder) Types() []string {
+	return []string{typeGif}
+}
+
+func (e GifEncoder) MimeType() string {
+	return "image/gif"
+}
+
 // EncoderForType returns the encoder for a given file type
 // or nil if the type is not supported.
-func EncoderForType(fileType string) Encoder {
+func EncoderForType(fileType string) (Encoder, error) {
 	switch strings.ToLower(fileType) {
-	case "png":
-		return PngEncoder{}
-	case "jpg", "jpeg":
-		return JpegEncoder{}
+	case typePng:
+		return PngEncoder{}, nil
+	case typeJpg, typeJpeg:
+		return JpegEncoder{}, nil
+	case typeGif:
+		return GifEncoder{}, nil
 	default:
-		return nil
+		return nil, ErrNoEncoderForType
 	}
 }