I want to retain previous value if current value is not valid in MQTT sensor

Configuration
1

So, I have a custom smart meter adapter, which reports the values via MQTT. My sensor is defined like this:

- platform: mqtt
  state_topic: '/smartmeter/sensor/low_tariff_energy'
  unit_of_measurement: 'Wh'
  icon: 'mdi:flash'
  name: 'Energy LT'
  device_class: energy
  state_class: total_increasing
  value_template: '{{ value | int(default=0) }}'

As the decryption code occasionaly has some hiccups, the MQTT sensor sometimes gets strange values - for example, the values come in a sequence like: 230000, 230021, 230048, 4545454545454, 230067
This of course screws the whole statistics. So, I would like to define the value_template in a way, that it disregards the new value (and the previous value reimains), if the new value is f.e. more than 1000 higher or lower than current value. What would I have to add to the value_template to achieve this?

I tried to experiment with example from THIS thread, but I couldn’t make it work - maybe because at the initialization the value is empty (no retain available) and compared to the new value the result is always false?.

Thx in advance for all your inputs.

2

Well, solved it myself and posting the solution here, if anyone might have the same problem.
For my scenario the following code works (eliminating unknown values at HA/MQTT/… restart):

- platform: mqtt
  state_topic: '/smartmeter/sensor/low_tariff_energy'
  unit_of_measurement: 'Wh'
  icon: 'mdi:flash'
  name: 'Energy LT'
  device_class: energy
  state_class: total_increasing
  value_template: >- 
    {% if  states('sensor.energy_lt') == 'unknown'  %}
    {{ value }}
    {% elif ( value | int (default=0) - states('sensor.energy_lt') | int (default=0)) | abs > 1000 %}
    {{ states('sensor.energy_lt') | int (default=0) }}
    {% else %}
    {{ value }}
    {% endif %}