1 4:22 6:22 7:44 8:12312
1 4:44 7:44
0 1:33 9:0.44
-1 1:55 4:0 8:12132

[(4, 3), (7, 2), (8, 2), (1, 2), (6, 1), (9, 1)]

import argparse
import multiprocessing as mp
import os
from operator import itemgetter
from collections import Counter
import functools
import json


def parse_args():
    parser = argparse.ArgumentParser()
    parser.add_argument("--input", required=True)
    parser.add_argument("--output", action='store_true', default=False)
    parser.add_argument("--no-stdout", action='store_true', default=False)
    parser.add_argument("--cores", type=int, default=None)

    return parser.parse_args()


def parse_libsvm_line(line: str) -> list:
    """
    Parses a line in a LibSVM file to return the indexes of non-zero features
    :param line: A line in LibSVM format: "1 5:22 7:44 99:0.88"
    :return: A list of ints, one for each index appearing in the line
    """
    features = line.split()[1:] # Get rid of the class value
    indexes = [int(pair.split(":")[0]) for pair in features]
    return indexes


def process_wrapper(arg_tuple):
    """
    Applies the process function to every line in a chunk of a file, to determine the frequency
    of features in the chunk.
    :param arg_tuple: A tuple that contains: line_process_fun, filename, chunk_start, chunk_size
    :return: A counter object that counts the frequency of each feature in the chunk
    """
    line_process_fun, filename, chunk_start, chunk_size = arg_tuple
    counter = Counter()
    with open(filename) as f:
        f.seek(chunk_start)
        lines = f.read(chunk_size).splitlines()
        for line in lines:
            indexes = line_process_fun(line)
            for index in indexes:
                counter[index] += 1

    return counter


def chunkify(fname, size=1024*1024):
    """
    Creates a generator that indicates how to chunk a file into parts.
    :param fname: The name of the file to be chunked
    :param size: The size of each chunk, in bytes.
    :return: A generator of (chunk_start, chunk_size) tuples for the file.
    """
    file_end = os.path.getsize(fname)
    with open(fname, 'r') as f:
        chunk_end = f.tell()
        while True:
            chunk_start = chunk_end
            f.seek(f.tell() + size, os.SEEK_SET)
            f.readline()
            chunk_end = f.tell()
            yield chunk_start, chunk_end - chunk_start
            if chunk_end > file_end:
                break


if __name__ == '__main__':
    args = parse_args()
    pool = mp.Pool(args.cores)
    jobs = []

    # Create one job argument tuple for each chunk of the file
    for chunk_start, chunk_size in chunkify(args.input):
        jobs.append((parse_libsvm_line, args.input, chunk_start, chunk_size))

    # Process chunks in parallel. The result is a list of Counter objects
    res_list = pool.map(process_wrapper, jobs)

    # Aggregate the chunk dictionaries and sort by decreasing value
    aggregated_count = sorted(functools.reduce(lambda a, b: a + b, res_list).items(),
                              key=itemgetter(1), reverse=True)
    # Print the result
    if not args.no_stdout:
        print(aggregated_count)

    # Write the result to a file as json (sorted list of [index, count] lists)
    if args.output:
        with open(args.input + "_frequencies.json", 'w') as out:
            json.dump(aggregated_count, out)

    # Close the pool workers
    pool.close()