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Whether compressing or expanding, once the counts have been loaded, it is time to build the Huffman tree. In HUFF.C, this is done in a function called build_tree(). Because some care was taken when creating the data structure, the actual process of creating the tree is the simple matter of sitting in a loop and combining the two free nodes with the lowest weight into a new internal node with the combined weight of the nodes. Once only one free node is left, the tree is done, and the free node is the root of the tree.
The logic of the build_tree() routine is fairly simple. When the routine is first entered, all nodes below 257 have a count value set to their frequency in the file. A nonzero value here means that this is an active node.
build_tree() also sets up a special node used as a straw man for comparison purposes. Node 513, which will never be used, is set to have a count value of 65535, which no normal node can ever exceed. When searching for the two minimum nodes, I will start by setting the minimum node to 513, knowing that any valid active node will fall below its value.
Finally, before the comparisons start, an index to the next free nodes initialized. The node array is in use from 0 to 256, so the next free node will be at 257.
After things have been set up, build_tree() goes into an infinite loop. On each pass through the loop, build_tree tries to find the two active nodes with the lowest weights. If only one node is found, the tree is complete and the loop is exited. If there are two good minimum values, a new node to the tree can be created. This new node is set up using the next_free node index. Its two child pointers are set to point to the two minimum nodes found before, and its weight is their sum. The two minimum nodes are now marked as being inactive by setting their weights to 0. Nodes with a weight of 0 are considered to be unused and will never again be selected to represent a minimum.
One piece of inefficient code is deliberately left in build_tree(). There is an extra member in the node structure called saved_count. When a node is taken off the active list by having its count set to zero, the previous count is stored in saved_count. Later, if the user has selected the -d option in order to print out the model, the saved_count can be printed. This helps when debugging the program and when trying to understand how the tree works.
During the expansion phase, it is easy to see how to use the Huffman tree. Starting at the root node, a single bit at a time is read in by the decoder. If the bit is a 0, the next node is the one pointed to by the child_0 index. If the bit is a 1, the next node is the one pointed to by the child_1 index. If the new node is 256 or less, we have reached a leaf of the tree and can output the corresponding symbol. If the symbol was the special end-of-stream symbol, we can exit instead of sending it out. This is what is done in the expand_node() function. It is just a few lines of code, and it decodes a compressed Huffman code file with relative ease.
Compressing the same file is a bit harder. Essentially, we want to work down the tree, outputting a 1 or a 0 bit at each node, till we get to the appropriate leaf node. Unfortunately, the tree structure makes this impossible. When we start at the root node, we have no idea whether to take the 0 or the 1 branch to arrive at a particular symbol.
One way to solve this problem when building the tree would be to add a parent member to the node structure. When combining the two minimum nodes to form a new internal node, each minimum node would have its parent structure set to point to the new node. With this new node, we could start at the leaf node and work our way up through the tree toward the root. The only problem with this procedure is that we would accumulate bits in reverse order as we went up the tree. We would have to rack them up till we reached the root node, then put them out in reverse order.
Fortunately, there is a better way to do this. Rather than trying to use the tree to code our symbols when compressing a file, we could build a code table by recursively traversing the entire tree one time only. This creates a table of codes, one for each symbol, along with the length of each code. Once the table is built, the file can be encoded by simply outputting the appropriate code for every character in the input file.
The code to convert the tree data structures into a table of codes is very simple, thanks to a recursive algorithm. We start at the root node of the tree with a zero. Then we begin working down the individual branches of the tree, adding a one or a zero to the code each time we travel down a branch. Whenever we reach a leaf, we store the code values for that leaf in the code array and back up to the previous node, where we can start searching down the other side of the tree.
The code to accomplish this is in function convert_tree_to_code(). This routine takes a fair amount of work to create the code table, but once it is done the actual file compression is very easy.
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