public void CountLeadinZeros()
{
Random r = new Random();
Assert.AreEqual(32, BitMath.CountLeadingZeros(0u));
Assert.AreEqual(0, BitMath.CountLeadingZeros(uint.MaxValue));
for (int k = 0; k < 10; k++)
{
for (int x = 0; x < 32; x++)
{
Assert.AreEqual(31 - x, BitMath.CountLeadingZeros(1u << x | Next(r, 1u << x)));
}
}
Assert.AreEqual(64, BitMath.CountLeadingZeros(0ul));
Assert.AreEqual(0, BitMath.CountLeadingZeros(ulong.MaxValue));
for (int k = 0; k < 10; k++)
{
for (int x = 0; x < 63; x++)
{
Assert.AreEqual(63 - x, BitMath.CountLeadingZeros(1ul << x | Next(r, 1ul << x)));
}
}
}
/// <summary>
/// Returns a cryptographically strong number that is less than the the supplied value
/// </summary>
/// <param name="maxValue">the max value to return exclusive</param>
/// <returns></returns>
/// <remarks>
/// if 0 is provided, 0 is returned
/// </remarks>
private static ulong GetRandomNumberLessThan(ulong maxValue)
{
//A crypto number cannot be achieved via *, /, or % since these
//operations have rounding and uneven redistribution properties.
//Method: In order to get a crypto number <= maxValue
//A random number must be generated. If the random number is in range, it
//may be kept, otherwise a new number must be generated.
//To increase the likelihood that the number is in range,
//bit masking can be used on the higher order bits to
//create a 75% likelihood (on average) that the number will be in range.
//Exception cases where algorithm doesn't work
if (maxValue == 0 || maxValue == 1)
{
return(0);
}
//Determine the number of random bits that I need
int leadingZeroes = BitMath.CountLeadingZeros(maxValue);
ulong value = UInt64;
//By shifting the value by the number of leading zeros, I'll have
//a number with the highest likelihood of being in range
while (value >> leadingZeroes >= maxValue)
{
//If the number is outside of range, all bits must
//be discarded and new ones generated. Not doing this
//technically alters the crypto-random nature of the value being generated.
value = UInt64;
}
return(value >> leadingZeroes);
}
public void BitMath_Should_Return_Count_Of_31()
{
//Integer as 32 digit binary number
Int32 actual = BitMath.CountLeadingZeros(1);
const Int32 expected = 31;
Assert.Equal(expected, actual);
}
public void BitMath_Should_Return_Count_Of_28()
{
//Integer as 32 digit binary number
//15 = 1111 in binary, so 32 - 4 = 28
Int32 actual = BitMath.CountLeadingZeros(15);
const Int32 expected = 28;
Assert.Equal(expected, actual);
}
/// <summary>
/// Defines a binary-to-text encoding.
/// Additional decode characters let you add aliases, and a filter callback can be used
/// to make decoding case-insensitive among other things.
/// </summary>
/// <param name="characterSet">The characters of the encoding.</param>
/// <param name="msbComesFirst">
/// <c>true</c> to begin with the most-significant bit of each byte.
/// Otherwise, the encoding begins with the least-significant bit.
/// </param>
/// <param name="additionalDecodeCharacters">
/// A dictionary of alias characters, or <c>null</c> if no aliases are desired.
/// </param>
/// <param name="decodeFilterCallback">
/// A callback to map arbitrary characters onto the characters that can be decoded.
/// </param>
public BaseEncoding(string characterSet, bool msbComesFirst,
IDictionary <char, int> additionalDecodeCharacters,
BaseEncodingDecodeFilterCallback decodeFilterCallback)
{
Check.Null("characterSet", characterSet);
if (!BitMath.IsPositivePowerOf2(characterSet.Length))
{
throw Exceptions.Argument("characterSet",
"Length must be a power of 2.");
}
if (characterSet.Length > 256)
{
throw Exceptions.Argument("characterSet",
"Character sets with over 256 characters are not supported.");
}
_bitCount = 31 - BitMath.CountLeadingZeros(characterSet.Length);
_bitMask = (1 << _bitCount) - 1;
_characters = characterSet;
_msbComesFirst = msbComesFirst;
_decodeFilterCallback = decodeFilterCallback;
_values = additionalDecodeCharacters != null
? new Dictionary <char, int>(additionalDecodeCharacters)
: new Dictionary <char, int>();
for (int i = 0; i < characterSet.Length; i++)
{
char ch = characterSet[i];
if (_values.ContainsKey(ch))
{
throw Exceptions.Argument("Duplicate characters are not supported.",
"characterSet");
}
_values.Add(ch, (byte)i);
}
}