List snapshots, accept snapshot id prefix

Example:

    $ ./khepri snapshots
    ID        Date                 Source      Directory
    --------------------------------------------------------------------------------
    fa31d65b  2014-11-24 19:45:11  kasimir     /home/user/testdata
    20bdc140  2014-11-24 20:00:47  kasimir     /home/user/testdata
    326cb59d  2014-11-24 20:01:40  kasimir     /home/user/testdata
    20ff988b  2014-11-24 20:35:35  kasimir     /home/user

backend/generic.go

@@ -4,12 +4,19 @@ import (
 	"bytes"
 	"compress/zlib"
 	"crypto/sha256"
+	"encoding/hex"
+	"errors"
 	"io/ioutil"
 	"sync"
 )
 
 var idPool = sync.Pool{New: func() interface{} { return ID(make([]byte, IDSize)) }}
 
+var (
+	ErrNoIDPrefixFound   = errors.New("no ID found")
+	ErrMultipleIDMatches = errors.New("multiple IDs with prefix found")
+)
+
 // Each lists all entries of type t in the backend and calls function f() with
 // the id and data.
 func Each(be Server, t Type, f func(id ID, data []byte, err error)) error {
@@ -85,3 +92,54 @@ func Hash(data []byte) ID {
 	copy(id, h[:])
 	return id
 }
+
+// Find loads the list of all blobs of type t and searches for IDs which start
+// with prefix. If none is found, nil and ErrNoIDPrefixFound is returned. If
+// more than one is found, nil and ErrMultipleIDMatches is returned.
+func Find(be Server, t Type, prefix string) (ID, error) {
+	p, err := hex.DecodeString(prefix)
+	if err != nil {
+		return nil, err
+	}
+
+	list, err := be.List(t)
+	if err != nil {
+		return nil, err
+	}
+
+	match := ID(nil)
+
+	// TODO: optimize by sorting list etc.
+	for _, id := range list {
+		if bytes.Equal(p, id[:len(p)]) {
+			if match == nil {
+				match = id
+			} else {
+				return nil, ErrMultipleIDMatches
+			}
+		}
+	}
+
+	if match != nil {
+		return match, nil
+	}
+
+	return nil, ErrNoIDPrefixFound
+}
+
+// FindSnapshot takes a string and tries to find a snapshot whose ID matches
+// the string as closely as possible.
+func FindSnapshot(be Server, s string) (ID, error) {
+	// parse ID directly
+	if id, err := ParseID(s); err == nil {
+		return id, nil
+	}
+
+	// find snapshot id with prefix
+	id, err := Find(be, Snapshot, s)
+	if err != nil {
+		return nil, err
+	}
+
+	return id, nil
+}