icns

commit 56bd4e8ba5a555d192d0bad277c3a736c9906991
parent a0554fe76cb70ad063a045dcbe2428a7f0c38dbd
Author: Jack Mordaunt <jackmordaunt.dev@gmail.com>
Date:   Fri, 18 Sep 2026 16:09:32 -0400

cmd/shell-extension: decode only the icon it needs

Explorer asks for one thumbnail at a time, and the provider decoded every
icon in the file to return one of them. Files now carry more elements than
they did, so the waste grew with them.

Diffstat:
M
cmd/shell-extension/internal/provider/provider.go
 | 
25
++++++++++++++++++++-----

1 file changed, 20 insertions(+), 5 deletions(-)
diff --git a/cmd/shell-extension/internal/provider/provider.go b/cmd/shell-extension/internal/provider/provider.go
@@ -198,16 +198,31 @@ func Thumbnail(r io.Reader, cx int) (*image.RGBA, error) {
 	if cx <= 0 {
 		return nil, fmt.Errorf("invalid thumbnail size %d", cx)
 	}
-	images, err := icns.DecodeAll(r) // Sorted largest first.
+	d, err := icns.NewDecoder(r)
 	if err != nil {
 		return nil, err
 	}
-	chosen := images[0]
-	for _, img := range images {
-		if side(img) < cx {
+	// The icons arrive largest first, so the last one still at least cx wide
+	// is the smallest that does not need upscaling. Only that one is decoded.
+	var (
+		best  icns.Entry
+		found bool
+	)
+	for _, icon := range d.Icons() {
+		if icon.ImageFormat == icns.ImageFormatJPEG2000 {
+			continue
+		}
+		if found && int(icon.Size) < cx {
 			break
 		}
-		chosen = img
+		best, found = icon, true
+	}
+	if !found {
+		return nil, fmt.Errorf("no icon in a format this build can decode")
+	}
+	chosen, err := best.Decode()
+	if err != nil {
+		return nil, err
 	}
 	// Normalise to premultiplied RGBA, which is what a GDI ARGB bitmap
 	// wants, shrinking the icon to fit the cx square on the way if needed.