move cmd/pruning to pruning, as it's independent of the command implementation

pruning/keep_last_n.go

@@ -0,0 +1,32 @@
+package pruning
+
+import (
+	"github.com/pkg/errors"
+	"sort"
+)
+
+type KeepLastN struct {
+	n int
+}
+
+func NewKeepLastN(n int) (*KeepLastN, error) {
+	if n <= 0 {
+		return nil, errors.Errorf("must specify positive number as 'keep last count', got %d", n)
+	}
+	return &KeepLastN{n}, nil
+}
+
+func (k KeepLastN) KeepRule(snaps []Snapshot) []Snapshot {
+
+	if k.n > len(snaps) {
+		return snaps
+	}
+
+	res := shallowCopySnapList(snaps)
+
+	sort.Slice(res, func(i, j int) bool {
+		return res[i].Date().After(res[j].Date())
+	})
+
+	return res[:k.n]
+}