feat: support asc/dec sorting

Closes coop-cloud/tagcmp#1.

README.md

@@ -57,7 +57,7 @@ func main() {
 		}
 	}
 
-	sort.Sort(tagcmp.ByTag(compatible))
+	sort.Sort(tagcmp.ByTagAsc(compatible))
 
 	fmt.Println(compatible)
 }

godoc.md

@@ -12,10 +12,14 @@ Package tagcmp provides image tag comparison operations\.
 
 - [Variables](<#variables>)
 - [func IsParsable(tag string) bool](<#func-isparsable>)
-- [type ByTag](<#type-bytag>)
+- [type ByTagAsc](<#type-bytagasc>)
-  - [func (t ByTag) Len() int](<#func-bytag-len>)
+  - [func (t ByTagAsc) Len() int](<#func-bytagasc-len>)
-  - [func (t ByTag) Less(i, j int) bool](<#func-bytag-less>)
+  - [func (t ByTagAsc) Less(i, j int) bool](<#func-bytagasc-less>)
-  - [func (t ByTag) Swap(i, j int)](<#func-bytag-swap>)
+  - [func (t ByTagAsc) Swap(i, j int)](<#func-bytagasc-swap>)
+- [type ByTagDesc](<#type-bytagdesc>)
+  - [func (t ByTagDesc) Len() int](<#func-bytagdesc-len>)
+  - [func (t ByTagDesc) Less(i, j int) bool](<#func-bytagdesc-less>)
+  - [func (t ByTagDesc) Swap(i, j int)](<#func-bytagdesc-swap>)
 - [type Tag](<#type-tag>)
   - [func Parse(tag string) (Tag, error)](<#func-parse>)
   - [func (t Tag) Equals(tag Tag) bool](<#func-tag-equals>)
@@ -65,30 +69,56 @@ func IsParsable(tag string) bool
 
 IsParsable determines if a tag is supported by this library
 
-## type ByTag
+## type ByTagAsc
 
-ByTag sorts tags in asc/desc order where the last element is the latest tag\.
+ByTagAsc sorts tags in ascending order where the last element is the latest tag\.
 
 ```go
-type ByTag []Tag
+type ByTagAsc []Tag
 ```
 
-### func \(ByTag\) Len
+### func \(ByTagAsc\) Len
 
 ```go
-func (t ByTag) Len() int
+func (t ByTagAsc) Len() int
 ```
 
-### func \(ByTag\) Less
+### func \(ByTagAsc\) Less
 
 ```go
-func (t ByTag) Less(i, j int) bool
+func (t ByTagAsc) Less(i, j int) bool
 ```
 
-### func \(ByTag\) Swap
+### func \(ByTagAsc\) Swap
 
 ```go
-func (t ByTag) Swap(i, j int)
+func (t ByTagAsc) Swap(i, j int)
+```
+
+## type ByTagDesc
+
+ByTagDesc sorts tags in descending order where the first element is the latest tag\.
+
+```go
+type ByTagDesc []Tag
+```
+
+### func \(ByTagDesc\) Len
+
+```go
+func (t ByTagDesc) Len() int
+```
+
+### func \(ByTagDesc\) Less
+
+```go
+func (t ByTagDesc) Less(i, j int) bool
+```
+
+### func \(ByTagDesc\) Swap
+
+```go
+func (t ByTagDesc) Swap(i, j int)
 ```
 
 ## type Tag

tagcmp.go

@@ -18,15 +18,24 @@ type Tag struct {
 	UsesV        bool   // whether or not the tag uses the "v" prefix
 }
 
-// ByTag sorts tags in asc/desc order where the last element is the latest tag.
+// ByTagAsc sorts tags in ascending order where the last element is the latest tag.
-type ByTag []Tag
+type ByTagAsc []Tag
 
-func (t ByTag) Len() int      { return len(t) }
+func (t ByTagAsc) Len() int      { return len(t) }
-func (t ByTag) Swap(i, j int) { t[i], t[j] = t[j], t[i] }
+func (t ByTagAsc) Swap(i, j int) { t[i], t[j] = t[j], t[i] }
-func (t ByTag) Less(i, j int) bool {
+func (t ByTagAsc) Less(i, j int) bool {
 	return t[i].IsLessThan(t[j])
 }
 
+// ByTagDesc sorts tags in descending order where the first element is the latest tag.
+type ByTagDesc []Tag
+
+func (t ByTagDesc) Len() int      { return len(t) }
+func (t ByTagDesc) Swap(i, j int) { t[j], t[i] = t[i], t[j] }
+func (t ByTagDesc) Less(i, j int) bool {
+	return t[j].IsLessThan(t[i])
+}
+
 // IsGreaterThan tests if a tag is greater than another. There are some
 // tag-isms to take into account here, shorter is bigger (i.e. 2.1 > 2.1.1 ==
 // true, 2 > 2.1 == true).

tagcmp_test.go

@@ -617,7 +617,7 @@ func TestIsCompatible(t *testing.T) {
 	}
 }
 
-func TestSort1(t *testing.T) {
+func TestSortAsc1(t *testing.T) {
 	rawTags := []string{
 		"v1.4.8",
 		"v1.3.9",
@@ -637,7 +637,7 @@ func TestSort1(t *testing.T) {
 		tags = append(tags, tag)
 	}
 
-	sort.Sort(tagcmp.ByTag(tags))
+	sort.Sort(tagcmp.ByTagAsc(tags))
 
 	expected := []string{
 		"v0.0.1",
@@ -656,7 +656,7 @@ func TestSort1(t *testing.T) {
 	}
 }
 
-func TestSort2(t *testing.T) {
+func TestSortAsc2(t *testing.T) {
 	rawTags := []string{
 		"10.0",
 		"10.6",
@@ -675,7 +675,7 @@ func TestSort2(t *testing.T) {
 		tags = append(tags, tag)
 	}
 
-	sort.Sort(tagcmp.ByTag(tags))
+	sort.Sort(tagcmp.ByTagAsc(tags))
 
 	expected := []string{
 		"5.5",
@@ -693,6 +693,43 @@ func TestSort2(t *testing.T) {
 	}
 }
 
+func TestSortDesc(t *testing.T) {
+	rawTags := []string{
+		"10.0",
+		"10.6",
+		"10.2",
+		"10.1",
+		"10.5",
+		"5.5",
+	}
+
+	var tags []tagcmp.Tag
+	for _, rawTag := range rawTags {
+		tag, err := tagcmp.Parse(rawTag)
+		if err != nil {
+			t.Errorf("'%s' should have parsed but didn't: %s", tag, err)
+		}
+		tags = append(tags, tag)
+	}
+
+	sort.Sort(tagcmp.ByTagDesc(tags))
+
+	expected := []string{
+		"10.6",
+		"10.5",
+		"10.2",
+		"10.1",
+		"10.0",
+		"5.5",
+	}
+
+	for idx, tag := range tags {
+		if tag.String() != expected[idx] {
+			t.Errorf("'%s' sorted out of order, saw '%s', expected '%s'", tag, tags, expected)
+		}
+	}
+}
+
 func TestString(t *testing.T) {
 	for _, tag := range supported {
 		p, err := tagcmp.Parse(tag)
@@ -730,7 +767,7 @@ func TestGiteaFilterCompatible(t *testing.T) {
 		}
 	}
 
-	sort.Sort(tagcmp.ByTag(filtered))
+	sort.Sort(tagcmp.ByTagAsc(filtered))
 
 	for idx, tag := range filtered {
 		if tag.String() != expected[idx] {