Add "getExpectedInput" to calculate token input amount

[stefanionescu]

Uniswap (both v1 and v2) allows developers to calculate how many input tokens should be sent for a certain amount of output tokens.

It would be helpful to include a similar feature here, especially if you intend to integrate Uni V2 once it's launched.

[PBRT]

Any news on this? That'd be very useful :)