Optimized BitArray.hash(hasher) (#5101)

akzhan · RX14 · commit 1d401e4ad352 · 2017-10-14T15:18:27.000+01:00
* Optimized BitArray.hash(hasher)

* Add note about different hashes for same bit arrays after downsizing.
Just like BitArray.==
diff --git a/src/bit_array.cr b/src/bit_array.cr
@@ -35,8 +35,8 @@ struct BitArray
   def ==(other : BitArray)
     return false if size != other.size
     # NOTE: If BitArray implements resizing, there may be more than 1 binary
-    # representation for equivalent BitArrays after a downsize as the discarded
-    # bits may not have been zeroed.
+    # representation and their hashes for equivalent BitArrays after a downsize as the
+    # discarded bits may not have been zeroed.
     return LibC.memcmp(@bits, other.@bits, malloc_size) == 0
   end
 
@@ -226,6 +226,13 @@ struct BitArray
     Slice.new(@bits.as(Pointer(UInt8)), (@size / 8.0).ceil.to_i)
   end
 
+  # See `Object#hash(hasher)`
+  def hash(hasher)
+    hasher = size.hash(hasher)
+    hasher = to_slice.hash(hasher)
+    hasher
+  end
+
   private def bit_index_and_sub_index(index)
     bit_index_and_sub_index(index) { raise IndexError.new }
   end
