# Given an arbitrarily large file containing integers, return the largest N numbers, highest first

I've written a top_n program in Ruby, which does pretty much exactly what the title says, as part of a coding exercise. I'm trying to learn about how to sort efficiently (i.e. deciding the best sorting algorithm and implementing them or their variations), to manage memory efficiently, and know whether I'm using the right language/constructs/ideas to tackle a sorting problem.

What I'm looking for is advice on the algorithm and the code in general. I feel it can be done MUCH, MUCH better (i.e. I'm doing splits and casting to integers... my intuition tells me something could be better).

I'd really appreciate if someone gave me some pointers on how could I improve this little program. Thanks in advance.

require 'set'

class File
def self.tail(path, n = 10)
result = File.open(path, 'r') do |file|
buffer_size = 512
line_count = 0

file.seek(0, IO::SEEK_END)

offset = file.pos

while line_count <= n && offset > 0
to_read = if (offset - buffer_size) < 0
offset
else
buffer_size
end

data.reverse.each_char do |c|
if line_count > n
offset += 1
break
end

offset -= 1

if c == "\n"
line_count += 1
end
end
end

file.seek(offset)
end

result
end

def each_chunk(chunk_size)
end
end

def top_n(filename, n = 100)
pre_sorted_chunks = Dir[".#{filename}_sorted_chunk_*"]

if pre_sorted_chunks.empty?
build_pre_sorted_chunks_for(filename)
end

top = SortedSet.new

# This reference takes ±0.141 seconds.
# pre_sorted_chunks.each do |file|
#   top << tail -n #{n} #{file}.strip.split.map(&:to_i)
# end

# This takes ±0.130 seconds. A little better.
top << File.tail(file_path, n).strip.split.map(&:to_i)
end
end

top.max(n)
end

# Current impl. takes ±5m. A lot of time.
def build_pre_sorted_chunks_for(filename)
File.open(filename) do |file|
n = 0

# Chunks of 500MB.
file.each_chunk(1024 ** 2 * 500) do |chunk|
numbers = chunk.split.map(&:to_i)

sorted_set = SortedSet.new(numbers)
sorted_set = sorted_set.to_a.join("\n")

File.open(".#{filename}_sorted_chunk_#{n}", 'w') do |f|
f.write(sorted_set)
end

n += 1
end
end
end

unless ARGV.empty?
filename = ARGV[0]
n = ARGV[1].to_i

puts top_n(filename, n)
else
puts "Please provide a filename and a N value to calculate the top N."
end


I've used this method to create a file with random numbers:

def generate_random_number_file(n = 15_000_000)
require 'set'

randoms = Set.new

loop do
randoms << rand(n)
break if randoms.size >= n
end

File.open('randos.txt', 'w') do |file|
random_list = randoms.to_a.join("\n")
file.write(random_list)
end
end

• Is there a reason why you wrote your own File tail, instead of using the 'file-tail' gem? And wrote your own benchmarking code instead of using the conventional Benchmark gem? – Mark Thomas Aug 21 '17 at 23:51
• @MarkThomas No particular reason. I just wanted to stick to the core/standard library as much as I could. As for the benchmarking code... I definitely should've used the benchmark module. I'll re-run it and update the benchmarks. Thanks. – Horacio Bertorello Aug 22 '17 at 0:06
• Since we're on Code Review, I must tell you that in the Ruby community it is considered wasteful and even foolish to 'stick to standard libraries' if there exists a gem which does what you want. – Mark Thomas Aug 22 '17 at 0:32
• @MarkThomas Yes, I agree entirely. I would never try to reinvent the wheel just to accomplish something already solved efficiently. Unless I'm playing around, trying to learn how things work, which is what I'm doing here right now. – Horacio Bertorello Aug 22 '17 at 0:45
• Is the output the largest n numbers or unique numbers? In other words is it OK if the top n are all the same 15_000_000? – Mark Thomas Aug 22 '17 at 1:55

Consider remembering the n largest numbers:

def top_n(filename, n = 100)
require 'set'

topn = SortedSet.new()

File.foreach(filename) do |line|
num = line.to_i
if topn.length < n
elsif num > topn.first
topn.delete(topn.first)
end
end
end

• Thanks @NetMage. I wonder how can I stress-test this to see if it's an apt solution for a multi-hundred GB file. – Horacio Bertorello Sep 1 '17 at 19:04

I wonder if a TopNSet class might be of use here. I'm not sure how it would perform, but it seems like separating the reading of the values from the file, and getting the Top-N are different activities, and a TopNSet might be of value more widely.

I implemented it as:

class TopNSet
def initialize(n)
@n      = n
@values = Set.new
end

def << number
values << number
values.delete(values.min) if values.size > n
to_a
end

def to_a
values.to_a
end

private

attr_accessor :values, :n
end


... and hence ...

2.2.5 :021 >     t = TopNSet.new(3)
=> #<TopNSet:0x000001145cca28 @n=3, @values=#<Set: {}>>
2.2.5 :022 >     t << 1
=> [1]
2.2.5 :023 >     t << 2
=> [1, 2]
2.2.5 :024 >     t << 3
=> [1, 2, 3]
2.2.5 :025 >     t << 4
=> [2, 3, 4]
2.2.5 :026 >     t << 4
=> [2, 3, 4]
2.2.5 :027 >     t << 3
=> [2, 3, 4]
2.2.5 :028 >     t << 7
=> [3, 4, 7]
2.2.5 :029 >     t << 6
=> [4, 7, 6]
2.2.5 :030 >     t << 1
=> [4, 7, 6]

• I used a SortedSet instead of a Set on the theory that inserting a number in the proper position and retrieving the minimum would be more efficient than scanning the entire set for the minimum when a deletion is necessary. – NetMage Sep 5 '17 at 20:51
• @NetMage Sounds sensible -- benchmarking Set vs SortedSet would be interesting. – David Aldridge Sep 5 '17 at 21:08