Exceptional Code: External Sorting for sorting large files in disk

1. Start reading the input file from the beginning.
2. Read M (or less if number of entries remaining in the file is less than M) numbers from the file and store it into a temp buffer.
3. Sort (using any good sorting method - Quicksort, for example) the numbers in the buffer stored in step 2.
4. Create a temp file in disk and write out the sorted numbers from step 3 to this temp file. Save the name of the temp file.
5. Repeat step 2 to 4 until all numbers from the input file has been read, sorted, and written out to temp files.

At this point, we have chunks of numbers of size M sorted and stored in temp files in disk. We need to merge all these sorted files into one single sorted file. We will apply the merging algorithm from Merge Sort to join the numbers from these sorted files together.

6. Open all the temp files (and set the read pointer to the beginning of the files).
7. Find the minimum number from the set of numbers currently pointed to by the file read pointer.
8. Write the number to disk. (To increase efficiency you could write the number to a buffer first and then flush the buffer out to disk when the buffer is full. But modern I/O libraries should be doing this anyway for you).
9. Read another number from the file that contained the minimum number at step 7.
10. Repeat step 7 to 9 until all numbers from all the temp files have been processed, merged, and written out to disk.

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