From aa81b0306c80d477e33e6e55156c631b45bf4bbc Mon Sep 17 00:00:00 2001
From: knoflook <knoflook@disroot.org>
Date: Fri, 1 Oct 2021 19:11:21 +0200
Subject: [PATCH] feat: add Tag.UpgradeElement(newTag Tag) which returns a
 difference between two tags

---
 tagcmp.go | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/tagcmp.go b/tagcmp.go
index c898c55..3d80f5c 100644
--- a/tagcmp.go
+++ b/tagcmp.go
@@ -166,6 +166,24 @@ func (t Tag) IsCompatible(tag Tag) bool {
 	return true
 }
 
+// UpgradeElement returns a Tag 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) {
+	if !curTag.IsCompatible(newTag) {
+		return Tag{}, fmt.Errorf("%s and %s are not compatible with each other", curTag.String(), newTag.String())
+	}
+	diff := curTag
+	diff.Major(strconv.Itoa(strconv.Atoi(newTag.Major) - strconv.Atoi(curTag.Major)))
+	if !curTag.MissingMinor {
+		diff.Minor(strconv.Itoa(strconv.Atoi(newTag.Minor) - strconv.Atoi(curTag.Minor)))
+	}
+	if !curTag.MissingPatch {
+		diff.Patch(strconv.Itoa(strconv.Atoi(newTag.Patch) - strconv.Atoi(curTag.Patch)))
+	}
+
+	return diff, nil
+}
+
 // CommitHashPattern matches commit-like hash tags
 var CommitHashPattern = "^[a-f0-9]{7,40}$"