Increment part of a string in Ruby - ruby

Question

I have a method in a Ruby script that is attempting to rename files before they are saved. It looks like this:
def increment (path)
if path[-3,2] == "_#"
print " Incremented file with that name already exists, renaming\n"
count = path[-1].chr.to_i + 1
return path.chop! << count.to_s
else
print " A file with that name already exists, renaming\n"
return path << "_#1"
end
end
Say you have 3 files with the same name being saved to a directory, we'll say the file is called example.mp3. The idea is that the first will be saved as example.mp3 (since it won't be caught by if File.exists?("#{file_path}.mp3") elsewhere in the script), the second will be saved as example_#1.mp3 (since it is caught by the else part of the above method) and the third as example_#2.mp3 (since it is caught by the if part of the above method).
The problem I have is twofold.
1) if path[-3,2] == "_#" won't work for files with an integer of more than one digit (example_#11.mp3 for example) since the character placement will be wrong (you'd need it to be path[-4,2] but then that doesn't cope with 3 digit numbers etc).
2) I'm never reaching problem 1) since the method doesn't reliably catch file names. At the moment it will rename the first to example_#1.mp3 but the second gets renamed to the same thing (causing it to overwrite the previously saved file).
This is possibly too vague for Stack Overflow but I can't find anything that addresses the issue of incrementing a certain part of a string.
Thanks in advance!
Edit/update:
Wayne's method below seems to work on it's own but not when included as part of the whole script - it can increment a file once (from example.mp3 to example_#1.mp3) but doesn't cope with taking example_#1.mp3 and incrementing it to example_#2.mp3. To provide a little more context - currently when the script finds a file to save it is passing the name to Wayne's method like this:
file_name = increment(image_name)
File.open("images/#{file_name}.jpeg", 'w') do |output|
open(image_url) do |input|
output << input.read
end
end
I've edited Wayne's script a little so now it looks like this:
def increment (name)
name = name.gsub(/\s{2,}|(http:\/\/)|(www.)/i, '')
if File.exists?("images/#{name}.jpeg")
_, filename, count, extension = *name.match(/(\A.*?)(?:_#(\d+))?(\.[^.]*)?\Z/)
count = (count || '0').to_i + 1
"#{name}_##{count}#{extension}"
else
return name
end
end
Where am I going wrong? Again, thanks in advance.

Answer 1

A regular expression will git 'er done:
#!/usr/bin/ruby1.8
def increment(path)
_, filename, count, extension = *path.match(/(\A.*?)(?:_#(\d+))?(\.[^.]*)?\Z/)
count = (count || '0').to_i + 1
"#{filename}_##{count}#{extension}"
end
p increment('example') # => "example_#1"
p increment('example.') # => "example_#1."
p increment('example.mp3') # => "example_#1.mp3"
p increment('example_#1.mp3') # => "example_#2.mp3"
p increment('example_#2.mp3') # => "example_#3.mp3"
This probably doesn't matter for the code you're writing, but if you ever may have multiple threads or processes using this algorithm on the same files, there's a race condition when checking for existence before saving: Two writers can both find the same filename unused and write to it. If that matters to you, then open the file in a mode that fails if it exists, rescuing the exception. When the exception occurs, pick a different name. Roughly:
loop do
begin
File.open(filename, File::CREAT | File::EXCL | File::WRONLY) do |file|
file.puts "Your content goes here"
end
break
rescue Errno::EEXIST
filename = increment(filename)
redo
end
end

Answer 2

Here's a variation that doesn't accept a file name with an existing count:
def non_colliding_filename( filename )
if File.exists?(filename)
base,ext = /\A(.+?)(\.[^.]+)?\Z/.match( filename ).to_a[1..-1]
i = 1
i += 1 while File.exists?( filename="#{base}_##{i}#{ext}" )
end
filename
end
Proof:
%w[ foo bar.mp3 jim.bob.mp3 ].each do |desired|
3.times{
file = non_colliding_filename( desired )
p file
File.open( file, 'w' ){ |f| f << "tmp" }
}
end
#=> "foo"
#=> "foo_#1"
#=> "foo_#2"
#=> "bar.mp3"
#=> "bar_#1.mp3"
#=> "bar_#2.mp3"
#=> "jim.bob.mp3"
#=> "jim.bob_#1.mp3"
#=> "jim.bob_#2.mp3"