/// <summary>
/// CopyToken Help derived bit masks are used by the Unpack CopyToken (section 2.4.1.3.19.2)
/// and the Pack CopyToken (section 2.4.1.3.19.3) algorithms. CopyToken Help also derives the
/// maximum length for a CopySequence (section 2.4.1.3.19) which is used by the Matching
/// algorithm (section 2.4.1.3.19.4).
/// The pseudocode uses the state variables described in State Variables (section 2.4.1.2):
/// DecompressedCurrent and DecompressedChunkStart.
/// </summary>
internal static CopyTokenHelpResult CopyTokenHelp(long difference)
{
var result = new CopyTokenHelpResult();
// SET BitCount TO the smallest integer that is GREATER THAN OR EQUAL TO LOGARITHM base 2
// of difference
result.BitCount = 0;
while ((1 << result.BitCount) < difference)
{
result.BitCount += 1;
}
// The number of bits used to encode Length MUST be greater than or equal to four. The
// number of bits used to encode Length MUST be less than or equal to 12
// SET BitCount TO the maximum of BitCount and 4
if (result.BitCount < 4)
{
result.BitCount = 4;
}
if (result.BitCount > 12)
{
throw new Exception();
}
// SET LengthMask TO 0xFFFF RIGHT SHIFT BY BitCount
result.LengthMask = (UInt16)(0xffff >> result.BitCount);
// SET OffsetMask TO BITWISE NOT LengthMask
result.OffsetMask = (UInt16)(~result.LengthMask);
// SET MaximumLength TO (0xFFFF RIGHT SHIFT BY BitCount) PLUS 3
result.MaximumLength = (UInt16)((0xffff >> result.BitCount) + 3);
return(result);
}
// page 69
public static CopyTokenHelpResult CopyTokenHelp(int DecompressedCurrent, int DecompressedChunkStart)
{
var difference = DecompressedCurrent - DecompressedChunkStart;
var result = new CopyTokenHelpResult();
result.BitCount = Math.Max((ushort)Math.Ceiling(Math.Log(difference, 2)), (ushort)4);
result.LengthMask = (ushort)(0xFFFF >> result.BitCount);
result.OffsetMask = (ushort)(~result.LengthMask); // bitwise not
result.MaximumLength = (ushort)((0xFFFF >> result.BitCount) + 3);
return(result);
}
