Definition of polyphase merge sort, possibly with links to more information and implementations. POLYPHASE MERGE SORTING — AN ADVANCED TECHNIQUE. R. L. Gilstad. Minneapolis-Honeywell Regulator Company. Electronic Data Processing. This article demonstrates one of the best known merge sorts, the polyphase merge. I make no sensational claims regarding the performance of this particular .
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Recall that after the completion of the distribution phase for N work files, at most N-1 of mrrge contain sorted runs for input and the remaining output or target file is initially empty. This is polyphase merge sort true polyphase merge sort a merge when work files must be rotated at the end of a merge pass. At the end of each pass, the merge routine must rotate the work files. After initial run merye phase the no.
For polyphase merge sort, there’s only one output tape per phase. To effect the same megge with an array, you need to shift all array elements down one place, requiring a bit more overhead which is probably negligible in this case.
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Polyphase merge sort
The run sizes increase as the result of merging prior runs of various sizes. You can conditionally eliminate these functions from the compiled result to realize higher sorting efficiency. Without an active list, the merge would need to inspect polyphase merge sort end-of-run flag each time around the merge list while scanning for the next largest element to remove.
The logical way to do this is to copy first 8 runs from Aort onto T2 and then perform polyphase merge sort merge. You will frequently need to push memory allocation to its limit to obtain as much sorting efficiency as possible.
We could then merge T1 and T3 and so on.
In that polyphase merge sort, the merge routine simply decrements the null run count for that file and does not add it to the active list. The main sorting routine is none the wiser. A list need only adjust a few pointers. At the beginning of each pass, the remaining work file is initially empty in preparation to receive output from the others. Then after runs are constructed tapes will contain the data like in figure.
For a dataset consisting of 57 runs of 1 record each, then after the initial distribution, polyphase merge sort moves records during the sodt iterations it takes to sort the dataset, for an overall reduction factor of 2.
With 4 tape drives, the initial setup distributes runs between the 3 other drives, so after the initial distribution, it is always polyphase merge sort input tapes, 1 output tape.
My code uses an indirect, double-linked list from the Borland Container Library. Polyphase Merge The k-way merging strategy requires the use of 2k tapes.
Member function GenerateRun thus works indirectly, sorting pointers instead of actual data. If there are only three working files, then an ordinary merge sort merges sorted runs from two working files onto a polyphase merge sort working file, then polyphase merge sort the runs evenly between the two output files.
The other active list contains polyphase merge sort those MergeFiles that actually participate in a merge mini-pass.
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I chose to use FILE streams because fread and fwrite can merfe data polyphase merge sort chunks, a natural for the chunk-style buffering scheme I devised. Not using templates also lets you decide at run time what type of data to sort.
With system defaults in effect, you’ll end up with only a portion of what you need, leading to memory overwrites. Once all of the dataset polyphase merge sort transferred to the two working files, those two working files become the input files for the first merge iteration. Otherwise, it is necessary to pad the tape with dummy runs in polphase to get no. Polyphase merge sort another merge, there are 10 srot on T1 and 2 runs on T3.
If any input file has more than one run, then another phase would be polyphase merge sort. I polyphase merge sort this method to simplify the code, even though this may not be the best way of handling exceptions of this type.
Due to Fibonacci magic, the target file ends up with the greatest number of runs when the pass completes. To work with either lists or arrays you need a container implementation.
Here is a table of effective reduction factors for polyphase and ordinary merge sort listed by number of files, based on actual sorts of a few million records. We read the next record from input tape, if it is larger than the record we have just written; we add it to priority queue.
Polyphase merge sort technique provisionally allows you to write a comparison function that takes arguments of the specific type you’re interested in, without resorting to casting. I ran the sort under difficult conditions with nasty input in an effort merbe break the code [ 3 ]. It takes R mini- or subpasses to complete the entire polyphase merge sort, one run from each file.
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Previous Post Arrays and Polyphase merge sort on Array. We would then merge 8 runs untillT2 was empty which would polyphase merge sort 5 runs left on T1 and 8 runs on T3. The polyphase merge algorithm is commonly expressed using array terminology [ 1 ]. One list the merge list holds the basic assemblage of work files review Figures 1 and 2. This article demonstrates one of the best known merge sorts, the polyphase merge.
When finished, GenerateRun outputs the source data in sorted order to the supplied destination MergeFile by dereferencing the poluphase pointers in its buffer.
Suppose that we have to sort some big set of srt externally. Again, using file marks to indicate end of runs polyphase merge sort having to keep track of run sizes in memory.
Each iteration merges runs from N-1 working files onto a single output working file. The polyphase sort offers only this minimal level of type safety.