Ensure bitset can handle single entries (#9)

Author: frankdekker

src/Util/BitSet.php

@@ -97,7 +97,7 @@ public function toBinaryString(): string
         $words = [];
 
         // ensure all slots between 0 and maxKey have a value
-        $maxKey = max(...array_keys($this->words));
+        $maxKey = max(0, ...array_keys($this->words));
         for ($i = 0; $i <= $maxKey; $i++) {
             $words[$i] = $this->words[$i] ?? 0;
         }

tests/Unit/Util/BitSetTest.php

@@ -142,6 +142,17 @@ public function testSerializeBinaryString(): void
         static::assertEquals($bitSet, $newBitSet);
     }
 
+    public function testSerializeBinaryStringSingleEntry(): void
+    {
+        $bitSet = new BitSet();
+        $bitSet->set(5, 6);
+
+        $binaryString = $bitSet->toBinaryString();
+        $newBitSet    = BitSet::fromBinaryString($binaryString);
+
+        static::assertEquals($bitSet, $newBitSet);
+    }
+
     public function testSerializeBinaryStringEmptyBitSet(): void
     {
         $bitSet = new BitSet();