template_renderer.rb

=begin
In this problem you will write a function called populate_template which will take in a template string e.g. " Hello {!first_name} - How are you?" and a hash of fields e.g { "first_name" : "John" } as arguments and will return the template string with the fields inserted in the correct place. 
The usecase for a function like this would be for instance for answer templates to be used by a support team at a company. 

So in the previous example you would get:

populate_template("Hello {!first_name} - How are you?", {"first_name" => "John"})

and you would get:

"Hello John - How are you?"

if you called populate_template("Hello {!first_name} - How are you?", {"first_name" => "Mike"}) 

You would get: 

"Hello Mike - How are you?"

-----------------------------------------------------------------------

A few details:

Write a function that takes in 2 arguments:
- template (String) 
- fields (hash / dictionary)

The populate_template function should support unlimited different fields. You should be able to call:
populate_template("Hello {!first_name} {!last_name}", {"first_name" => "Mike", "last_name" => "Smith"}) 

A template might use the same field several times:
populate_template("Hello {!first_name} - I hope everything is going well with you {!first_name}", {"first_name" => "Mike"}) 

You should print helpful error messages (e.g. missing field in hash)
=end


class TemplateRenderer
  def initialize(template, value)
    @template = template
    @value = value
  end

  def populate_template
    extract_fields.inject(@template.dup) do |new_template, field|
      val = @value.fetch clear_field(field)
      new_template.gsub! field, val
    end
  rescue => error
    puts "Ops! There's no #{error.key} field in #{@value}"
  end

  private

  def extract_fields
    @template.scan(/{!\w+}/).uniq
  end

  def clear_field(field)
    field.gsub /{!|}/, ''
  end
end


def populate_template(template, value)
  TemplateRenderer.new(template, value).populate_template
end