Neptune Coins

Neptune Coins refers to two things

• the native currency coin type for Neptune;
• the unit in which quantities of the former are measured.

In the code, the struct NativeCurrencyAmount defines the unit. The native currency type script is encapsulated as a struct NativeCurrency implementing trait ConsensusProgram in native_currency.rs.

The Unit

One Neptune Coin equals $10^{30} \times 2^2$ nau, which stands for Neptune Atomic Unit. The conversion factor is such that

• The largest possible amount, corresponding to 42'000'000 Neptune Coins, can be represented in 127 bits.
• It can represent a number of Neptune Coins with up to 30 decimal symbols after the point exactly.

The struct NativeCurrencyAmount is a wrapper around a u128. It leaves 1 bit for testing positivity.

The Type Script

The Neptune Coins type script

• computes the sum of all inputs, plus coinbase if it is set;
• computes the sum of all outputs plus fee;
• equates the two quantities.

Additional Features

Transactions have two features that make the native currency type script special. The first is the fee field, which is the excess of the transaction balance that can be captured by the miner. The second is the option coinbase field, which stipulates by how much a transaction is allowed to exceed the sum of input amounts because it is the only transaction in a block.