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

pruning/pruning.go

@@ -0,0 +1,39 @@
+package pruning
+
+import (
+	"time"
+)
+
+type KeepRule interface {
+	KeepRule(snaps []Snapshot) []Snapshot
+}
+
+type Snapshot interface {
+	Name() string
+	Replicated() bool
+	Date() time.Time
+}
+
+func PruneSnapshots(snaps []Snapshot, keepRules []KeepRule) []Snapshot {
+
+	if len(keepRules) == 0 {
+		return snaps
+	}
+
+	remCount := make(map[Snapshot]int, len(snaps))
+	for _, r := range keepRules {
+		ruleRems := r.KeepRule(snaps)
+		for _, ruleRem := range ruleRems {
+			remCount[ruleRem]++
+		}
+	}
+
+	remove := make([]Snapshot, 0, len(snaps))
+	for snap, rc := range remCount {
+		if rc == len(keepRules) {
+			remove = append(remove, snap)
+		}
+	}
+
+	return remove
+}