result_set.cr

module DB
  # The response of a query performed on a `Database`.
  #
  # See `DB` for a complete sample.
  #
  # Each `#read` call consumes the result and moves to the next column.
  # Each column must be read in order.
  # At any moment a `#move_next` can be invoked, meaning to skip the
  # remaining, or even all the columns, in the current row.
  # Also it is not mandatory to consume the whole `ResultSet`, hence an iteration
  # through `#each` or `#move_next` can be stopped.
  #
  # **Note:** depending on how the `ResultSet` was obtained it might be mandatory an
  # explicit call to `#close`. Check `QueryMethods#query`.
  #
  # ### Note to implementors
  #
  # 1. Override `#move_next` to move to the next row.
  # 2. Override `#read` returning the next value in the row.
  # 3. (Optional) Override `#read(t)` for some types `t` for which custom logic other than a simple cast is needed.
  # 4. Override `#column_count`, `#column_name`.
  abstract class ResultSet
    include Disposable

    # :nodoc:
    getter statement

    def initialize(@statement : DB::Statement)
    end

    protected def do_close
      statement.release_connection
    end

    # TODO add_next_result_set : Bool

    # Iterates over all the rows
    def each
      while move_next
        yield
      end
    end

    # Iterates over all the columns
    def each_column
      column_count.times do |x|
        yield column_name(x)
      end
    end

    # Move the next row in the result.
    # Return `false` if no more rows are available.
    # See `#each`
    abstract def move_next : Bool

    # TODO def empty? : Bool, handle internally with move_next (?)

    # Returns the number of columns in the result
    abstract def column_count : Int32

    # Returns the name of the column in `index` 0-based position.
    abstract def column_name(index : Int32) : String

    # Returns the name of the columns.
    def column_names
      Array(String).new(column_count) { |i| column_name(i) }
    end

    # Reads the next column value
    abstract def read

    # Returns the column index that corresponds to the next `#read`.
    #
    # If the last column of the current row has been read, it must return `#column_count`.
    abstract def next_column_index : Int32

    # Reads the next columns and maps them to a class
    def read(type : DB::Mappable.class)
      type.new(self)
    end

    # Reads the next column value as a **type**
    def read(type : T.class) : T forall T
      col_index = next_column_index
      value = read
      if value.is_a?(T)
        value
      else
        raise DB::ColumnTypeMismatchError.new(
          context: "#{self.class}#read",
          column_index: col_index,
          column_name: column_name(col_index),
          column_type: value.class.to_s,
          expected_type: T.to_s
        )
      end
    end

    # Reads the next columns and returns a tuple of the values.
    def read(*types : Class)
      internal_read(*types)
    end

    # Reads the next columns and returns a named tuple of the values.
    def read(**types : Class)
      internal_read(**types)
    end

    private def internal_read(*types : *T) forall T
      {% begin %}
        Tuple.new(
          {% for type in T %}
            read({{type.instance}}),
          {% end %}
        )
      {% end %}
    end

    private def internal_read(**types : **T) forall T
      {% begin %}
        NamedTuple.new(
          {% for name, type in T %}
            {{ name }}: read({{type.instance}}),
          {% end %}
        )
      {% end %}
    end

    # def read_blob
    #   yield ... io ....
    # end

    # def read_text
    #   yield ... io ....
    # end
  end
end