External sort

From Rosetta Code
External sort is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.
Sort a huge file too large to fit into memory. The algorithm consists in reading a large file to be sorted in chunks of data small enough to fit in main memory, sort each of the chunks, write them out to a temporary file, and finally combined the smaller subfiles into a single larger file. For more info see: https://en.wikipedia.org/wiki/External_sorting

The sorting algorithm can be any popular sort, like quicksort. For simplicity one can assume that the file consists of fixed length integers and that the sort function is less-than (<).

j[edit]

Untested on a memory mapped file.

 
NB. Apply an in-place sorting algorithm to a memory mapped file
NB. in-place sort is translation of in-place python quicksort.
 
require 'jmf'
JCHAR map_jmf_ 'DATA'; 'file.huge'
NB. The noun DATA now refers to the memory mapped file.
NB. Use: quicksort DATA
 
 
NB. use: quicksort DATA
quicksort=: 3 :'qsinternal 0 , <:@:# ARRAY=: y' NB. ARRAY is global
 
qsinternal=: 3 :0
'start stop'=. y
if. 0 < stop - start do.
'left right pivot'=. start, stop, start{ARRAY NB. pivot, left, right = array[start], start, stop
while. left <: right do. NB. while left <= right:
while. pivot > left { ARRAY do. NB. while array[left] < pivot:
left=. >: left
end.
while. pivot < right { ARRAY do. NB. while array[right] > pivot:
right=. <: right NB. right -= 1
end.
if. left <: right do. NB. if left <= right:
 
NB. mapped files work by reference, assignment not required, but for testing.
ARRAY=: (left, right) {`(|.@:[)`]} ARRAY NB. array[left], array[right] = array[right], array[left]
 
left=. >: left NB. left += 1
right=. <: right NB. right -= 1
end.
end.
qsinternal start , right NB. _quicksort(array, start, right)
qsinternal left , stop NB. _quicksort(array, left, stop)
end.
i. 0 0 NB. verbs return the final noun
)
 

Demonstration the sorting works:

   quicksort ?~10
   ARRAY
0 1 2 3 4 5 6 7 8 9
   

python[edit]

A technique demonstrated with a short string character data.

 
#! /usr/bin/python3
 
'''
$ # example session in bash
$ python3 external_sort.py
expect 123456789
memory size 1 passed
memory size 2 passed
memory size 3 passed
memory size 4 passed
memory size 5 passed
memory size 6 passed
memory size 7 passed
memory size 8 passed
memory size 9 passed
memory size 10 passed
memory size 11 passed
'''

 
import io
 
def sort_large_file(n: int, source: open, sink: open, file_opener = open)->None:
 
'''
approach:
break the source into files of size n
sort each of these files
merge these onto the sink
'''

 
# store sorted chunks into files of size n
mergers = []
while True:
text = list(source.read(n))
if not len(text):
break;
text.sort()
merge_me = file_opener()
merge_me.write(''.join(text))
mergers.append(merge_me)
merge_me.seek(0)
 
# merge onto sink
stack_tops = [f.read(1) for f in mergers]
while stack_tops:
c = min(stack_tops)
sink.write(c)
i = stack_tops.index(c)
t = mergers[i].read(1)
if t:
stack_tops[i] = t
else:
del stack_tops[i]
mergers[i].close()
del mergers[i] # __del__ method of file_opener should delete the file
 
def main():
'''
test case
sort 6,7,8,9,2,5,3,4,1 with several memory sizes
'''

 
# load test case into a file like object
input_file_too_large_for_memory = io.StringIO('678925341')
 
# generate the expected output
t = list(input_file_too_large_for_memory.read())
t.sort()
expect = ''.join(t)
print('expect', expect)
 
# attempt to sort with several memory sizes
for memory_size in range(1,12):
input_file_too_large_for_memory.seek(0)
output_file_too_large_for_memory = io.StringIO()
sort_large_file(memory_size, input_file_too_large_for_memory, output_file_too_large_for_memory, io.StringIO)
output_file_too_large_for_memory.seek(0)
assert(output_file_too_large_for_memory.read() == expect)
print('memory size {} passed'.format(memory_size))
 
if __name__ == '__main__':
example = main
example()