feat(users): offline UUD generation support

src/main/java/me/itzg/helpers/users/ManageUsersCommand.java

@@ -14,6 +14,7 @@
 import java.util.HashSet;
 import java.util.List;
 import java.util.Set;
+import java.util.UUID;
 import java.util.concurrent.Callable;
 import lombok.extern.slf4j.Slf4j;
 import me.itzg.helpers.errors.GenericException;
@@ -25,7 +26,10 @@
 import me.itzg.helpers.json.ObjectMappers;
 import me.itzg.helpers.users.model.JavaOp;
 import me.itzg.helpers.users.model.JavaUser;
+
+import org.apache.commons.codec.digest.DigestUtils;
 import org.apache.maven.artifact.versioning.ComparableVersion;
+
 import picocli.CommandLine.ArgGroup;
 import picocli.CommandLine.Command;
 import picocli.CommandLine.ExitCode;
@@ -46,6 +50,9 @@ public class ManageUsersCommand implements Callable<Integer> {
     @Option(names = {"--help", "-h"}, usageHelp = true)
     boolean help;
 
+    @Option(names = {"--offline"}, required = false, description = "Use for offline server, UUIDs are generated")
+    boolean offline;
+
     @Option(names = "--output-directory", defaultValue = ".")
     Path outputDirectory;
 
@@ -240,6 +247,10 @@ private JavaUser resolveJavaUserId(SharedFetch sharedFetch, List<? extends JavaU
                     }
                 }
 
+                if (offline) {
+                    return getOfflineUUID(input);
+                }
+
                 final UserApi userApi;
                 switch (userApiProvider) {
                     case mojang:
@@ -329,4 +340,32 @@ private void processInputAsFile(SharedFetch sharedFetch, String filePathUrl) thr
     private boolean usesTextUserList() {
         return version != null && new ComparableVersion(version).compareTo(MIN_VERSION_USES_JSON) < 0;
     }
+
+    private static JavaUser getOfflineUUID(String username) {
+        byte[] bytes = DigestUtils.md5("OfflinePlayer:"+username);
+
+        // Force version = 3 (bits 12-15 of time_hi_and_version)
+        bytes[6] &= 0x0F;
+        bytes[6] |= 0x30;
+
+        // Force variant = 2 (bits 6-7 of clock_seq_hi_and_reserved)
+        bytes[8] &= 0x3F;
+        bytes[8] |= 0x80;
+
+        long msb = 0;
+        long lsb = 0;
+
+        for (int i = 0; i < 8; i++) {
+            msb = (msb << 8) | (bytes[i] & 0xFF);
+        }
+
+        for (int i = 8; i < 16; i++) {
+            lsb = (lsb << 8) | (bytes[i] & 0xFF);
+        }
+
+        return JavaUser.builder()
+        .name(username)
+        .uuid(new UUID(msb, lsb).toString())
+        .build();
+    }
 }

src/test/java/me/itzg/helpers/users/ManageUsersCommandTest.java

@@ -25,8 +25,10 @@ class ManageUsersCommandTest {
 
     private static final String USER1_ID = "3f5f20286a85445fa7b46100e70c2b3a";
     private static final String USER1_UUID = "3f5f2028-6a85-445f-a7b4-6100e70c2b3a";
+    private static final String USER1_OFFLINE_UUID = "fb4cdad9-642b-358f-8f6f-717981c9f42b";
     private static final String USER2_ID = "5e5a1b2294b14f5892466062597e4c91";
     private static final String USER2_UUID = "5e5a1b22-94b1-4f58-9246-6062597e4c91";
+    private static final String USER2_OFFLINE_UUID = "6e7d9aa0-0da2-390c-ab6a-377df9d77518";
 
     @TempDir
     Path tempDir;
@@ -406,6 +408,45 @@ void givenUuidsAndAllExist(WireMockRuntimeInfo wmInfo) throws IOException {
         }
     }
 
+    @Nested
+    public class whitelistOffline {
+        @Test
+        void givenNames(WireMockRuntimeInfo wmInfo) {
+            setupUserStubs();
+
+            final int exitCode = new CommandLine(
+                new ManageUsersCommand()
+            )
+                .execute(
+                    "--mojang-api-base-url", wmInfo.getHttpBaseUrl(),
+                    "--user-api-provider", "mojang",
+                    "--offline",
+                    "--type", "JAVA_WHITELIST",
+                    "--output-directory", tempDir.toString(),
+                    "user1", "user2"
+                );
+
+            assertThat(exitCode).isEqualTo(0);
+
+            final Path expectedFile = tempDir.resolve("whitelist.json");
+
+            assertThat(expectedFile).exists();
+
+            assertJson(expectedFile)
+                .isArrayContainingExactlyInAnyOrder(
+                    conditions()
+                        .satisfies(conditions()
+                            .at("/name").hasValue("user1")
+                            .at("/uuid").hasValue(USER1_OFFLINE_UUID)
+                        )
+                        .satisfies(conditions()
+                            .at("/name").hasValue("user2")
+                            .at("/uuid").hasValue(USER2_OFFLINE_UUID)
+                        )
+                );
+        }
+    }
+
     @Nested
     public class whitelistOrOpsText {