Working on requires

.gitignore

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+*.gem
+.bundle
+Gemfile.lock
+pkg/*

Gemfile

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+source "http://rubygems.org"
+
+# Specify your gem's dependencies in capistrano-conditional.gemspec
+gemspec

Rakefile

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+require "bundler/gem_tasks"

capistrano-conditional.gemspec

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+# -*- encoding: utf-8 -*-
+$:.push File.expand_path("../lib", __FILE__)
+require "capistrano-conditional/version"
+
+Gem::Specification.new do |s|
+  s.name        = "capistrano-conditional"
+  s.version     = Capistrano::Conditional::VERSION
+  s.authors     = ["Kali Donovan"]
+  s.email       = ["kali@deviantech.com"]
+  s.homepage    = ""
+  s.summary     = %q{Adds support for conditional deployment tasks}
+  s.description = %q{Allows making tasks for git-based projects conditional based on the specific files to be deployed.}
+
+  s.rubyforge_project = "capistrano-conditional"
+
+  s.files         = `git ls-files`.split("\n")
+  s.test_files    = `git ls-files -- {test,spec,features}/*`.split("\n")
+  s.executables   = `git ls-files -- bin/*`.split("\n").map{ |f| File.basename(f) }
+  s.require_paths = ["lib"]
+
+  # specify any dependencies here; for example:
+  # s.add_development_dependency "rspec"
+  s.add_runtime_dependency "git"
+end

lib/capistrano-conditional.rb

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+require "capistrano-conditional/version"
+require "capistrano-conditional/unit"
+require "capistrano-conditional/deploy"
+
+require "capistrano-conditional/integration"

lib/capistrano-conditional/deploy.rb

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+puts "Loading Deploy"
+
+module Capistrano
+  class Configuration
+    class ConditionalDeploy
+
+      @@conditionals = []
+
+      def self.register(name, opts, &block)
+        @@conditionals << ConditionalUnit.new(name, opts, block)
+      end
+
+
+      def self.apply_conditions!(deployed)
+        conditional = self.new(deployed)
+        conditional.ensure_local_up_to_date
+        conditional.run_conditionals
+        conditional.report_plan
+
+        abort "Done"
+      end
+
+
+
+      def initialize(compare_to = 'HEAD^')
+        @verbose = true
+        @git = Git.open('.')
+        @diff = @git.diff('HEAD', compare_to)
+        @changed = @diff.stats[:files].keys.sort
+        @to_run = []
+      end
+
+      def ensure_local_up_to_date
+        s = @git.status
+        no_changes = %w(changed added deleted untracked).all? { |attrib| s.send(attrib).empty? }
+
+        unless no_changes
+          abort "Your working copy contains local changes not yet committed to git. \nPlease commit all changes before deploying.\n\n"
+        end
+      end
+
+      def report_plan
+        if @verbose
+          puts "\n" * 3
+          puts "Conditional Deploy -- Files Modified:"
+          @changed.each {|f| puts "\t- #{f}"}
+        end
+        puts "\n" * 3
+        puts "Conditional Deploy Plan:"
+        if @to_run.empty?
+          puts "\t* No conditional tasks have been added"
+        else
+          @to_run.each do |job|
+            puts "\t* Running #{job.name}"
+          end
+        end
+        puts "\n" * 3
+      end
+
+      def run_conditionals
+        @@conditionals.each do |job|
+          force = job.name && ENV["RUN_#{job.name.to_s.upcase}"]
+          next unless force || job.applies?(@changed)
+          @to_run << job
+        end
+      end  
+    end
+
+  end
+end

lib/capistrano-conditional/integration.rb

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+Capistrano::Configuration.instance(:must_exist).load do
+  abort "capistrano-conditional is not compatible with Capistrano 1.x." unless respond_to?(:namespace)
+
+  require 'git'
+  abort "Git is not defined (are you in a git repository, with the Git gem installed?)" unless defined?(Git)
+
+  namespace :conditional do
+    desc "Initializes the conditional deployment functionality"
+    task :apply do
+      log = capture("cd #{current_path} && git log --format=oneline -n 1", :pty => false)
+      hash = log.split[0]
+      puts "\nLast deployed git commit: #{log.gsub(hash, '')}\n"
+      ConditionalDeploy.apply_conditions!(hash)
+    end
+  end
+
+  before 'deploy', 'conditional:apply'
+  before 'deploy:migrations', 'conditional:apply'
+end

lib/capistrano-conditional/unit.rb

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+puts "Loading Unit"
+
+module Capistrano
+  module Conditional
+    class Unit
+      attr_accessor :name, :conditions, :block
+
+      def initialize(name, opts, block)
+        @name = name
+        @block = block
+        @conditions = {}
+        opts.each do |k,v|
+          @conditions[k] = v
+        end
+      end
+
+      def applies?(changed)
+        @changed = changed
+        watchlist_applies? && if_applies? && unless_applies?
+      end  
+
+      protected
+
+        def watchlist_applies?
+          Array(conditions[:watchlist]).any? do |watched| 
+            @changed.any? do |path| 
+              path[watched]
+            end
+          end
+        end
+
+        def if_applies?
+          return true if conditions[:if].nil? 
+          condition_true?(:if)
+        end
+
+        def unless_applies?
+          return true if conditions[:unless].nil?
+          !condition_true?(:unless)
+        end
+
+        def condition_true?(label)
+          c = conditions[label]
+          case c.arity
+          when 0 then c.call
+          when 1 then c.call(@changed)
+          else 2 
+            c.call(@changed, @git)
+          end
+        end 
+
+    end 
+  end
+end

lib/capistrano-conditional/version.rb

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+module Capistrano
+  module Conditional
+    VERSION = "0.0.1"
+  end
+end