/// <summary>
/// Builds a Huffman tree based on symbols quantity dictionary.
/// </summary>
/// <param name="valueComparer">
/// Comparer used as a last resort to define
/// relation between two huffman tree nodes. It is used to define the
/// relation only if other means based on Quantity and Subtree depth fail.
/// It should never return 0 (equality) for different elements.
/// </param>
/// <param name="symbolQuantityDic">dicrionary containing quantity
/// of symbol occurences for this tree to build.</param>
/// <returns>Huffman tree.</returns>
public IHuffmanTreeNode <T> BuildTree(IComparer <T> valueComparer, Dictionary <T, int> symbolQuantityDic)
{
var nodeComparer = new HuffmanTreeNodeComparer <T>(valueComparer);
if (symbolQuantityDic.Keys.Count == 0)
{
throw new Exception("This builder requires at least one symbol in quantity dictionary.");
}
if (symbolQuantityDic.Keys.Count == 1)
{
var only = symbolQuantityDic.Keys.First();
return(new HuffmanTreeNode <T>(
value: only,
quantity: 1));
}
var priorityQueue = GetHuffmanNodePriorityQueue(nodeComparer, symbolQuantityDic);
while (priorityQueue.Count() > 1)
{
var first = priorityQueue.DeleteMin();
var second = priorityQueue.DeleteMin();
var mergedNode = Merge(first, second);
priorityQueue.Add(mergedNode);
}
return(priorityQueue.DeleteMin());
}
/// <summary>
/// </summary>
/// <returns>An initial priority queue created for building Huffman tree.</returns>
private IntervalHeap <HuffmanTreeNode <T> > GetHuffmanNodePriorityQueue(
HuffmanTreeNodeComparer <T> nodeComparer, Dictionary <T, int> symbolQuantityDic)
{
var priorityQueue = new IntervalHeap <HuffmanTreeNode <T> >(nodeComparer);
priorityQueue.AddAll(symbolQuantityDic.Select(
s => new HuffmanTreeNode <T>(
value: s.Key,
quantity: s.Value
)
));
return(priorityQueue);
}