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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:

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 (<).


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
while. pivot < right { ARRAY do. NB. while array[right] > pivot:
right=. <: right NB. right -= 1
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
qsinternal start , right NB. _quicksort(array, start, right)
qsinternal left , stop NB. _quicksort(array, left, stop)
i. 0 0 NB. verbs return the final noun

Demonstration the sorting works:

   quicksort ?~10
0 1 2 3 4 5 6 7 8 9


A technique demonstrated with a short string character data.

#! /usr/bin/python3
$ # example session in bash
$ python3
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:
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(
if not len(text):
merge_me = file_opener()
# merge onto sink
stack_tops = [ for f in mergers]
while stack_tops:
c = min(stack_tops)
i = stack_tops.index(c)
t = mergers[i].read(1)
if t:
stack_tops[i] = t
del stack_tops[i]
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(
expect = ''.join(t)
print('expect', expect)
# attempt to sort with several memory sizes
for memory_size in range(1,12):
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)
assert( == expect)
print('memory size {} passed'.format(memory_size))
if __name__ == '__main__':
example = main