CODEWITHSUNDEEP
rubyrakerake-task
Lars Haugseth
Lars Haugseth

Reputation: 14881

Rake task with multiple prerequisites generating multiple outputs

I have a method that depends on two files for input, and that produces two or more files as output. There is no one-to-one mapping between each input file and each output file - the method performs operations that combines the input data to produce various output files.

Here's a (somewhat contrived) example:

class Util
  def self.perform
    `cat source1.txt source2.txt > target1.txt`
    `cat source2.txt source1.txt > target2.txt`
  end
end

Edit: The actual code in my case is much more complicated than this, and it is not viable to separate the work done to produce each target file into distinct parts. Hence setting up one file task per target file is not really an option in my case.

How do I set up a rake task for running this method once when needed and only then, ie. if one or more of the target files are missing, or if any of the source files are newer than any of the target files?

My current solution looks like this:

task :my_task => ['source1.txt', 'source2.txt'] do |t|
  targets = ['target1.txt', 'target2.txt']
  if targets.all? { |f| File.exist? f }
    mtime_newest_source = t.prerequisites.map {|f| File.mtime(f) }.max
    mtime_oldest_target = targets.map {|f| File.mtime(f) }.min
    # Stop task early if all targets are up to date
    next if mtime_newest_source < mtime_oldest_target
  end
  # Code that actually produces target files goes here
end

What I am looking for is a way to define the task so that none of the code within its block will be run unless the target files need to be rebuilt.

Upvotes: 3

Views: 1310

Answers (2)

Andy
Andy

Reputation: 135

I don't believe the above answers are correct. The comment from Lars

['target1.txt', 'target2.txt'].each { |target| file target => ['source1.txt', 'source2.txt'] { do_stuff_here } }

Will run { do stuff here } twice if rake 'target1.txt', 'target2.txt' is called.

I believe a correct resolution of the dependencies is achieved by

file 'target1.txt' => ['source1.txt', 'source2.txt'] { do_stuff_here } file 'target2.txt' => 'target1.txt'

Upvotes: 0

James Mason
James Mason

Reputation: 4296

FileTasks are designed for exactly this. Here's a good old long explanation, and here's a quick example:

file 'target1.txt' => ['source1.txt', 'source2.txt'] do
  # do something to generate target1.txt
  `cat source1.txt source2.txt > target1.txt`
end

file 'target2.txt' => ['source1.txt', 'source2.txt'] do
  # again, generate the file
  `cat source2.txt source1.txt > target2.txt`
end

File tasks can depend on other rake tasks or files on disk. If your tasks depends on a file, rake will compare the timestamps and only run your task if the dependent file is newer.

Update

Here's a working example of generating all of targets from both tasks. Rake is smart enough to skip the second task when the first one creates both files.

def make_the_targets
  `cat source1.txt source2.txt > target1.txt`
  `cat source2.txt source1.txt > target2.txt`
end

file 'target1.txt' => ['source1.txt', 'source2.txt'] do
  puts "making target1.txt"
  make_the_targets
end

file 'target2.txt' => ['source1.txt', 'source2.txt'] do
  puts "making target2.txt"
  make_the_targets
end

task :make_targets => ['target1.txt', 'target2.txt']

Upvotes: 4

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