Add a utility method for testing if a number begins with a decimal prefix

service/src/main/java/org/whispersystems/textsecuregcm/util/Util.java

@@ -144,6 +144,35 @@ public class Util {
     }
   }
 
+  /**
+   * Tests whether the decimal form of the given number (without leading zeroes) begins with the decimal form of the
+   * given prefix (without leading zeroes).
+   *
+   * @param number the number to check for the given prefix
+   * @param prefix the prefix
+   *
+   * @return {@code true} if the given number starts with the given prefix or {@code false} otherwise
+   *
+   * @throws IllegalArgumentException if {@code number} is negative or if {@code prefix} is zero or negative
+   */
+  public static boolean startsWithDecimal(final long number, final long prefix) {
+    if (number < 0) {
+      throw new IllegalArgumentException("Number must be non-negative");
+    }
+
+    if (prefix <= 0) {
+      throw new IllegalArgumentException("Prefix must be positive");
+    }
+
+    long workingCopy = number;
+
+    while (workingCopy > prefix) {
+      workingCopy /= 10;
+    }
+
+    return workingCopy == prefix;
+  }
+
   public static byte[] truncate(byte[] element, int length) {
     byte[] result = new byte[length];
     System.arraycopy(element, 0, result, 0, result.length);

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

@@ -5,16 +5,15 @@
 
 package org.whispersystems.textsecuregcm.util;
 
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
 import com.google.i18n.phonenumbers.PhoneNumberUtil;
-import org.junit.jupiter.api.Test;
+import java.util.List;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.CsvSource;
 import org.junit.jupiter.params.provider.MethodSource;
 
-import java.util.List;
-
-import static org.junit.jupiter.api.Assertions.*;
-
 class UtilTest {
 
   @ParameterizedTest
@@ -39,4 +38,15 @@ class UtilTest {
         Arguments.of(oldFormatBeninE164, List.of(oldFormatBeninE164, newFormatBeninE164))
     );
   }
+
+  @ParameterizedTest
+  @CsvSource({
+      "0, 1, false",
+      "123456789, 1, true",
+      "123456789, 123, true",
+      "123456789, 456, false",
+  })
+  void startsWithDecimal(final long number, final long prefix, final boolean expectStartsWithPrefix) {
+    assertEquals(expectStartsWithPrefix, Util.startsWithDecimal(number, prefix));
+  }
 }