Implementing external mergesort. How to get started?

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For a project I have to implement an external mergesort algorithm. It will be used to sort a file with mostly numbers or strings and will be some GBs in size. This is the definition of the mergesort that I've been given

void MergeSort (char *infile,
                unsigned char field,
                block_t *buffer,
                unsigned int nmem_blocks,
                char *outfile,
                unsigned int *nsorted_segs,
                unsigned int *npasses,
                unsigned int *nios);

I'm not allowed to change that. The first argument is the file that I'm going to sort. The second the field according to which I want to sort the file (doesn't interest me right now), the third argument is the buffer. Which is a struct. Here is the definition of a block

typedef struct
{
    unsigned int blockid;
    unsigned int nreserved; // how many reserved entries
    record_t entries[MAX_RECORDS_PER_BLOCK]; // array of records
    bool valid;  // if set, then this block is valid
    unsigned char misc;
    unsigned int next_blockid;
    unsigned int dummy;

} block_t;

the fourth argument is the number of blocks in memory. The last three arguments can be set by me.

My questions are:

  1. Do I take the file and cut it into two files?

  2. Is the buffer a file stored in the harddrive or does it stay in the memory? Do I have to create a new file? I'm a little confused with this part.

These are my thoughts to start right now. First I get the file and split it in two parts. I also create a buffer, which I don't know what size it should have. Then I read the first block of records from the first file, and compare the numbers to the first block of records of the second file. Whenever the number is lesser or equal to another I will send it to the output file. Can you evaluate my stream of thoughts? Or am I thinking it wrong?

I think the solution depends on the size of input file.

What I would do is to check the size of the file first, if it's smaller than certain size, for example, 1GB (providing 1GB is a small amount of memory on your machine), then I'll read the whole file, store the content in memory, merge sort them, then write to the new file.

Otherwise, I'll have to divide the original file to K temp files less than 1GB, merge sort each of them, then do a K way merge sort between the files, and finally concats the K files together. Basically you need to divide and conquer in two levels, first the files on disc, then the contents in memory.

External Sorting, One example of external sorting is the external merge sort algorithm, which Following is C++ implementation of the above steps. to get index of right child. External Merge Sort uses a hybrid sort-merge technique. The chunks of data small enough to fit in the RAM are read, sorted, and written out to a temporary file during the sorting phase. In the merge phase, the sorted sub-files are combined into a single larger file. In other words, external External Merge Sort sorts..

  1. If you are using any reasonable OS (anything with an X in it or BSD) and have enough memory rely on 'sort'. If you hit the limit on file size, use 'split' and then the --merge option of sort to merge the already sorted files.

  2. If you really need to write code for an external sort you can save yourself a lot of trouble by starting with a thorough reading of Knuth's TAOCP Vol III, Chp 5 on external sorting.

External sorting, Then by this, we start merging a pair of intermediate files in the main memory to get a sorted output. Two-Way Merge Sort. Two-way merge sort is  I am aware of External merge sort and how it works. But currently i'm stuck while implementing it. I've written code to sort and merge the arrays but I'm facing problem while reading and writing the data from/into the file, i want to implement the following methods in C++:

Refer my github repo - https://github.com/melvilgit/external-Merge-Sort/blob/master/README.md

Problem Stement

All Sorting Algorithm works within the RAM .When the data to be sorted does not fit into the RAM and instead they resides in the slower external memory (usually a hard drive) , this technique is used . Example , If we have to Sort 100 numbers with each number 1KB and our RAM size is 10KB ,external merge sort works like a charm !.

How to ?

Split Phase

Split the 100 KB file into 10 files each 10kb Sort the 10KB files using some efficient Sorting Algo in O(nlogn) Stores each of the smaller files to disk . Merge Phase

Do a K-way merge with each smaller files one by one. Inline the details .

After the Split Phase , A list of file handler of all the splitted files will be stored - sortedTempFileHandlerList

Now, We creates a list of heapnode - heapnodes. Each heapnode will stores the actual entry read from the file and also the file which owns it . The heapnodes is heapified and it will be a min-heap.

Assuming there 10 files , heapnodes will takes 10KB only (each number assume 1KB) .

Loop While Least Element (the top of heap ) is INT_MAX Picks the node with least element from heapnodes . ( 0(1) since heapnodes is a min heap ) Write the element to sortedLargeFile (it will be the sorted number) Find the filehandler of the corresponding element by looking at heapnode.filehandler . Read the next item from the file . If it's EOF, mark the item as INT_MAX Put the item to heap top . Again Heapify to persist min heap property . Continue ; At the end of the Merge Phase sortedLargeFile will have all the elements in sorted order .

