I’ve created a “software” thermostat. Using OpenHAB with a relay for heating and temperature sensor for the temperature.
You can make it as advanced as you want. For example you can use an IR blaster to control the AC.
But a little warning, it takes a lot of time. If you don’t have the time or skills, you’ll be better of with an of the shelf product. For example: a full blown independent, but controllable by OpenHAB, system you can use Honeywell Evohome. If you want little more controle, you can use a Zwave thermostat.
Every solution depends on your country your live in, kind of system you use (zwave, zigbee, knx, dmx, etc etc) and your time/wishes.