Introduce a username validator

service/src/test/java/org/whispersystems/textsecuregcm/util/UsernameValidatorTest.java

@@ -0,0 +1,56 @@
+/*
+ * Copyright 2013-2021 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.whispersystems.textsecuregcm.util;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.stream.Stream;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+class UsernameValidatorTest {
+
+  @ParameterizedTest
+  @MethodSource
+  void isValid(final String username, final boolean expectValid) {
+    final UsernameValidator usernameValidator = new UsernameValidator();
+
+    assertEquals(expectValid, usernameValidator.isValid(username, null));
+  }
+
+  private static Stream<Arguments> isValid() {
+    return Stream.of(
+        Arguments.of("test", true),
+        Arguments.of("_test", true),
+        Arguments.of("test123", true),
+        Arguments.of("a", false), // Too short
+        Arguments.of("thisIsAReallyReallyReallyLongUsernameThatWeWouldNotAllow", false),
+        Arguments.of("Illegal character", false),
+        Arguments.of("0test", false), // Illegal first character
+        Arguments.of("pаypal", false), // Unicode confusable characters
+        Arguments.of("test\uD83D\uDC4E", false), // Emoji
+        Arguments.of(" ", false),
+        Arguments.of("", false),
+        Arguments.of(null, false)
+    );
+  }
+
+  @ParameterizedTest
+  @MethodSource
+  void getCanonicalUsername(final String username, final String expectedCanonicalUsername) {
+    assertEquals(expectedCanonicalUsername, UsernameValidator.getCanonicalUsername(username));
+  }
+
+  private static Stream<Arguments> getCanonicalUsername() {
+    return Stream.of(
+        Arguments.of("test", "test"),
+        Arguments.of("TEst", "test"),
+        Arguments.of("t_e_S_T", "t_e_s_t"),
+        Arguments.of(null, null)
+    );
+  }
+}