update basics rent program

Author: ZYJLiu

basics/rent/solang/solidity/minimum_balance.sol

@@ -0,0 +1,33 @@
+// SPDX-License-Identifier: Apache-2.0
+
+// Disclaimer: This library provides functions for working with storage rent. Although it is production ready,
+// it has not been audited for security, so use it at your own risk.
+
+// This is the Solidity version of the rust module rent:
+// https://github.com/solana-labs/solana/blob/master/sdk/program/src/rent.rs
+// As rent is currently not implemented on Solana, only the minimum balance is required.
+
+/// 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
+uint64 constant DEFAULT_LAMPORTS_PER_BYTE_YEAR = 1_000_000_000 / 100 * 365 / (1024 * 1024);
+
+/// Default amount of time (in years) the balance has to include rent for the
+/// account to be rent exempt.
+uint64 constant DEFAULT_EXEMPTION_THRESHOLD = 2;
+
+/// Account storage overhead for calculation of base rent.
+///
+/// This is the number of bytes required to store an account with no data. It is
+/// added to an accounts data length when calculating [`Rent::minimum_balance`].
+uint64 constant ACCOUNT_STORAGE_OVERHEAD = 128;
+
+/// Minimum balance due for rent-exemption of a given account data size.
+function minimum_balance(uint64 data_len) pure returns (uint64) {
+    return ((ACCOUNT_STORAGE_OVERHEAD + data_len) * DEFAULT_LAMPORTS_PER_BYTE_YEAR)
+        * DEFAULT_EXEMPTION_THRESHOLD;
+}

basics/rent/solang/solidity/rent.sol

@@ -1,14 +1,17 @@
 
-
 import "./system_instruction.sol";
+import 'minimum_balance.sol';
 
 @program_id("F1ipperKF9EfD821ZbbYjS319LXYiBmjhzkkf5a26rC")
 contract rent {
 
     @payer(payer) // The "payer" pays for the data account creation
     constructor(address payer) {}
 
-    function createSystemAccount(address payer, address newAccount, uint64 lamports, uint64 space) public view {
+    function createSystemAccount(address payer, address newAccount, uint64 space) public view {
+        // The minimum lamports required for the amount of space allocated to the account
+        uint64 lamports = minimum_balance(space);
+
         SystemInstruction.create_account(
             payer,        // lamports sent from this account (payer)
             newAccount, // lamports sent to this account (account to be created)

basics/rent/solang/tests/rent.ts

@@ -29,15 +29,12 @@ describe("rent", () => {
     const newAccount = anchor.web3.Keypair.generate()
     // Number of bytes of space to allocate for the account
     const space = 100
-    // Get the minimum balance required for the account for the given space
-    const lamports = await connection.getMinimumBalanceForRentExemption(space)
 
     // Create a new account via a Cross Program Invocation to the system program
     const tx = await program.methods
       .createSystemAccount(
         wallet.publicKey, // payer
         newAccount.publicKey, // new account
-        new anchor.BN(lamports), // lamports
         new anchor.BN(space) // space
       )
       .accounts({ dataAccount: dataAccount.publicKey })