bin/generate usage instructions. (#461)

Give usage instructions when bin/generate is called with no arguments.
The instructions also provide a list of the available generators.

Return 0 on exit when providing usage information.

It wasn't REALLY an error situation, the script completed correctly, you
just didn't provide enough arguments for it to do anything else.

Move 'require's up to the top of the file.

Send error component of the message to stderr.

Author: Insti

bin/generate

@@ -1,10 +1,29 @@
 #!/usr/bin/env ruby
 
+require_relative '../lib/helper'
+require 'generator'
+
+def available_generators
+  cases = File.join( __dir__, '../lib/*_cases.rb')
+  Dir[cases].map {|filename| /([^\/]*)_cases\.rb$/.match(filename).captures}
+end
+
+def usage
+  "Usage: #{$PROGRAM_NAME} exercise_generator\n\n" +
+  "Available exercise generators:\n" +
+  available_generators.sort.join(' ')
+end
+
 exercise = ARGV[0]
+
+unless exercise
+  STDERR.puts "Exercise name required!\n"
+  puts usage
+  exit
+end
+
 cases = "#{exercise.tr('-','_')}_cases"
 
-require_relative '../lib/helper'
-require 'generator'
 begin
   require "#{cases}"
 rescue LoadError
@@ -14,3 +33,4 @@ end
 
 klass = Object.const_get(cases.split('_').map(&:capitalize).join)
 Generator.new(exercise, klass).generate
+