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Using Value Notes in Aztec.nr

ValueNotes hold one main property - a value - and have utils useful for manipulating this value, such as incrementing and decrementing it similarly to an integer.

ValueNote​

This is the ValueNote struct:

value-note-def
#[aztec(note)]
struct ValueNote {
    value: Field,
   // The nullifying public key hash is used with the nsk_app to ensure that the note can be privately spent.
    npk_m_hash: Field,
    randomness: Field,
}
Source code: noir-projects/aztec-nr/value-note/src/value_note.nr#L16-L24

Importing ValueNote​

In Nargo.toml​

value_note = { git="https://github.com/AztecProtocol/aztec-packages/", tag="aztec-packages-v0.54.0", directory="noir-projects/aztec-nr/value-note" }

In your contract​

import_valuenote
use dep::value_note::value_note::ValueNote;
Source code: noir-projects/noir-contracts/contracts/crowdfunding_contract/src/main.nr#L12-L14

Working with ValueNote​

Creating a new note​

Creating a new ValueNote takes the following args:

  • value (Field): the value of the ValueNote
  • npk_m_hash (Field): the master nullifier public key hash of the user
valuenote_new
let mut note = ValueNote::new(amount as Field, donor_keys.npk_m.hash());
Source code: noir-projects/noir-contracts/contracts/crowdfunding_contract/src/main.nr#L74-L76

In this example, amount is the value and the npk_m_hash of the donor was computed earlier.

Getting a balance​

A user may have multiple notes in a set that all refer to the same content (e.g. a set of notes representing a single token balance). By using the ValueNote type to represent token balances, you do not have to manually add each of these notes and can instead use a helper function get_balance().

It takes one argument - the set of notes.

get_balance
// Return the sum of all notes in the set.
balance_utils::get_balance(owner_balance)
Source code: noir-projects/noir-contracts/contracts/stateful_test_contract/src/main.nr#L94-L97

This can only be used in an unconstrained function.

Incrementing and decrementing​

Both increment and decrement functions take the same args:

increment_args
balance: PrivateSet<ValueNote, &mut PrivateContext>,
amount: Field,
recipient: AztecAddress,
Source code: noir-projects/aztec-nr/value-note/src/utils.nr#L16-L20

Note that this will create a new note in the set of notes passed as the first argument. For example:

increment_valuenote
increment(storage.notes.at(owner), value, owner, outgoing_viewer);
Source code: noir-projects/noir-contracts/contracts/benchmarking_contract/src/main.nr#L21-L23

The decrement function works similarly except the amount is the number that the value will be decremented by, and it will fail if the sum of the selected notes is less than the amount.

Learn more​