Week of Year Calculation (logic)

We know that in sql server we have a lib function for week of year calculation.
But we need to know the calculation behind that. Here is the actual calculation given below.

DECLARE @Inputdate DATE='2015-01-01'

DECLARE @ORDINAL INT

DECLARE @RESULT FLOAT

IF (datepart(yy,@Inputdate)%100=0 and datepart(yy,@Inputdate)%4=0)

      BEGIN

            SET @ORDINAL=

                CASE WHEN datepart(mm,@Inputdate)=1 then 0

                         WHEN datepart(mm,@Inputdate)=2 then 31

                         WHEN datepart(mm,@Inputdate)=3 then 60

                       WHEN datepart(mm,@Inputdate)=4 then 91

                       WHEN datepart(mm,@Inputdate)=5 then 121

                       WHEN datepart(mm,@Inputdate)=6 then 152

                       WHEN datepart(mm,@Inputdate)=7 then 182

                       WHEN datepart(mm,@Inputdate)=8 then 213

                       WHEN datepart(mm,@Inputdate)=9 then 244

                       WHEN datepart(mm,@Inputdate)=10 then 274

                       WHEN datepart(mm,@Inputdate)=11 then 305

                  ELSE 335 END

      SET @ORDINAL=@ORDINAL+datepart(DD,@Inputdate)

      SET @RESULT=(@ORDINAL-(datepart(DW,@Inputdate)-1)+10)/7

     

      SELECT CAST(@RESULT AS INT) WEEK_OF_YEAR

      END

ELSE

      BEGIN

      SET @ORDINAL=

               CASE WHEN datepart(mm,@Inputdate)=1 then 0

                        WHEN datepart(mm,@Inputdate)=2 then 31

                        WHEN datepart(mm,@Inputdate)=3 then 59

                        WHEN datepart(mm,@Inputdate)=4 then 90

                        WHEN datepart(mm,@Inputdate)=5 then 120

                        WHEN datepart(mm,@Inputdate)=6 then 151

                        WHEN datepart(mm,@Inputdate)=7 then 181

                        WHEN datepart(mm,@Inputdate)=8 then 212

                        WHEN datepart(mm,@Inputdate)=9 then 243

                        WHEN datepart(mm,@Inputdate)=10 then 273

                        WHEN datepart(mm,@Inputdate)=11 then 304

                  ELSE  334 END

      SET @ORDINAL=@ORDINAL+datepart(DD,@Inputdate)       

      SET @RESULT=(@ORDINAL-(datepart(DW,@Inputdate)-1)+10)/7

     

      SELECT CAST(@RESULT AS INT) WEEK_OF_YEAR

      END