module SyntaxTree::HashKeyFormatter

def self.for(container)
  container.assocs.each do |assoc|
    if assoc.is_a?(AssocSplat)
      # Splat nodes do not impact the formatting choice.
    elsif assoc.value.nil?
      # If the value is nil, then it has been omitted. In this case we have
      # to match the existing formatting because standardizing would
      # potentially break the code. For example:
      #
      #     { first:, "second" => "value" }
      #
      return Identity.new
    else
      # Otherwise, we need to check the type of the key. If it's a label or
      # dynamic symbol, we can use labels. If it's a symbol literal then it
      # needs to match a certain pattern to be used as a label. If it's
      # anything else, then we need to use hash rockets.
      case assoc.key
      when Label, DynaSymbol
        # Here labels can be used.
      when SymbolLiteral
        # When attempting to convert a hash rocket into a hash label,
        # you need to take care because only certain patterns are
        # allowed. Ruby source says that they have to match keyword
        # arguments to methods, but don't specify what that is. After
        # some experimentation, it looks like it's:
        value = assoc.key.value.value
        if !value.match?(/^[_A-Za-z]/) || value.end_with?("=")
          return Rockets.new
        end
      else
        # If the value is anything else, we have to use hash rockets.
        return Rockets.new
      end
    end
  end
  Labels.new
end