create a protocol-parsing package to distinguish dir,docker, etc...

Signed-off-by: Alfredo Deza <adeza@anchore.com>

imgbom/protocol.go

@@ -0,0 +1,58 @@
+package imgbom
+
+import "strings"
+
+// Potentially consider moving this out into a generic package that parses user input.
+// Aside from scope, this is the 2nd package that looks at a string to parse the input
+// and return an Option type.
+
+const (
+	UnknownProtocol ProtocolType = iota
+	ImageProtocol
+	DirProtocol
+)
+
+var optionStr = []string{
+	"UnknownProtocol",
+	"image",
+	"dir",
+}
+
+type ProtocolType int
+
+type Protocol struct {
+	Type  ProtocolType
+	Value string
+}
+
+func NewProtocol(userStr string) Protocol {
+	candidates := strings.Split(userStr, "://")
+
+	switch len(candidates) {
+	case 2:
+		if strings.HasPrefix(userStr, "dir://") {
+			return Protocol{
+				Type:  DirProtocol,
+				Value: strings.TrimPrefix(userStr, "dir://"),
+			}
+		}
+		// default to an Image for anything else since stereoscope can handle this
+		return Protocol{
+			Type:  ImageProtocol,
+			Value: userStr,
+		}
+	default:
+		return Protocol{
+			Type:  ImageProtocol,
+			Value: userStr,
+		}
+	}
+}
+
+func (o ProtocolType) String() string {
+	if int(o) >= len(optionStr) || o < 0 {
+		return optionStr[0]
+	}
+
+	return optionStr[o]
+}