fix hashCode function in BitArray

src/commonMain/kotlin/com/github/quillraven/fleks/collection/bitArray.kt

@@ -154,16 +154,12 @@ class BitArray(
     }
 
     override fun hashCode(): Int {
-        if (bits.isEmpty()) {
-            return 0
+        var result = 1
+        for (i in bits.size - 1 downTo 0) {
+            val word = bits[i]
+            result = 31 * result + (word xor (word ushr 32)).toInt()
         }
-
-        val word = length() ushr 6
-        var hash = 0
-        for (i in 0..word) {
-            hash = 127 * hash + (bits[i] xor (bits[i] ushr 32)).toInt()
-        }
-        return hash
+        return result
     }
 
     override fun equals(other: Any?): Boolean {

src/commonTest/kotlin/com/github/quillraven/fleks/collection/BitArrayTest.kt

@@ -189,4 +189,33 @@ internal class BitArrayTest {
         assertEquals(expected3, bits3.toString())
         assertEquals(expected4, bits4.toString())
     }
+
+    @Test
+    fun testHashCode() {
+        // empty BitArray
+        val bits = BitArray()
+        bits.hashCode()
+
+        // test for single long
+        bits.set(0)
+        bits.hashCode()
+        bits.set(1)
+        bits.hashCode()
+        bits.set(63)
+        bits.hashCode()
+
+        // BitArray resizes to two longs in the LongArray
+        bits.set(64)
+        bits.hashCode()
+        bits.set(65)
+        bits.hashCode()
+        bits.set(127)
+        bits.hashCode()
+
+        // BitArray resizes to more than two longs
+        bits.set(128)
+        bits.hashCode()
+        bits.set(129)
+        bits.hashCode()
+    }
 }