Optimise usage of UTXOs in transaction.create

[kalouo]

In line with the objective of reducing the amount of UTXOs on the network, implement logic in /transaction.create to select the maximum number of UTXOs as transaction inputs.

[kalouo]

Planning to implement following logic:

• Start with the derivation of minimum inputs required for a transaction.
• Continue with selection of extra inputs as per specification below.

[kalouo]

Desired behaviour in selection of extra UTXOs

Case	Desired Outcome	Tests
Available UTXOs cannot cover amount (incl. fee)	Error message	1
Available UTXOs can cover amount (incl. fee) but merge required	Error Message	1
U = 4 and payable amounts are covered	No action required	1
U = 3 (single currency); input amounts match corresponding currency payables exactly	Add a fourth input for code simplicity even though no actual merge will take place	1
U = 3 (multi-currency); input amounts match corresponding currency payables exactly	Add a fourth input for code simplicity even though no actual merge will take place	1
U < 3 (single currency); input amounts match corresponding currency payables exactly	If 2 or more UTXOs of the same currency available, stealth merge. If not, add inputs for which no actual merge will take place	2
U < 3 (multi currency); input amounts match corresponding currency payables exactly	If 2 or more UTXOs of the same currency available, stealth merge. If not, add inputs for which no actual merge will take place	2
U < 4; input amounts exceed corresponding currency payables	Stealth merge if 1 or more UTXOs available on a currency where we expect "change"	?

U denotes the minimum number of UTXOs to cover transfer and fee amounts

• U = (number of currencies in transaction) x (amount required to cover each currency amount)
• U ≤ 4

[kalouo]

Current UTXO selection flow:

Step	To Do	Tested
1. get_sorted_grouped_utxos: fetch UTXOs from database and order them by currency and amount.		✅
2. needed_funds: generates required funds per currency (incl. fee)		✅
3. select_utxos: if no input is matching the payable exactly, collects inputs in descending order (starting from the largest amount) until the payment is covered or until the spender runs out of possible inputs		✅
4. funds_sufficient?: checks if the inputs selected by select_utxos are sufficient to cover the transaction		✅
5. if number of inputs exceeds 4 -> create_merge	- Remove this and return an error.
6. if number of inputs is 4 or less -> create_transaction	- Put in new module

[unnawut]

☝️ These analyses are pure gold. Thanks!

[kalouo]

Pseudo Code

stealth_merge(utxos_currently_in_tx, user_utxos_ordered_by_currency_with_largest_number_of_utxos) do
   base_case: = maximum_inputs
   else
        - take first item in list of first currency and add it to current utxos/remove from user utxos
   stealth_merge(new_current_utxos, new_user_utxos)
end