Write a non recursive version of find-set with path compression

The second pass is in reverse as the recursion unwinds. We are using a recursive buffer-less version of LZ77 with back references to the compressed stream, but for various reasons we would like to limit the depth of recursion that we are allowed to use during decompression.

The recursive method is doing the same thing. In such cases, the parent chain can be O n in size for n elements. The compressor replaces substring at the current position with the pointer back to the already decompressed text or a literal copy of a single character.

However, this is not a problem. How would one attack this kind of problem? To implement highest ancestor compression without recursion you need to maintain your own list. Then make a second pass to update the necessary parent pointers.

My real question is: Consider a standard LZtype compression algorithm.


Make one pass up the chain to get pointers to the sets that need their parent pointers changed and also to learn the root. There are cases where many unions are performed in a row before a find.

This can be done with iteration in constant storage. I differ with those who say the recursive method is always best. In this case the algorithm becomes recursive - references to previous positions in the compressed stream can require one to follow further references before in the compressed stream.

In a reasonable number systems especially embedded onesthe program stack is of limited size. The easiest case would be to prohibit recursion completely. Since you are working in Objective C, this may be iOS. For example, the maximum recursion depth of 1 would mean that a referred substring can involve further references to previous substrings, but neither of these substrings would be able to have any further back references.

The graph is now dynamic and the path-finding algorithm is not "local" in the sense that it can no longer work in a "ripple"-type fashion, because any rush decision early on may disallow much greater savings later on.

In principle, one does not need to store the decompressed text. Now, suppose that we have an additional complication. Each position of the text becomes a vertex, each edge contains the length of the code needed to encode the corresponding substring.

Overlapping substrings would not be allowed in this case. This lecture at Princeton talks about this algorithm and implements it in a loop with 5 lines of C. It is clearly not possible to implement something similar to a Dijkstra algorithm mentioned earlier, because the graph is no longer static: It might be smaller than you think.

Instead, one can make references back to the positions in compressed stream and decompress it as needed on the fly. Early compressors tended to use greedy algorithms for choosing specific substrings, but better in fact, optimal for a fixed LZ77 code results can be obtained by generating a set of several all encoding options at each position and then treating the problem as a problem of finding the shortest path on a weighted graph.

I came across an interesting problem, which I believe to be pretty hard and would love to get references or any other advice to know what to search for in the academic literature on this topic. Here collapsing by recursion can blow out the stack.

All this can be done and in fact I have a working implementation of a greedy algorithm for this problem. I am not a computer scientist, so my use of terminology will be off.

The resulting problem is routinely solved by using the Dijkstra algorithm. Of course, it is much less efficient compared with the classic LZ The first pass is the "winding up" of the recursion, which stores the sets needing parent pointer updates on the program stack.

This article implies K for secondary threads and 1Mb for the main thread. Dealing with overlapping substrings in this case can be a bit tricky, but otherwise, the logic of the algorithm remains mostly the same.WeightedQuickUnionPathCompressionUF code in Java.

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LZ-type compression with a limited recursion depth. specific substrings, but better (in fact, optimal for a fixed LZ77 code) results can be obtained by generating a set of several (all) encoding options at each position and then treating the problem as a problem of finding the shortest path on a weighted graph.

and the answer to the. Nov 20,  · accommodate even non-identical pigeons, analogically speaking." So, he's a crank, not a fraudster. Re: “Recursive Lossless Compression of Random Data” Now a Reality RDM (Recursive Digital Management Ltd.), a Hong Kong-based software company: and yes plenty of people did write their own versions of LHA.

Following is union by rank and path compression based implementation to find a cycle in a graph. Disjoint Set (Or Union-Find) | Set 1 (Detect Cycle in an Undirected Graph) Union-Find Algorithm | Set 2 (Union By Rank and Path Compression).

Remember, every node on the path is going to have its parent pointer changed to be the ultimate representative in the partition. Imagine that you use the potential function that you described in .

Write a non recursive version of find-set with path compression
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