/// <summary>
/// This method searches through the slidingWindow
/// dictionary for the longest sequence matching the MAX_CODED
/// long string stored in the uncodedLookahead
/// </summary>
/// <param name="uncodedHead">Index of the character from which the coding begins</param>
private static EncodedString FindMatch(int uncodedHead)
{
var matchData = new EncodedString();
var i = HashTable[GetHashKey(uncodedHead, true)];
var j = 0;
while (i != Constants.NullIndex)
{
// We've matched the first symbol
if (SlidingWindow[i] == UncodedLookahead[uncodedHead])
{
j = 1;
while (SlidingWindow[(i + j) % Constants.WindowSize] ==
UncodedLookahead[(uncodedHead + j) % Constants.MaxCoded])
{
if (j >= Constants.MaxCoded)
{
break;
}
j++;
}
if (j > matchData.Length)
{
matchData.Length = j;
matchData.Offset = i;
}
}
if (j >= Constants.MaxCoded)
{
matchData.Length = Constants.MaxCoded;
break;
}
i = Next[i];
}
return(matchData);
}
/// <summary>
/// Performs LZSS algorithm decoding
/// </summary>
/// <param name="inputStream">Input file</param>
/// <param name="outputStream">Output file</param>
public static void Decode(Stream inputStream, Stream outputStream)
{
var reader = new BinaryReader(inputStream);
var writer = new BinaryWriter(outputStream);
InitializeDataStructures();
var flags = 0; // Encoded flag
var flagsUsed = 8; // Not encoded flag
var nextChar = Constants.WindowSize - 16 - 2; // Next char in sliding window
var code = new EncodedString();
while (true)
{
// Shifted out all the flag bits -> read a new flag
byte readChar = 0;
if (flagsUsed >= 8)
{
if (reader.BaseStream.Position >= reader.BaseStream.Length)
{
break;
}
readChar = reader.ReadByte();
flags = readChar;
flagsUsed = 0;
}
// Uncoded character
if ((flags & 1) != 0)
{
if (reader.BaseStream.Position >= reader.BaseStream.Length)
{
break;
}
readChar = reader.ReadByte();
// Write out byte and put it in sliding window
writer.Write(readChar);
SlidingWindow[nextChar] = readChar;
nextChar = (nextChar + 1) % Constants.WindowSize;
}
else
{
if (reader.BaseStream.Position >= reader.BaseStream.Length)
{
break;
}
code.Offset = reader.ReadByte();
if (reader.BaseStream.Position >= reader.BaseStream.Length)
{
break;
}
code.Length = reader.ReadByte();
// Unpack offset and length
code.Offset |= ((code.Length & 0x00F0) << 4);
code.Length = (code.Length & 0x000F) + Constants.MaxUncoded + 1;
// Write out decoded string to file and lookahead
for (var i = 0; i < code.Length; i++)
{
readChar = SlidingWindow[(code.Offset + i) % Constants.WindowSize];
writer.Write(readChar);
SlidingWindow[(nextChar + i) % Constants.WindowSize] = readChar;
}
nextChar = (nextChar + code.Length) % Constants.WindowSize;
// PrintSlidingWindow();
// PrintUncodedLookahead();
// Console.WriteLine();
}
flags >>= 1;
flagsUsed++;
}
}
/// <summary>
/// Performs LZSS algorithm decoding
/// </summary>
/// <param name="inputFileNameForDecode">Input file</param>
/// <param name="outputFileNameForDecode">Output file</param>
public static void Decode(string inputFileNameForDecode, string outputFileNameForDecode)
{
using (var reader = new StreamReader(inputFileNameForDecode))
{
using (var writer = new StreamWriter(outputFileNameForDecode))
{
InitializeDataStructures();
var flags = 0; // Encoded flag
var flagsUsed = 7; // Not encoded flag
var nextChar = 0; // Next char in sliding window
var code = new EncodedString();
while (true)
{
flags >>= 1;
flagsUsed++;
// Shifted out all the flag bits -> read a new flag
var readChar = 0;
if (flagsUsed == 8)
{
if ((readChar = reader.Read()) == -1)
{
break;
}
flags = readChar & 0xFF;
flagsUsed = 0;
}
// Uncoded character
if ((flags & 1) != 0)
{
if ((readChar = reader.Read()) == -1)
{
break;
}
// Write out byte and put it in sliding window
writer.Write((char)readChar);
SlidingWindow[nextChar] = (char)readChar;
nextChar = (nextChar + 1) % Constants.WindowSize;
}
else
{
if ((code.Offset = reader.Read()) == -1)
{
break;
}
if ((code.Length = reader.Read()) == -1)
{
break;
}
// Unpack offset and length
code.Offset <<= 4;
code.Offset |= (code.Length & 0x00F0) >> 4;
code.Length = (code.Length & 0x000F) + Constants.MaxUncoded + 1;
// Write out decoded string to file and lookahead
for (var i = 0; i < code.Length; i++)
{
readChar = SlidingWindow[(code.Offset + i) % Constants.WindowSize];
writer.Write((char)readChar);
UncodedLookahead[i] = (char)readChar;
}
// Write out decoded string to sliding window
for (var i = 0; i < code.Length; i++)
{
SlidingWindow[(nextChar + i) % Constants.WindowSize] = UncodedLookahead[i];
}
nextChar = (nextChar + code.Length) % Constants.WindowSize;
// PrintSlidingWindow();
// PrintUncodedLookahead();
// Console.WriteLine();
}
}
}
}
}
