[Mono-dev] BitVector32 patch

[Miguel de Icaza]

Hello,

    Scott, would you mind providing NUnit test cases to ensure that this
bug does not ever creep up again, and in particular, test cases for the
current failure?

> OK, this patch just spruces up the HighestSetBit method and fixes the
> bug.
> 
> Index: class/System/System.Collections.Specialized/ChangeLog
> ===================================================================
> --- class/System/System.Collections.Specialized/ChangeLog    (revision
> 109665)
> +++ class/System/System.Collections.Specialized/ChangeLog    (working
> copy)
> @@ -1,3 +1,10 @@
> +2008-08-06  Scott Peterson  <lunchtimemama at gmail.com>
> +
> +    * BitVector32.cs: Fixed a bug which allowed for invalid sections
> +    to be created with CreateSection. Also simlpified HighestSetBit
> +    algorithm and got rid ofNumberOfSetBits (using HighestSetBit
> +    works just fine).
> +
>  2008-07-31  Jb Evain  <jbevain at novell.com>
>  
>      * StringDictionary.cs: remove ComponentModel bits for NET_2_1.
> Index: class/System/System.Collections.Specialized/BitVector32.cs
> ===================================================================
> --- class/System/System.Collections.Specialized/BitVector32.cs
> (revision 109665)
> +++ class/System/System.Collections.Specialized/BitVector32.cs
> (working copy)
> @@ -184,11 +184,11 @@
>              if (maxValue < 1)
>                  throw new ArgumentException ("maxValue");
>              
> -            int bit = HighestSetBit(maxValue) + 1;
> +            int bit = HighestSetBit(maxValue);
>              int mask = (1 << bit) - 1;
> -            int offset = previous.Offset + NumberOfSetBits
> (previous.Mask);
> +            int offset = previous.Offset + HighestSetBit
> (previous.Mask);
>  
> -            if (offset > 32) {
> +            if (offset + bit > 32) {
>                  throw new ArgumentException ("Sections cannot exceed
> 32 bits in total");
>              }
>  
> @@ -227,27 +227,12 @@
>          }
>  
>          // Private utilities
> -        private static int NumberOfSetBits (int i) 
> +        private static int HighestSetBit (int i) 
>          {
>              int count = 0;
> -            for (int bit = 0; bit < 32; bit++) {
> -                int mask = 1 << bit;
> -                if ((i & mask) != 0) 
> -                    count++;
> -            }
> +            while(i >> count != 0)
> +                count++;
>              return count;
>          }
> -
> -        private static int HighestSetBit (int i) 
> -        {
> -            for (int bit = 31; bit >= 0; bit--) {
> -                int mask = 1 << bit;
> -                if ((mask & i) != 0) {
> -                    return bit;
> -                }
> -            }
> -
> -            return -1;
> -        }
>      }
>  }
> 
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