add ResolveRelationship

part.go

@@ -1,8 +1,10 @@
 package opc
 
 import (
+	"fmt"
 	"mime"
 	"net/url"
+	"path/filepath"
 	"strings"
 )
 
@@ -28,6 +30,21 @@ func (p *Part) validate() error {
 
 var defaultRef, _ = url.Parse("http://defaultcontainer/")
 
+// ResolveRelationship returns the absolute URI (from the package root) of the part pointed by a relationship of a source part.
+// This method should be used in places where we have a target relationship URI and we want to get the
+// name of the part it targets with respect to the source part.
+// The source can be a valid part URI, for part relationships, or "/", for package relationships.
+func ResolveRelationship(source string, rel string) string {
+	if !strings.HasPrefix(rel, "/") && !strings.HasPrefix(rel, "\\") {
+		sourceDir := strings.Replace(filepath.Dir(source), "\\", "/", -1)
+		if sourceDir == "/" {
+			return "/" + rel
+		}
+		return fmt.Sprintf("%s/%s", sourceDir, rel)
+	}
+	return rel
+}
+
 // NormalizePartName transforms the input name so it follows the constrains specified in the ISO/IEC 29500-2 §9.1.1:
 //     part-URI = 1*( "/" segment )
 //     segment = 1*( pchar )

part_test.go

@@ -100,3 +100,29 @@ func TestPart_validate(t *testing.T) {
 		})
 	}
 }
+
+func TestResolveRelationship(t *testing.T) {
+	type args struct {
+		source string
+		rel    string
+	}
+	tests := []struct {
+		name string
+		args args
+		want string
+	}{
+		{"package", args{"/", "c.xml"}, "/c.xml"},
+		{"packageWin", args{"\\", "c.xml"}, "/c.xml"},
+		{"rel", args{"/3D/3dmodel.model", "c.xml"}, "/3D/c.xml"},
+		{"rel", args{"/3D/3dmodel.model", "/3D/box1.model"}, "/3D/box1.model"},
+		{"rel", args{"/3D/box3.model", "/2D/2dmodel.model"}, "/2D/2dmodel.model"},
+		{"relChild", args{"/3D/box3.model", "2D/2dmodel.model"}, "/3D/2D/2dmodel.model"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := ResolveRelationship(tt.args.source, tt.args.rel); got != tt.want {
+				t.Errorf("ResolveRelationship() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}