Function to count set bits in a signed integer

-- =============================================
-- Author:		Jonathan Roberts
-- Create date: 24/02.2014
-- Description:	Counts the number of set bits in a signed integer
-- Sample calls: 
--   SELECT [dbo].[BitsIn] (-9223372036854775808) -- Returns 1
--   SELECT [dbo].[BitsIn] (-1) -- Returns 64
--   SELECT [dbo].[BitsIn] (0)  -- Returns 0
--   SELECT [dbo].[BitsIn] (3)  -- Returns 2
--   SELECT [dbo].[BitsIn] (15) -- Returns 4
--   SELECT [dbo].[BitsIn] (16) -- Returns 1
--   SELECT [dbo].[BitsIn] (9223372036854775807)  -- Returns 63
-- =============================================
CREATE FUNCTION [dbo].[BitsIn]
(
	@Input bigint
)
RETURNS int
AS
BEGIN

	DECLARE @BitsIn tinyint = 0

	IF @Input < 0 BEGIN -- To cope with negative numbers
		SELECT @Input &= ~CONVERT(bigint, -9223372036854775808),
		       @BitsIn = 1
	END

	WHILE @Input > 0 BEGIN
		SET @Input &= @Input & (@Input - 1)
		SET @BitsIn += 1
	END
		
	RETURN @BitsIn

END
GO