Calculate rent

The base account size is 128 bytes. We need to figure out the amount of Solana required to be rent excempt.

This number is determined by the size of the data we want to store in the account (on top of the base) and the rent we would pay for a minimum of two years.

There are two ways to calculate this:

1. Ask an RPC to fetch this from the chain
2. Calculate it offline ourselves

To fetch it from an RPC we can just ask:

const rpc = createSolanaRpc("http://127.0.0.1:8899")
const rent = await rpc.getMinimumBalanceForRentExemption(space).send()

However there can be times we want to calculate this offline and we are able to estimate it given just the space.

// https://github.com/DecalLabs/gill/blob/master/packages/gill/src/core/accounts.ts
/**
 * Calculate the total rent needed for to create an account, with or without extra data stored in it
 */
export function getMinimumBalanceForRentExemption(space: bigint | number = 0) {
  /**
   * Default values for Rent calculations
   *
   * Values taken from: https://github.com/anza-xyz/solana-sdk/blob/c07f692e41d757057c8700211a9300cdcd6d33b1/rent/src/lib.rs#L93-L97
   *
   */
  const RENT = {
    /**
     * Account storage overhead for calculation of base rent. (aka the number of bytes required to store an account with no data.
     */
    ACCOUNT_STORAGE_OVERHEAD: 128n,
    /**
     * Amount of time (in years) a balance must include rent for the account to
     * be rent exempt.
     */
    DEFAULT_EXEMPTION_THRESHOLD: BigInt(Math.floor(2.0 * 1000)) / 1000n,
    /**
     * Default rental rate in lamports/byte-year. This calculation is based on:
     * - 10^9 lamports per SOL
     * - $1 per SOL
     * - $0.01 per megabyte day
     * - $3.65 per megabyte year
     */
    DEFAULT_LAMPORTS_PER_BYTE_YEAR: BigInt(
      Math.floor(((1_000_000_000 / 100) * 365) / (1024 * 1024)),
    ),
  };

  return (
    ((RENT.ACCOUNT_STORAGE_OVERHEAD + BigInt(space)) *
      RENT.DEFAULT_LAMPORTS_PER_BYTE_YEAR *
      RENT.DEFAULT_EXEMPTION_THRESHOLD) /
    1n
  );
}