Added cover path utilities

* Image part hashing based on @shermp's Kobo-UNCaGED code.
* Qt resizing algorithm re-implementation based on our (@NiLuJe, @geek1011) implementation for KU.
* Path/ImageID/ContentID conversion based on my seriesmeta code from kepubify.

kobo/device.go

@@ -322,7 +322,7 @@ func (c CoverType) String() string {
 	return c.NickelString()
 }
 
-// Resize returnes the dimensions to resize sz to for the cover type and target size.
+// Resize returns the dimensions to resize sz to for the cover type and target size.
 func (c CoverType) Resize(target image.Point, sz image.Point) image.Point {
 	switch c {
 	case CoverTypeLibList:
@@ -333,6 +333,13 @@ func (c CoverType) Resize(target image.Point, sz image.Point) image.Point {
 	panic("unknown cover type")
 }
 
+// GeneratePath generates the path for the cover of an ImageID. The path is always
+// separated with forward slashes.
+func (c CoverType) GeneratePath(iid string) string {
+	dir1, dir2, base := hashedImageParts(iid)
+	return fmt.Sprintf(".kobo-images/%s/%s/%s - %s.jpg", dir1, dir2, base, c.NickelString())
+}
+
 func (d Device) StorageGB() int {
 	switch d {
 	case DeviceTouchAB, DeviceTouchC, DeviceMini:

kobo/util.go

@@ -1,11 +1,29 @@
 package kobo
 
 import (
+	"fmt"
 	"image"
+	"path/filepath"
 	"strconv"
 	"strings"
 )
 
+// PathToContentID generates the Kobo ContentId for a path relative to the
+// internal storage root (slashes are converted to forward slashes automatically).
+func PathToContentID(relpath string) string {
+	return fmt.Sprintf("file:///mnt/onboard/%s", filepath.ToSlash(relpath))
+}
+
+// ContentIDToImageID converts the Kobo ContentId to the ImageId.
+func ContentIDToImageID(contentID string) string {
+	return strings.NewReplacer(
+		" ", "_",
+		"/", "_",
+		":", "_",
+		".", "_",
+	).Replace(contentID)
+}
+
 // resizeKeepAspectRatio resizes sz to fill bounds while keeping the aspect
 // ratio. It is based on the code for QSize::scaled with the modes
 // Qt::KeepAspectRatio and Qt::KeepAspectRatioByExpanding.
@@ -30,6 +48,20 @@ func resizeKeepAspectRatio(sz image.Point, bounds image.Point, expand bool) imag
 	return image.Pt(bounds.X, int(float64(bounds.X)/ar))
 }
 
+// hashedImageParts returns the parts needed for constructing the path to the
+// cached image. The result can be applied like:
+// .kobo-images/{dir1}/{dir2}/{basename} - N3_SOMETHING.jpg
+func hashedImageParts(imageID string) (dir1, dir2, basename string) {
+	imgID := []byte(imageID)
+	h := uint32(0x00000000)
+	for _, x := range imgID {
+		h = (h << 4) + uint32(x)
+		h ^= (h & 0xf0000000) >> 23
+		h &= 0x0fffffff
+	}
+	return fmt.Sprintf("%d", h&(0xff*1)), fmt.Sprintf("%d", (h&(0xff00*1))>>8), imageID
+}
+
 func strSplitInt(str string) []int64 {
 	spl := strings.Split(str, ".")
 	ints := make([]int64, len(spl))