private static LinkedNode Decode(BitStream input, LinkedNode head)
{
LinkedNode node = head;
while (node.Child0 != null)
{
int bit = input.ReadBits(1);
if (bit == -1)
{
throw new Exception("Unexpected end of file");
}
node = bit == 0 ? node.Child0 : node.Child1;
}
return(node);
}
private static LinkedNode BuildList(byte[] primeData)
{
LinkedNode root;
root = new LinkedNode(256, 1);
root = root.Insert(new LinkedNode(257, 1));
for (int i = 0; i < primeData.Length; i++)
{
if (primeData[i] != 0)
{
root = root.Insert(new LinkedNode(i, primeData[i]));
}
}
return(root);
}
public static MemoryStream Decompress(Stream data)
{
int comptype = data.ReadByte();
if (comptype == 0)
{
throw new NotImplementedException("Compression type 0 is not currently supported");
}
LinkedNode tail = BuildList(sPrime[comptype]);
LinkedNode head = BuildTree(tail);
MemoryStream outputstream = new MemoryStream();
BitStream bitstream = new BitStream(data);
int decoded;
do
{
LinkedNode node = Decode(bitstream, head);
decoded = node.DecompressedValue;
switch (decoded)
{
case 256:
break;
case 257:
int newvalue = bitstream.ReadBits(8);
outputstream.WriteByte((byte)newvalue);
tail = InsertNode(tail, newvalue);
break;
default:
outputstream.WriteByte((byte)decoded);
break;
}
} while (decoded != 256);
outputstream.Seek(0, SeekOrigin.Begin);
return(outputstream);
}
private static LinkedNode BuildTree(LinkedNode tail)
{
LinkedNode current = tail;
while (current != null)
{
LinkedNode child0 = current;
LinkedNode child1 = current.Prev;
if (child1 == null)
{
break;
}
LinkedNode parent = new LinkedNode(0, child0.Weight + child1.Weight);
parent.Child0 = child0;
child0.Parent = parent;
child1.Parent = parent;
current.Insert(parent);
current = current.Prev.Prev;
}
return(current);
}
// This increases the weight of the new node and its antecendants
// and adjusts the tree if needed
private static void AdjustTree(LinkedNode newNode)
{
LinkedNode current = newNode;
while (current != null)
{
current.Weight++;
LinkedNode insertpoint;
LinkedNode prev;
// Go backwards thru the list looking for the insertion point
insertpoint = current;
while (true)
{
prev = insertpoint.Prev;
if (prev == null)
{
break;
}
if (prev.Weight >= current.Weight)
{
break;
}
insertpoint = prev;
}
// No insertion point found
if (insertpoint == current)
{
current = current.Parent;
continue;
}
// The following code basicly swaps insertpoint with current
// remove insert point
if (insertpoint.Prev != null)
{
insertpoint.Prev.Next = insertpoint.Next;
}
insertpoint.Next.Prev = insertpoint.Prev;
// Insert insertpoint after current
insertpoint.Next = current.Next;
insertpoint.Prev = current;
if (current.Next != null)
{
current.Next.Prev = insertpoint;
}
current.Next = insertpoint;
// remove current
current.Prev.Next = current.Next;
current.Next.Prev = current.Prev;
// insert current after prev
LinkedNode temp = prev.Next;
current.Next = temp;
current.Prev = prev;
temp.Prev = current;
prev.Next = current;
// Set up parent/child links
LinkedNode currentparent = current.Parent;
LinkedNode insertparent = insertpoint.Parent;
if (currentparent.Child0 == current)
{
currentparent.Child0 = insertpoint;
}
if (currentparent != insertparent && insertparent.Child0 == insertpoint)
{
insertparent.Child0 = current;
}
current.Parent = insertparent;
insertpoint.Parent = currentparent;
current = current.Parent;
}
}
