From 5294d8d4844351d6169a1650d8f6ccc70120b07c Mon Sep 17 00:00:00 2001
From: knoflook <knoflook@disroot.org>
Date: Fri, 1 Oct 2021 18:56:46 +0200
Subject: [PATCH] feat: add Tag.UpgradeElement(newTag Tag) which returns a
 difference between two tags

---
 tagcmp.go | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/tagcmp.go b/tagcmp.go
index c898c55..825c016 100644
--- a/tagcmp.go
+++ b/tagcmp.go
@@ -166,6 +166,25 @@ 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 := Tag{
+		Major:        Itoa(Atoi(newTag.Major) - Atoi(curTag.Major)),
+		Minor:        Itoa(Atoi(newTag.Minor) - Atoi(curTag.Minor)),
+		MissingMinor: false,
+		Patch:        Itoa(Atoi(newTag.Patch) - Atoi(curTag.Patch)),
+		MissingPatch: false,
+		UsesV:        false,
+		Suffix:       "",
+	}
+
+	return diff, nil
+}
+
 // CommitHashPattern matches commit-like hash tags
 var CommitHashPattern = "^[a-f0-9]{7,40}$"