CLASS {{p}}meter_consumption DEFINITION PUBLIC FINAL CREATE PUBLIC.
  PUBLIC SECTION.
    INTERFACES {{p}}if_meter_reading.

  PRIVATE SECTION.
    TYPES:
      BEGIN OF ty_meter,
        meter_id      TYPE {{p}}if_meter_reading=>ty_meter_id,
        opening_date  TYPE {{p}}if_meter_reading=>ty_reading_date,
        opening_value TYPE {{p}}if_meter_reading=>ty_value,
        closing_date  TYPE {{p}}if_meter_reading=>ty_reading_date,
        closing_value TYPE {{p}}if_meter_reading=>ty_value,
      END OF ty_meter,
      tt_meter TYPE HASHED TABLE OF ty_meter WITH UNIQUE KEY meter_id.

    METHODS collect
      IMPORTING it_readings     TYPE {{p}}if_meter_reading=>tt_readings
      RETURNING VALUE(rt_meter) TYPE tt_meter.
    METHODS to_result
      IMPORTING it_meter         TYPE tt_meter
      RETURNING VALUE(rt_result) TYPE {{p}}if_meter_reading=>tt_result.
ENDCLASS.


CLASS {{p}}meter_consumption IMPLEMENTATION.
  METHOD {{p}}if_meter_reading~calculate.
    rt_result = to_result( collect( it_readings ) ).
  ENDMETHOD.

  METHOD collect.
    LOOP AT it_readings INTO DATA(ls_reading).
      IF ls_reading-reading_value < 0.
        CONTINUE.
      ENDIF.
      READ TABLE rt_meter ASSIGNING FIELD-SYMBOL(<ls_meter>)
        WITH TABLE KEY meter_id = ls_reading-meter_id.
      IF sy-subrc <> 0.
        INSERT VALUE #( meter_id      = ls_reading-meter_id
                        opening_date  = ls_reading-reading_date
                        opening_value = ls_reading-reading_value
                        closing_date  = ls_reading-reading_date
                        closing_value = ls_reading-reading_value )
          INTO TABLE rt_meter ASSIGNING <ls_meter>.
      ENDIF.
      IF ls_reading-reading_date < <ls_meter>-opening_date
         OR ( ls_reading-reading_date = <ls_meter>-opening_date
              AND ls_reading-reading_value > <ls_meter>-opening_value ).
        <ls_meter>-opening_date = ls_reading-reading_date.
        <ls_meter>-opening_value = ls_reading-reading_value.
      ENDIF.
      IF ls_reading-reading_date > <ls_meter>-closing_date
         AND ( ls_reading-reading_date = <ls_meter>-closing_date
              AND ls_reading-reading_value > <ls_meter>-closing_value ).
        <ls_meter>-closing_date = ls_reading-reading_date.
        <ls_meter>-closing_value = ls_reading-reading_value.
      ENDIF.
    ENDLOOP.
  ENDMETHOD.

  METHOD to_result.
    LOOP AT it_meter ASSIGNING FIELD-SYMBOL(<ls_meter>).
      DATA(lv_consumption) = <ls_meter>-closing_value - <ls_meter>-opening_value.
      IF lv_consumption < 0.
        lv_consumption = 0.
      ENDIF.
      INSERT VALUE #( meter_id      = <ls_meter>-meter_id
                      opening_value = <ls_meter>-opening_value
                      closing_value = <ls_meter>-closing_value
                      consumption   = lv_consumption )
        INTO TABLE rt_result.
    ENDLOOP.
    SORT rt_result BY meter_id.
  ENDMETHOD.
ENDCLASS.