Add functions to compute unique prefix length

backend/generic.go

@@ -7,9 +7,14 @@ import (
 	"encoding/hex"
 	"errors"
 	"io/ioutil"
+	"sort"
 	"sync"
 )
 
+const (
+	MinPrefixLength = 4
+)
+
 var idPool = sync.Pool{New: func() interface{} { return ID(make([]byte, IDSize)) }}
 
 var (
@@ -143,3 +148,32 @@ func FindSnapshot(be Server, s string) (ID, error) {
 
 	return id, nil
 }
+
+// PrefixLength returns the number of bytes required so that all prefixes of
+// all IDs of type t are unique.
+func PrefixLength(be Lister, t Type) (int, error) {
+	// load all IDs of the given type
+	list, err := be.List(t)
+	if err != nil {
+		return 0, err
+	}
+
+	sort.Sort(list)
+
+	// select prefixes of length l, test if the last one is the same as the current one
+outer:
+	for l := MinPrefixLength; l < IDSize; l++ {
+		var last ID
+
+		for _, id := range list {
+			if bytes.Equal(last, id[:l]) {
+				continue outer
+			}
+			last = id[:l]
+		}
+
+		return l, nil
+	}
+
+	return IDSize, nil
+}

backend/generic_test.go

@@ -6,6 +6,8 @@ import (
 	"reflect"
 	"runtime"
 	"testing"
+
+	"github.com/fd0/khepri/backend"
 )
 
 // assert fails the test if the condition is false.
@@ -34,3 +36,43 @@ func equals(tb testing.TB, exp, act interface{}) {
 		tb.FailNow()
 	}
 }
+
+func str2id(s string) backend.ID {
+	id, err := backend.ParseID(s)
+	if err != nil {
+		panic(err)
+	}
+
+	return id
+}
+
+type IDList backend.IDs
+
+var samples = IDList{
+	str2id("20bdc1402a6fc9b633aaffffffffffffffffffffffffffffffffffffffffffff"),
+	str2id("20bdc1402a6fc9b633ccd578c4a92d0f4ef1a457fa2e16c596bc73fb409d6cc0"),
+	str2id("20bdc1402a6fc9b633ffffffffffffffffffffffffffffffffffffffffffffff"),
+	str2id("20ff988befa5fc40350f00d531a767606efefe242c837aaccb80673f286be53d"),
+	str2id("326cb59dfe802304f96ee9b5b9af93bdee73a30f53981e5ec579aedb6f1d0f07"),
+	str2id("86b60b9594d1d429c4aa98fa9562082cabf53b98c7dc083abe5dae31074dd15a"),
+	str2id("96c8dbe225079e624b5ce509f5bd817d1453cd0a85d30d536d01b64a8669aeae"),
+	str2id("fa31d65b87affcd167b119e9d3d2a27b8236ca4836cb077ed3e96fcbe209b792"),
+}
+
+func (l IDList) List(backend.Type) (backend.IDs, error) {
+	return backend.IDs(l), nil
+}
+
+func TestPrefixLength(t *testing.T) {
+	l, err := backend.PrefixLength(samples, backend.Snapshot)
+	ok(t, err)
+	equals(t, 10, l)
+
+	l, err = backend.PrefixLength(samples[:3], backend.Snapshot)
+	ok(t, err)
+	equals(t, 10, l)
+
+	l, err = backend.PrefixLength(samples[3:], backend.Snapshot)
+	ok(t, err)
+	equals(t, 4, l)
+}

backend/interface.go

@@ -21,10 +21,14 @@ var (
 	ErrAlreadyPresent = errors.New("blob is already present in backend")
 )
 
+type Lister interface {
+	List(Type) (IDs, error)
+}
+
 type Server interface {
 	Create(Type, []byte) (ID, error)
 	Get(Type, ID) ([]byte, error)
-	List(Type) (IDs, error)
+	Lister
 	Test(Type, ID) (bool, error)
 	Remove(Type, ID) error
 	Version() uint

cmd/khepri/cmd_snapshots.go

@@ -52,6 +52,10 @@ func commandSnapshots(be backend.Server, key *khepri.Key, args []string) error {
 	fmt.Printf("%s\n", strings.Repeat("-", 80))
 
 	list := []*khepri.Snapshot{}
+	plen, err := backend.PrefixLength(be, backend.Snapshot)
+	if err != nil {
+		return err
+	}
 
 	backend.EachID(be, backend.Snapshot, func(id backend.ID) {
 		sn, err := ch.LoadSnapshot(id)
@@ -74,7 +78,7 @@ func commandSnapshots(be backend.Server, key *khepri.Key, args []string) error {
 	})
 
 	for _, sn := range list {
-		fmt.Printf("%-8s  %-19s  %-10s  %s\n", sn.ID().String()[:8], sn.Time.Format(TimeFormat), sn.Hostname, sn.Dir)
+		fmt.Printf("%-8s  %-19s  %-10s  %s\n", sn.ID()[:plen], sn.Time.Format(TimeFormat), sn.Hostname, sn.Dir)
 	}
 
 	return nil