refactor!: create and use TagDelta struct et al

add a TagDelta struct with accompanying function String()
rename UpgradeElement() to UpgradeDelta() and change the return type
move UpgradeType() to be a function of TagDelta
add ParseDelta() that returns a TagDelta from string

tagcmp.go

@@ -19,6 +19,12 @@ type Tag struct {
 	Metadata     string // metadata: what's after + and after the first "-"
 }
 
+type TagDelta struct {
+	Major int // major semver difference
+	Minor int // minor semver difference
+	Patch int // patch semver difference
+}
+
 // ByTagAsc sorts tags in ascending order where the last element is the latest tag.
 type ByTagAsc []Tag
 
@@ -148,6 +154,12 @@ func (t Tag) String() string {
 	return repr
 }
 
+func (t TagDelta) String() string {
+	var repr string
+	repr = fmt.Sprintf("%d.%d.%d", t.Major, t.Minor, t.Patch)
+	return repr
+}
+
 // IsCompatible determines if two tags can be compared together
 func (t Tag) IsCompatible(tag Tag) bool {
 	if t.UsesV && !tag.UsesV || tag.UsesV && !t.UsesV {
@@ -175,43 +187,30 @@ func (t Tag) IsCompatible(tag Tag) bool {
 	return true
 }
 
-// UpgradeElement returns a Tag object which is the difference between an old and new tag
+// UpgradeDelta returns a TagDelta object which is the difference between an old and new tag
 // It can contain negative numbers if comparing with an older tag.
-func (curTag Tag) UpgradeElement(newTag Tag) (Tag, error) {
+func (curTag Tag) UpgradeDelta(newTag Tag) (TagDelta, error) {
 	if !curTag.IsCompatible(newTag) {
-		return Tag{}, fmt.Errorf("%s and %s are not compatible with each other", curTag.String(), newTag.String())
+		return TagDelta{}, fmt.Errorf("%s and %s are not compatible with each other", curTag.String(), newTag.String())
 	}
-	diff := curTag
-	curMajor, err := strconv.Atoi(curTag.Major)
-	if err != nil {
-		return Tag{}, err
+	diff := TagDelta{
+		Major: 0,
+		Minor: 0,
+		Patch: 0,
 	}
-	newMajor, err := strconv.Atoi(newTag.Major)
-	if err != nil {
-		return Tag{}, err
-	}
-	diff.Major = strconv.Itoa(newMajor - curMajor)
+	// assuming tags are correctly formatted
+	curMajor, _ := strconv.Atoi(curTag.Major)
+	newMajor, _ := strconv.Atoi(newTag.Major)
+	diff.Major = newMajor - curMajor
 	if !curTag.MissingMinor {
-		curMinor, err := strconv.Atoi(curTag.Minor)
-		if err != nil {
-			return Tag{}, err
-		}
-		newMinor, err := strconv.Atoi(newTag.Minor)
-		if err != nil {
-			return Tag{}, err
-		}
-		diff.Minor = strconv.Itoa(newMinor - curMinor)
+		curMinor, _ := strconv.Atoi(curTag.Minor)
+		newMinor, _ := strconv.Atoi(newTag.Minor)
+		diff.Minor = newMinor - curMinor
 	}
 	if !curTag.MissingPatch {
-		curPatch, err := strconv.Atoi(curTag.Patch)
-		if err != nil {
-			return Tag{}, err
-		}
-		newPatch, err := strconv.Atoi(newTag.Patch)
-		if err != nil {
-			return Tag{}, err
-		}
-		diff.Patch = strconv.Itoa(newPatch - curPatch)
+		curPatch, _ := strconv.Atoi(curTag.Patch)
+		newPatch, _ := strconv.Atoi(newTag.Patch)
+		diff.Patch = newPatch - curPatch
 	}
 
 	return diff, nil
@@ -219,35 +218,23 @@ func (curTag Tag) UpgradeElement(newTag Tag) (Tag, error) {
 
 // UpgradeType takes exit from UpgradeElemene and returns a numeric representation of upgrade or downgrade
 // 1/-1: patch 2/-2: minor 4/-4: major 0: no change
-func UpgradeType(t Tag) int {
-	var major, minor, patch int
-	major, _ = strconv.Atoi(t.Major)
-	if t.MissingMinor {
-		minor = 0
-	} else {
-		minor, _ = strconv.Atoi(t.Minor)
-	}
-	if t.MissingPatch {
-		patch = 0
-	} else {
-		patch, _ = strconv.Atoi(t.Patch)
-	}
-	if major > 0 {
+func (d TagDelta) UpgradeType() int {
+	if d.Major > 0 {
 		return 4
 	}
-	if major < 0 {
+	if d.Major < 0 {
 		return -4
 	}
-	if minor > 0 {
+	if d.Minor > 0 {
 		return 2
 	}
-	if minor < 0 {
+	if d.Minor < 0 {
 		return -2
 	}
-	if patch > 0 {
+	if d.Patch > 0 {
 		return 1
 	}
-	if patch < 0 {
+	if d.Patch < 0 {
 		return -1
 	}
 	return 0
@@ -306,6 +293,42 @@ func parseVersionPart(part string) (int, error) {
 	return p, nil
 }
 
+// ParseDelta converts a tag difference in the format of X, X.Y or X.Y.Z where
+// X, Y, Z are positive or negative integers or 0
+func ParseDelta(delta string) (TagDelta, error) {
+	tagDelta := TagDelta{
+		Major: 0,
+		Minor: 0,
+		Patch: 0,
+	}
+
+	splits := strings.Split(delta, ".")
+	if len(splits) > 3 {
+		return TagDelta{}, fmt.Errorf("'%s' has too much dots", delta)
+	}
+	major, err := strconv.Atoi(splits[0])
+	if err != nil {
+		return TagDelta{}, fmt.Errorf("Major part of '%s' is not an integer", delta)
+	}
+	tagDelta.Major = major
+
+	if len(splits) > 1 {
+		minor, err := strconv.Atoi(splits[1])
+		if err != nil {
+			return TagDelta{}, fmt.Errorf("Minor part of '%s' is not an integer", delta)
+		}
+		tagDelta.Minor = minor
+	}
+	if len(splits) > 2 {
+		patch, err := strconv.Atoi(splits[2])
+		if err != nil {
+			return TagDelta{}, fmt.Errorf("Minor part of '%s' is not an integer", delta)
+		}
+		tagDelta.Patch = patch
+	}
+	return tagDelta, nil
+}
+
 // Parse converts an image tag into a structured data format. It aims to to
 // support the general case of tags which are "semver-like" and/or stable and
 // parseable by heuristics. Image tags follow no formal specification and