1 files changed, 85 insertions, 0 deletions
diff --git a/src/post/asset/loader_image.go b/src/post/asset/loader_image.go
new file mode 100644
index 0000000..dc996a9
--- /dev/null
+++ b/src/post/asset/loader_image.go
@@ -0,0 +1,85 @@
+package asset
+
+import (
+	"bytes"
+	"fmt"
+	"image"
+	"image/jpeg"
+	"image/png"
+	"io"
+	"path/filepath"
+	"strings"
+
+	"golang.org/x/image/draw"
+)
+
+// IsImageResizable returns whether or not an image can be resized, based on its
+// extension.
+func IsImageResizable(path string) bool {
+	switch strings.ToLower(filepath.Ext(path)) {
+	case ".jpg", ".jpeg", ".png":
+		return true
+	default:
+		return false
+	}
+}
+
+type imageLoader struct {
+	loader Loader
+}
+
+// NewImageLoader wraps an existing Loader in order to perform various
+// image-related transformations on any image assets being loaded. Non-image
+// assets are loaded as-is.
+func NewImageLoader(loader Loader) Loader {
+	return &imageLoader{loader}
+}
+
+func (l *imageLoader) Load(path string, into io.Writer, opts LoadOpts) error {
+
+	if opts.ImageWidth == 0 {
+		return l.loader.Load(path, into, opts)
+	}
+
+	if !IsImageResizable(path) {
+		return ErrCannotResize
+	}
+
+	buf := new(bytes.Buffer)
+
+	if err := l.loader.Load(path, buf, opts); err != nil {
+		return fmt.Errorf("loading image into buffer: %w", err)
+	}
+
+	img, format, err := image.Decode(buf)
+	if err != nil {
+		return fmt.Errorf("decoding image: %w", err)
+	}
+
+	maxWidth := float64(opts.ImageWidth)
+	imgRect := img.Bounds()
+	imgW, imgH := float64(imgRect.Dx()), float64(imgRect.Dy())
+
+	if imgW > maxWidth {
+
+		newH := imgH * maxWidth / imgW
+		newImg := image.NewRGBA(image.Rect(0, 0, int(maxWidth), int(newH)))
+
+		// Resize
+		draw.BiLinear.Scale(
+			newImg, newImg.Bounds(), img, img.Bounds(), draw.Over, nil,
+		)
+
+		img = newImg
+	}
+
+	switch format {
+	case "jpeg":
+		return jpeg.Encode(into, img, nil)
+	case "png":
+		return png.Encode(into, img)
+	default:
+		return fmt.Errorf("unknown image format %q", format)
+	}
+
+}