How clear is this ruby code?

1. Took around a 100 seconds to understand. Would be twice faster if I had a commentary: # this code merges two presorted files.

2. All is pretty ok, but what you forgot here is to close all opened files. And it is the only reason to have a first variable declared, otherwise I would mix both maps together.

3. If you pass filenames via ARGV, there should be also the merged_filename as the first or last parameter. Or at any position and optional if you start to use some named options parsing.

So we have this:

files_to_merge = ARGV.map &File.method(:open)
lines = files_to_merge.map &:gets
merged_file = File.open "merge_out.txt", "w"
while lines.any?
    merged_file.print(next_line = lines.compact.min)
    file_id = lines.index next_line
    lines[file_id] = files_to_merge[file_id].gets
end
merged_file.close
files_to_merge.each &:close

But what if try to solve the problem of mindblowing file_id = .index?
You could put file handler and current line together into an Array or Hash, but I don't feel like it makes code better:

files_and_lines = ARGV.map(&File.method(:open)).map{ |file| {file:file, line:file.gets} }
...
loop do
    files_and_lines.select!{ |tuple| tuple[:line] }
    break if files_and_lines.empty?
    next_tuple = files_and_lines.min_by{ |tuple| tuple[:line] }
    merged_file.print next_tuple[:line]
    next_tuple[:line] = next_tuple[:file].gets
end
...

Trying to make it shorter didn't work for me: converting tuples into Hash pair file->line can't easily return file for line and line->file doesn't look better either because to edit a key you have to have some temporal array variable:

files_and_lines = Hash[ ARGV.map(&File.method(:open)).map{ |file| [file.gets, file] } ]
...
loop do
    break if files_and_lines.keep_if{ |line, file| line }.empty?
    array = files_and_lines.to_a.sort_by &:first
    merged_file.print array[0][0]
    array[0][0] = array[0][1].gets
    files_and_lines = Hash[array]
end
...

So just leave Britney alone don't touch it -- .index is ok.

It took me about five minutes to figure out, then I found that I got it wrong. At first, I thought that it would take lines from each named file in turn. Then, I thought that it would take the first paragraph from each file, before realizing that an "empty" line still consists of a "\n" and therefore wouldn't get compacted out. In the end, I decided that it's just concatenating all the files. Then I ran the code to discover that it does one round of mergesort.