Add unit test to readU16

service/src/main/java/org/whispersystems/textsecuregcm/providers/MultiRecipientMessageProvider.java

@@ -5,6 +5,7 @@
 
 package org.whispersystems.textsecuregcm.providers;
 
+import com.google.common.annotations.VisibleForTesting;
 import io.dropwizard.util.DataSizeUnit;
 import java.io.IOException;
 import java.io.InputStream;
@@ -132,7 +133,8 @@ public class MultiRecipientMessageProvider implements MessageBodyReader<MultiRec
    * Reads two bytes with most significant byte first. Treats the value as unsigned so the range returned is
    * {@code [0, 65535]}.
    */
-  private int readU16(InputStream stream) throws IOException {
+  @VisibleForTesting
+  static int readU16(InputStream stream) throws IOException {
     int b1 = stream.read();
     if (b1 == -1) {
       throw new IOException("Missing byte 1 of U16");

service/src/test/java/org/whispersystems/textsecuregcm/providers/MultiRecipientMessageProviderTest.java

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2021 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.whispersystems.textsecuregcm.providers;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.jupiter.params.provider.Arguments.arguments;
+
+import java.io.ByteArrayInputStream;
+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;
+
+public class MultiRecipientMessageProviderTest {
+
+  static byte[] createTwoByteArray(int b1, int b2) {
+    return new byte[]{(byte) b1, (byte) b2};
+  }
+
+  static Stream<Arguments> readU16TestCases() {
+    return Stream.of(
+        arguments(0xFFFE, createTwoByteArray(0xFF, 0xFE)),
+        arguments(0x0001, createTwoByteArray(0x00, 0x01)),
+        arguments(0xBEEF, createTwoByteArray(0xBE, 0xEF)),
+        arguments(0xFFFF, createTwoByteArray(0xFF, 0xFF)),
+        arguments(0x0000, createTwoByteArray(0x00, 0x00)),
+        arguments(0xF080, createTwoByteArray(0xF0, 0x80))
+    );
+  }
+
+  @ParameterizedTest
+  @MethodSource("readU16TestCases")
+  void testReadU16(int expectedValue, byte[] input) throws Exception {
+    try (final ByteArrayInputStream stream = new ByteArrayInputStream(input)) {
+      assertThat(MultiRecipientMessageProvider.readU16(stream)).isEqualTo(expectedValue);
+    }
+  }
+  
+  
+
+}