add relative fetcher

This helps reduce any folder structures, weather on disk or in archives, to their relative root paths; i.e. ignore all file-prefixes that are given and go directly to the underlying files, relative to the common folders that contain it

lib/inspec/plugins/fetcher.rb

@@ -27,6 +27,66 @@ module Inspec
       def read(_file)
         fail "Fetcher #{self} does not implement `read(...)`. This is required."
       end
+
+      def relative_target
+        RelFetcher.new(self)
+      end
+    end
+
+    class RelFetcher
+      attr_reader :files
+
+      def initialize(fetcher)
+        @parent = fetcher
+        @prefix = get_prefix(fetcher.files)
+        @files = fetcher.files.find_all { |x| x.start_with? @prefix }
+                        .map { |x| x[@prefix.length..-1] }
+      end
+
+      def read(file)
+        @parent.read(@prefix + file)
+      end
+
+      private
+
+      def get_prefix(files)
+        return '' if files.empty?
+        sorted = files.sort_by(&:length)
+        get_folder_prefix(sorted)
+      end
+
+      def get_folder_prefix(files, first_iteration = true)
+        return get_files_prefix(files) if files.length == 1
+        pre = files[0] + File::SEPARATOR
+        rest = files[1..-1]
+        if rest.all? { |i| i.start_with? pre }
+          return get_folder_prefix(rest, false)
+        end
+        return get_files_prefix(files) if first_iteration
+        files
+      end
+
+      def get_files_prefix(files)
+        return '' if files.empty?
+
+        file = files[0]
+        bn = File.basename(file)
+        # no more prefixes
+        return '' if bn == file
+
+        i = file.rindex(bn)
+        prefix = file[0..i-1]
+
+        rest = files.find_all { |f| !f.start_with?(prefix) }
+        return prefix if rest.empty?
+
+        nu_prefix = get_prefix(rest)
+        return nu_prefix if prefix.start_with? nu_prefix
+        # edge case: completely different prefixes; retry prefix detection
+        a = File.dirname(prefix + 'a')
+        b = File.dirname(nu_prefix + 'b')
+        get_prefix([a, b])
+      end
     end
   end
 end

test/unit/fetchers.rb

@@ -10,3 +10,52 @@ describe Inspec::Fetcher do
     res.must_be_kind_of Fetchers::Local
   end
 end
+
+describe Inspec::Plugins::RelFetcher do
+  def fetcher
+    src_fetcher.expects(:files).returns(in_files).at_least_once
+    Inspec::Plugins::RelFetcher.new(src_fetcher)
+  end
+
+  let(:src_fetcher) { mock() }
+
+  IN_AND_OUT = {
+    []                        => [],
+    %w{file}                  => %w{file},
+    # don't prefix just by filename
+    %w{file file_a}           => %w{file file_a},
+    %w{path/file path/file_a} => %w{file file_a},
+    %w{path/to/file}          => %w{file},
+    %w{/path/to/file}         => %w{file},
+    %w{alice bob}             => %w{alice bob},
+    # mixed paths
+    %w{x/a y/b}               => %w{x/a y/b},
+    %w{/x/a /y/b}             => %w{x/a y/b},
+    %w{z/x/a z/y/b}           => %w{x/a y/b},
+    %w{/z/x/a /z/y/b}         => %w{x/a y/b},
+    # mixed with relative path
+    %w{a path/to/b}           => %w{a path/to/b},
+    %w{path/to/b a}           => %w{path/to/b a},
+    %w{path/to/b path/a}      => %w{to/b a},
+    %w{path/to/b path/a c}    => %w{path/to/b path/a c},
+    # mixed with absolute paths
+    %w{/path/to/b /a}         => %w{path/to/b a},
+    %w{/path/to/b /path/a}    => %w{to/b a},
+    %w{/path/to/b /path/a /c} => %w{path/to/b path/a c},
+    # mixing absolute and relative paths
+    %w{path/a /path/b}        => %w{path/a /path/b},
+    %w{/path/a path/b}        => %w{/path/a path/b},
+    # extract folder structure buildup
+    %w{/a /a/b /a/b/c}        => %w{c},
+    %w{/a /a/b /a/b/c/d/e}    => %w{e},
+  }.each do |ins, outs|
+    describe 'empty profile' do
+      let(:in_files) { ins }
+
+      it 'also has no files' do
+        fetcher.files.must_equal outs
+      end
+    end
+  end
+
+end

test/unit/fetchers/url_test.rb

@@ -32,7 +32,8 @@ describe Fetchers::Url do
     end
 
     it 'must contain all files' do
-      _(res.files).must_equal ["inspec.yml", "libraries", "libraries/testlib.rb", "controls", "controls/filesystem_spec.rb"]
+      _(res.files).must_equal %w{inspec.yml libraries libraries/testlib.rb
+        controls controls/filesystem_spec.rb}
     end
 
     it 'must not read if the file isnt included' do
@@ -60,7 +61,8 @@ describe Fetchers::Url do
     end
 
     it 'must contain all files' do
-      _(res.files).must_equal ["inspec.yml", "controls", "controls/filesystem_spec.rb"]
+      _(res.files).must_equal %w{inspec.yml libraries libraries/testlib.rb
+        controls controls/filesystem_spec.rb}
     end
 
     it 'must not read if the file isnt included' do