[#373] pruning: add optional regex field to last_n rule

fixes #373

pruning/keep_last_n.go

@@ -1,6 +1,7 @@
 package pruning
 
 import (
+	"regexp"
 	"sort"
 	"strings"
 
@@ -8,14 +9,27 @@ import (
 )
 
 type KeepLastN struct {
-	n int
+	n  int
+	re *regexp.Regexp
 }
 
-func NewKeepLastN(n int) (*KeepLastN, error) {
+func MustKeepLastN(n int, regex string) *KeepLastN {
+	k, err := NewKeepLastN(n, regex)
+	if err != nil {
+		panic(err)
+	}
+	return k
+}
+
+func NewKeepLastN(n int, regex string) (*KeepLastN, error) {
 	if n <= 0 {
 		return nil, errors.Errorf("must specify positive number as 'keep last count', got %d", n)
 	}
-	return &KeepLastN{n}, nil
+	re, err := regexp.Compile(regex)
+	if err != nil {
+		return nil, errors.Errorf("invalid regex %q: %s", regex, err)
+	}
+	return &KeepLastN{n, re}, nil
 }
 
 func (k KeepLastN) KeepRule(snaps []Snapshot) (destroyList []Snapshot) {
@@ -24,17 +38,25 @@ func (k KeepLastN) KeepRule(snaps []Snapshot) (destroyList []Snapshot) {
 		return []Snapshot{}
 	}
 
-	res := shallowCopySnapList(snaps)
+	matching, notMatching := partitionSnapList(snaps, func(snapshot Snapshot) bool {
+		return k.re.MatchString(snapshot.Name())
+	})
+	// snaps that don't match the regex are not kept by this rule
+	destroyList = append(destroyList, notMatching...)
 
-	sort.Slice(res, func(i, j int) bool {
+	if len(matching) == 0 {
+		return destroyList
+	}
+
+	sort.Slice(matching, func(i, j int) bool {
 		// by date (youngest first)
-		id, jd := res[i].Date(), res[j].Date()
+		id, jd := matching[i].Date(), matching[j].Date()
 		if !id.Equal(jd) {
 			return id.After(jd)
 		}
 		// then lexicographically descending (e.g. b, a)
-		return strings.Compare(res[i].Name(), res[j].Name()) == 1
+		return strings.Compare(matching[i].Name(), matching[j].Name()) == 1
 	})
-
-	return res[k.n:]
+	destroyList = append(destroyList, matching[k.n:]...)
+	return destroyList
 }