Add Coin Control suggestions

[nopara73]

In HiddenWallet I added coin control suggestions (user builds tx and wallet says, hey, if you use the wallet this way, your privacy will be better.)

This could be figured out properly and implemented into Wasabi, too.

Here're some ideas I implemented into HiddenWallet: https://medium.com/@nopara73/coin-control-is-must-learn-if-you-care-about-your-privacy-in-bitcoin-33b9a5f224a2

[MaxHillebrand]

I wanted to open a new issue about this, but since you have already, I'll simply comment my idea:

Problem

When the user wants to send a specific amount, say 0.23456 btc, then currently he has to manually select different coins, until the inputs are sufficiently large for the sending amount. There is no GUI support which coins would sum up to the value send. It's a trial by error until the fitting UTXOs are selected.

Solution

If a specific value is desired, Wasabi could highlight the coin selection that bitcoind would use, [Knapsack, I think]. This would only be a highlight, and not the final coin selection, which should be done by the user manually. Knapsack optimizes for transaction size / fees, but it is not concerned with privacy, thus hand selection considering labels, anonset, etc.

It might also be helpful, that in the case where the user wants to spend specifically UTXO C, that the GUI then calculates which other UTXOs might be suitable to consolidate.

List of UTXOs:

A 0.12
B 0.0156
C 0.131
D 0.561

Workflow would be:

• User selects the desired value to transact 0.23456 btc
• Wasabi highlights a first suggestion D
• User selects UTXO C
• Wasabi adapts the highlight to A, consider user choice of C
• User selects UTXO A
• Wasabi enables the Send button, since now Inputs > Outputs

Alternatives

There's maybe a more privacy focused coin selection algorithm?

@nopara73 has already proposed something like this for the coin join tab.

When the GUI shows the total amount selected, then this would improve UX of trial by error, but it would still be trial by error.

[nopara73]

WW2 obsolates this.