Refactor duplicate code for grouping snapshots

This commit is a followup to the addition of the --group-by flag for the
snapshots command. Adding the grouping code there introduced duplicated
code (the forget command also does grouping). This commit refactors
boths sides to only use shared code.

internal/restic/snapshot_group.go

@@ -0,0 +1,76 @@
+package restic
+
+import (
+	"encoding/json"
+	"sort"
+	"strings"
+
+	"github.com/restic/restic/internal/errors"
+)
+
+// SnapshotGroupKey is the structure for identifying groups in a grouped
+// snapshot list. This is used by GroupSnapshots()
+type SnapshotGroupKey struct {
+	Hostname string   `json:"hostname"`
+	Paths    []string `json:"paths"`
+	Tags     []string `json:"tags"`
+}
+
+// GroupSnapshots takes a list of snapshots and a grouping criteria and creates
+// a group list of snapshots.
+func GroupSnapshots(snapshots Snapshots, options string) (map[string]Snapshots, bool, error) {
+	// group by hostname and dirs
+	snapshotGroups := make(map[string]Snapshots)
+
+	var GroupByTag bool
+	var GroupByHost bool
+	var GroupByPath bool
+	var GroupOptionList []string
+
+	GroupOptionList = strings.Split(options, ",")
+
+	for _, option := range GroupOptionList {
+		switch option {
+		case "host":
+			GroupByHost = true
+		case "paths":
+			GroupByPath = true
+		case "tags":
+			GroupByTag = true
+		case "":
+		default:
+			return nil, false, errors.Fatal("unknown grouping option: '" + option + "'")
+		}
+	}
+
+	for _, sn := range snapshots {
+		// Determining grouping-keys
+		var tags []string
+		var hostname string
+		var paths []string
+
+		if GroupByTag {
+			tags = sn.Tags
+			sort.StringSlice(tags).Sort()
+		}
+		if GroupByHost {
+			hostname = sn.Hostname
+		}
+		if GroupByPath {
+			paths = sn.Paths
+		}
+
+		sort.StringSlice(sn.Paths).Sort()
+		var k []byte
+		var err error
+
+		k, err = json.Marshal(SnapshotGroupKey{Tags: tags, Hostname: hostname, Paths: paths})
+
+		if err != nil {
+			return nil, false, err
+		}
+		snapshotGroups[string(k)] = append(snapshotGroups[string(k)], sn)
+	}
+
+	return snapshotGroups, GroupByTag || GroupByHost || GroupByPath, nil
+}