move implementation to internal/ directory (#828)

internal/pruning/keep_last_n.go

@@ -0,0 +1,62 @@
+package pruning
+
+import (
+	"regexp"
+	"sort"
+	"strings"
+
+	"github.com/pkg/errors"
+)
+
+type KeepLastN struct {
+	n  int
+	re *regexp.Regexp
+}
+
+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)
+	}
+	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) {
+	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...)
+
+	if len(matching) == 0 {
+		return destroyList
+	}
+
+	sort.Slice(matching, func(i, j int) bool {
+		// by date (youngest first)
+		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(matching[i].Name(), matching[j].Name()) == 1
+	})
+
+	n := k.n
+	if n > len(matching) {
+		n = len(matching)
+	}
+	destroyList = append(destroyList, matching[n:]...)
+	return destroyList
+}