Example Say We have a file largefile with the following Contents

5 8 6 3 7 1 4 9 10 2

In Split Phase ,We Split them into the Sorted chunks in 5 separate temp files.

temp1 - 5 ,8 temp2 - 3 ,6 temp3 - 1, 7 temp4 -4 , 9 temp5 - 2 ,10

Next Construct a Min Heap with top element from each files

                     1                       
                   /  \
                  2    5
                /  \                         
               4    3     

Now picks the least Element from the min heap and write it to sortedOutputFile - 1. Finds the next element of the file which owns min element 1 . The no is 7 from temp3 . Move it to heap.

  7                                    2
/  \                                 /  \

2 5 Heapify --> 3 5 / \ / \ 4 3 4 7 Picks the least element 2 and moves it to sortedOutputFile - 1 2. Finds the next element of the file which owns min element 2 . The no is 10 from temp5 . Move it to heap.

  10                                   3
/  \                                 /  \

3 5 Heapify --> 4 5 / \ / \ 4 7 10 7 Picks the least element 3 and moves it to sortedOutputFile - 1 2 3. Finds the next element of the file which owns min element 3 . The no is 6 from temp2 . Move it to heap.

  6                                   4
/  \                                 /  \

4 5 Heapify --> 6 5 / \ / \ 10 7 10 7 Picks the least element 4 and moves it to sortedOutputFile - 1 2 3 4. Finds the next element of the file which owns min element 4 . The no is 9 from temp4 . Move it to heap.

  9                                   5
/  \                                 /  \

6 5 Heapify --> 6 9 / \ / \ 10 7 10 7 Picks the least element 5 and moves it to sortedOutputFile - 1 2 3 4 5. Finds the next element of the file which owns min element 5. The no is 8 from temp1 . Move it to heap

  8                                   6
/  \                                 /  \

6 9 Heapify --> 7 9 / \ / \ 10 7 10 8 Picks the least element 6 and moves it to sortedOutputFile - 1 2 3 4 5 6 . Finds the next element of the file which owns min element 5 . . We have see EOF . So mark the read no as INT_MAX .

INT_MAX 7 / \ / \ 7 9 Heapify --> 8 9 / \ / \ 10 8 10 INT_MAX Picks the least element 6 and moves it to sortedOutputFile - 1 2 3 4 5 6 7 . If we loop this process , we would reaches a point where , the heap would looks like below and the sortedOutputFile - 1 2 3 4 5 6 7 8 9 10 . We would also breaks at this point when the min element from heap becomes INT_MAX .

                   INT_MAX                       
                    /   \
                INT_MAX  INT_MAX
                /    \                         
             INT_MAX INT_MAX        

External Merge Sorting Algorithm, Implement CLOCK! 3. Midterm Do not forget to go over the activities as well! 4. Our usual files. Output: One merged sorted file. F. 1. F. 2. External Merge Algorithm With B+1 buffer pages, we can now start with B+1-length initial runs. C++ Merge sort is an efficient and comparison-based algorithm to sort an array or a list of integers you can say. It is a Divide and Conquer algorithm which was invented by John von Neumann in 1945.

[PDF] Lecture 11: External Sorting, The answer is quite simple, indeed: read File1; sort it; save it as File1Sorted. Start Now. You dismissed this ad. The feedback you provide will help us show input files are too large to internally sort them, so you need a merge-sort program to If the files are already sorted, many problems will have disappeared and you   MergeSort(A, p, r): if p > r return q = (p+r)/2 mergeSort(A, p, q) mergeSort(A, q+1, r) merge(A, p, q, r) To sort an entire array, we need to call MergeSort(A, 0, length(A)-1) . As shown in the image below, the merge sort algorithm recursively divides the array into halves until we reach the base case of array with 1 element.

How to implement external merge sort, If you're seeing this message, it means we're having trouble loading external Let's start with array holding [14, 7, 3, 12, 9, 11, 6, 2], so that the first subarray is It's the combine step, where you have to merge two sorted subarrays, where the real In the next challenge, you'll focus on implementing the overall merge sort  MergeSort(arr[], l, r) If r > l 1. Find the middle point to divide the array into two halves: middle m = (l+r)/2 2. Call mergeSort for first half: Call mergeSort(arr, l, m) 3. Call mergeSort for second half: Call mergeSort(arr, m+1, r) 4.

Merge sort algorithm overview (article), External Merge Sort uses a hybrid sort-merge technique. The chunks of int count = 0;. // Now one by one get the minimum element from min heap and replace. Mergesort is another comparison-based algorithm also known as a "divide and conquer" algorithm and its main focus is on how to merge together two pre-sorted arrays such that the resulting array is also sorted. You may be interested to know that it was invented by the marvellous John von Neumann in 1945.

Comments
  • Don't use ..._t as typedef'd names, since they are used by the standard built-in library, and that would lead to confusion.
  • First learn how mergesort works: it's a documented algorithm and it isn't clear from the question that you know it.
  • The file will for sure be more than 1GB. How do I use the buffer?Any ideas?
  • buffer will store the entries you are trying to sort in memory, you can start with a small file, implement your merge sort algorithm, then worry about bigger files.