server: Handle empty set on GolombEncoding

server/app/com/xsn/explorer/gcs/GolombEncoding.scala

@@ -17,9 +17,18 @@ class GolombEncoding(p: Int, m: Int, key: SipHashKey) {
   private val hasher = Hashing.sipHash24(key.k0, key.k1)
 
   /**
-   * Encodes the given word list.
+   * Encodes the given word set.
    */
-  def encode(words: Set[String]): GolombCodedSet = {
+  def encode(words: Set[String]): Option[GolombCodedSet] = {
+    if (words.isEmpty) {
+      Option.empty
+    } else {
+      val gcs = encodeNonEmptySet(words)
+      Option(gcs)
+    }
+  }
+
+  private def encodeNonEmptySet(words: Set[String]): GolombCodedSet = {
     val sortedHashes = hashes(words)
     val diffList = differences(sortedHashes)
     val encodedBits = diffList.flatMap(golombEncode)

server/test/com/xsn/explorer/gcs/GolombEncodingSpec.scala

@@ -1,11 +1,13 @@
 package com.xsn.explorer.gcs
 
 import com.google.common.io.BaseEncoding
-import org.scalatest.{MustMatchers, WordSpec}
+import org.scalatest.MustMatchers._
+import org.scalatest.OptionValues._
+import org.scalatest.WordSpec
 
-class GolombEncodingSpec extends WordSpec with MustMatchers {
+class GolombEncodingSpec extends WordSpec {
 
-  val words = List(
+  val words = Set(
     "Alex",
     "Bob",
     "Charlie",
@@ -32,10 +34,15 @@ class GolombEncodingSpec extends WordSpec with MustMatchers {
 
     val key = SipHashKey.fromBtcutil(keyBytes)
     val golomb = GolombEncoding.default(key)
-    val encoded = golomb.encode(words.toSet)
+    val encoded = golomb.encode(words).value
+
+    "succeed on empty set" in {
+      val result = golomb.encode(Set.empty)
+      result must be(empty)
+    }
 
     "decode the same hashes" in {
-      val hashes = golomb.hashes(words.toSet)
+      val hashes = golomb.hashes(words)
       val bytes = BaseEncoding
           .base16()
           .decode(encoded.hex.string.toUpperCase)