Use faster bit counting algorithm

Author: yegct

lib/bitarray-array.rb

@@ -43,8 +43,9 @@ def each_byte
   end
 
   # Returns the total number of bits that are set
-  # (The technique used here is about 6 times faster than using each or inject direct on the bitfield)
+  # Use Brian Kernighan's way, see
+  # https://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetKernighan
   def total_set
-    @field.each_byte.inject(0) { |a, byte| a += byte & 1 and byte >>= 1 until byte == 0; a }
+    @field.each_byte.inject(0) { |a, byte| (a += 1; byte &= byte - 1) while byte > 0 ; a }
   end
 end

lib/bitarray.rb

@@ -47,9 +47,10 @@ def each_byte
   end
 
   # Returns the total number of bits that are set
-  # (The technique used here is about 6 times faster than using each or inject direct on the bitfield)
+  # Use Brian Kernighan's way, see
+  # https://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetKernighan
   def total_set
-    @field.each_byte.inject(0) { |a, byte| a += byte & 1 and byte >>= 1 until byte == 0; a }
+    @field.each_byte.inject(0) { |a, byte| (a += 1; byte &= byte - 1) while byte > 0 ; a }
   end
 
   def byte_position(position)