/// <summary>
/// Builds the prefix code tree.
/// </summary>
/// <returns>An optimal prefix code tree for given characters and their frequencies.</returns>
private PrefixCodeTree BuildPrefixCodeTree()
{
// A MinPriorityQueue is used for selecting two nodes with minimal weights at a time
MinPriorityQueue <PrefixCodeTree> pq = new MinPriorityQueue <PrefixCodeTree>(this.symbols.Length);
// Add all leaf nodes to the priority queue separately
for (int i = 0; i < this.symbols.Length; ++i)
{
PrefixCodeTree node = new PrefixCodeTree();
node.Symbol = this.symbols[i];
node.Weight = this.frequencies[i];
pq.Enqueue(node);
}
// Select two nodes with minimal weights and join them under a new node until a root tree is built
while (pq.Size > 1)
{
PrefixCodeTree first = pq.Dequeue();
PrefixCodeTree second = pq.Dequeue();
PrefixCodeTree newNode = new PrefixCodeTree();
newNode.Symbol = null;
newNode.Weight = first.Weight + second.Weight;
newNode.Left = first;
newNode.Right = second;
pq.Enqueue(newNode);
}
// The last node in the queue is the root of the prefix code tree
return(pq.Dequeue());
}
/// <summary>
/// Traverses the code tree recursively and construct prefix codes at leaf nodes.
/// </summary>
/// <param name="tree">Prefix code tree.</param>
/// <param name="path">Path to the current node from the tree root.</param>
private void BuildCodes(PrefixCodeTree tree, string path)
{
if (tree.Symbol != null)
{
// This is a leaf node, a path has been found
this.codes[tree.Symbol.Value] = path;
}
else
{
// Traverse left and right children with updated paths
this.BuildCodes(tree.Left, path + "0");
this.BuildCodes(tree.Right, path + "1");
}
}