[#294] user: Implement Cmp() function for ID struct

Signed-off-by: Alexander Chuprov <a.chuprov@yadro.com>

user/id.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"errors"
 	"fmt"
+	"strings"
 
 	"git.frostfs.info/TrueCloudLab/frostfs-sdk-go/api/refs"
 	"github.com/mr-tron/base58"
@@ -123,3 +124,9 @@ func (x *ID) setUserID(w []byte) error {
 
 	return nil
 }
+
+// Cmp returns an integer comparing two base58 encoded user ID lexicographically.
+// The result will be 0 if id1 == id2, -1 if id1 < id2, and +1 if id1 > id2.
+func (x ID) Cmp(x2 ID) int {
+	return strings.Compare(x.EncodeToString(), x2.EncodeToString())
+}

user/id_test.go

@@ -3,6 +3,8 @@ package user_test
 import (
 	"bytes"
 	"crypto/rand"
+	"slices"
+	"strings"
 	"testing"
 
 	"git.frostfs.info/TrueCloudLab/frostfs-sdk-go/api/refs"
@@ -132,3 +134,16 @@ func TestID_Equal(t *testing.T) {
 	require.True(t, id3.Equals(id1)) // commutativity
 	require.False(t, id1.Equals(id2))
 }
+
+func TestID_Cmp(t *testing.T) {
+	id1 := usertest.ID()
+	id2 := usertest.ID()
+	id3 := usertest.ID()
+
+	arr := []ID{id1, id2, id3}
+
+	slices.SortFunc(arr, ID.Cmp)
+	for i := 1; i < len(arr); i++ {
+		require.NotEqual(t, strings.Compare(arr[i-1].EncodeToString(), arr[i].EncodeToString()), 1, "array is not sorted correctly")
+	}
+}