Ethereum Smart Contact

I would like to have you create a web3 with a smart contract to do a simple function:

Sell half of the token every time it double from the purchase price.

I think the logic would work like this:

*Keep selling half on the double, loop, always open*
$sell(var2)
var2 = var2*2
If price = var2
Execute order
If success
Strike price=var2
Call $sell function
End

If you can record the process so I know for future changes.