diff --git a/internal/epub/sortpath/core.go b/internal/epub/sortpath/core.go
new file mode 100644
index 0000000..17b3eaa
--- /dev/null
+++ b/internal/epub/sortpath/core.go
@@ -0,0 +1,83 @@
+package sortpath
+
+import (
+	"path/filepath"
+	"regexp"
+	"strconv"
+	"strings"
+)
+
+var split_path_regex = regexp.MustCompile(`^(.*?)(\d+)$`)
+
+type part struct {
+	name   string
+	number int
+}
+
+func (a part) Compare(b part) int {
+	if a.name == b.name {
+		return a.number - b.number
+	} else {
+		return strings.Compare(a.name, b.name)
+	}
+}
+
+func parsePart(p string) part {
+	r := split_path_regex.FindStringSubmatch(p)
+	if len(r) == 0 {
+		return part{p, 0}
+	}
+	n, err := strconv.ParseInt(r[2], 10, 32)
+	if err != nil {
+		return part{p, 0}
+	}
+	return part{r[1], int(n)}
+}
+
+func parse(filename string) []part {
+	pathname, name := filepath.Split(strings.ToLower(filename))
+	pathname = strings.TrimSuffix(pathname, string(filepath.Separator))
+	ext := filepath.Ext(name)
+	name = name[0 : len(name)-len(ext)]
+
+	f := []part{}
+	for _, p := range strings.Split(pathname, string(filepath.Separator)) {
+		f = append(f, parsePart(p))
+	}
+	f = append(f, parsePart(name))
+	return f
+}
+
+func comparePart(a, b []part) int {
+	m := len(a)
+	if m > len(b) {
+		m = len(b)
+	}
+	for i := 0; i < m; i++ {
+		c := a[i].Compare(b[i])
+		if c != 0 {
+			return c
+		}
+	}
+	return len(a) - len(b)
+}
+
+type by struct {
+	filenames []string
+	paths     [][]part
+}
+
+func (b by) Len() int           { return len(b.filenames) }
+func (b by) Less(i, j int) bool { return comparePart(b.paths[i], b.paths[j]) < 0 }
+func (b by) Swap(i, j int) {
+	b.filenames[i], b.filenames[j] = b.filenames[j], b.filenames[i]
+	b.paths[i], b.paths[j] = b.paths[j], b.paths[i]
+}
+
+func By(filenames []string) by {
+	p := [][]part{}
+	for _, filename := range filenames {
+		p = append(p, parse(filename))
+	}
+	return by{filenames, p}
+}