docs(token 22): guide for interest bearing tokens (#17)

* docs(token 22): guide for interest bearing tokens

* Add Solana Playground

* proofread

* refactor: changed dir

* refactor: editorial changes

---------

Co-authored-by: John <[email protected]>
Co-authored-by: nickfrosty <[email protected]>

content/guides/token-extensions/interest-bearing-tokens.md

@@ -0,0 +1,331 @@
+---
+date: Dec 7, 2023
+seoTitle: "Token Extensions: Interest-Bearing"
+title: How to use the Interest-Bearing extension
+description:
+  "Interest-bearing tokens are tokens that can either increase or decrease in
+  value over time. Similar to how a bank savings account or a loan accumulates
+  interest."
+keywords:
+  - token 2022
+  - token extensions
+  - token program
+difficulty: beginner
+tags:
+  - token 2022
+  - token extensions
+---
+
+The `InterestBearingConfig` extension enables developers to set an interest rate
+stored directly on the Mint Account. Interest is compounded continuously, based
+on the network's timestamp.
+
+In other words, the accrued interest is simply a visual UI conversion, but the
+underlying token quantity remains unchanged. This design eliminates the need for
+frequent rebase or update operations to adjust for accrued interest.
+
+<Callout type="info">
+
+Note that the interest accrual is only a
+[calculation](https://github.com/solana-labs/solana-program-library/blob/master/token/program-2022/src/extension/interest_bearing_mint/mod.rs#L85),
+and does not involve minting new tokens.
+
+</Callout>
+
+In this guide, we'll walk through an example of using Solana Playground. Here is
+the [final script](https://beta.solpg.io/65724856fb53fa325bfd0c53).
+
+## Getting Started
+
+Start by opening this Solana Playground
+[link](https://beta.solpg.io/656e19acfb53fa325bfd0c46) with the following
+starter code.
+
+```javascript
+// Client
+console.log("My address:", pg.wallet.publicKey.toString());
+const balance = await pg.connection.getBalance(pg.wallet.publicKey);
+console.log(`My balance: ${balance / web3.LAMPORTS_PER_SOL} SOL`);
+```
+
+If it is your first time using Solana Playground, you'll first need to create a
+Playground Wallet and fund the wallet with devnet SOL.
+
+<Callout type="info">
+
+If you do not have a Playground wallet, you may see a type error within the
+editor on all declarations of `pg.wallet.publicKey`. This type error will clear
+after you create a Playground wallet.
+
+</Callout>
+
+To get devnet SOL, run the `solana airdrop` command in the Playground's
+terminal, or visit this [devnet faucet](https://faucet.solana.com/).
+
+```
+solana airdrop 5
+```
+
+Once you've created and funded the Playground wallet, click the "Run" button to
+run the starter code.
+
+## Add Dependencies
+
+Let's start by setting up our script. We'll be using the `@solana/web3.js` and
+`@solana/spl-token` libraries.
+
+Replace the starter code with the following:
+
+```javascript
+import {
+  Connection,
+  Keypair,
+  SystemProgram,
+  Transaction,
+  clusterApiUrl,
+  sendAndConfirmTransaction,
+} from "@solana/web3.js";
+import {
+  ExtensionType,
+  updateRateInterestBearingMint,
+  createInitializeInterestBearingMintInstruction,
+  createInitializeMintInstruction,
+  getMintLen,
+  TOKEN_2022_PROGRAM_ID,
+  amountToUiAmount,
+  getInterestBearingMintConfigState,
+  getMint,
+} from "@solana/spl-token";
+
+// Playground wallet
+const payer = pg.wallet.keypair;
+
+// Connection to devnet cluster
+const connection = new Connection(clusterApiUrl("devnet"), "confirmed");
+
+// Transaction signature returned from sent transaction
+let transactionSignature: string;
+```
+
+## Mint Setup
+
+First, let's define the properties of the Mint Account we'll be creating in the
+following step.
+
+```javascript
+// Generate new keypair for Mint Account
+const mintKeypair = Keypair.generate();
+// Address for Mint Account
+const mint = mintKeypair.publicKey;
+// Decimals for Mint Account
+const decimals = 2;
+// Authority that can mint new tokens
+const mintAuthority = pg.wallet.publicKey;
+// Authority that can update the interest rate
+const rateAuthority = pg.wallet.keypair;
+// Interest rate basis points (100 = 1%)
+// Max value = 32,767 (i16)
+const rate = 32_767;
+```
+
+Next, let's determine the size of the new Mint Account and calculate the minimum
+lamports needed for rent exemption.
+
+```javascript
+// Size of Mint Account with extension
+const mintLen = getMintLen([ExtensionType.InterestBearingConfig]);
+// Minimum lamports required for Mint Account
+const lamports = await connection.getMinimumBalanceForRentExemption(mintLen);
+```
+
+With Token Extensions, the size of the Mint Account will vary based on the
+extensions enabled.
+
+## Build Instructions
+
+Next, let's build the set of instructions to:
+
+- Create a new account
+- Initialize the `InterestBearingConfig` extension
+- Initialize the remaining Mint Account data
+
+First, build the instruction to invoke the System Program to create an account
+and assign ownership to the Token Extensions Program.
+
+```javascript
+// Instruction to invoke System Program to create new account
+const createAccountInstruction = SystemProgram.createAccount({
+  fromPubkey: payer.publicKey, // Account that will transfer lamports to created account
+  newAccountPubkey: mint, // Address of the account to create
+  space: mintLen, // Amount of bytes to allocate to the created account
+  lamports, // Amount of lamports transferred to created account
+  programId: TOKEN_2022_PROGRAM_ID, // Program assigned as owner of created account
+});
+```
+
+Next, build the instruction to initialize the `InterestBearingConfig` extension
+for the Mint Account.
+
+```javascript
+// Instruction to initialize the InterestBearingConfig Extension
+const initializeInterestBearingMintInstruction =
+  createInitializeInterestBearingMintInstruction(
+    mint, // Mint Account address
+    rateAuthority.publicKey, // Designated Rate Authority
+    rate, // Interest rate basis points
+    TOKEN_2022_PROGRAM_ID, // Token Extension Program ID
+  );
+```
+
+Lastly, build the instruction to initialize the rest of the Mint Account data.
+This is the same as with the original Token Program.
+
+```javascript
+// Instruction to initialize Mint Account data
+const initializeMintInstruction = createInitializeMintInstruction(
+  mint, // Mint Account Address
+  decimals, // Decimals of Mint
+  mintAuthority, // Designated Mint Authority
+  null, // Optional Freeze Authority
+  TOKEN_2022_PROGRAM_ID, // Token Extension Program ID
+);
+```
+
+## Send Transaction
+
+Next, let's add the instructions to a new transaction and send it to the
+network. This will create a Mint Account with the `InterestBearingConfig`
+extension enabled.
+
+```javascript
+// Add instructions to new transaction
+const transaction = new Transaction().add(
+  createAccountInstruction,
+  initializeInterestBearingMintInstruction,
+  initializeMintInstruction,
+);
+
+// Send transaction
+transactionSignature = await sendAndConfirmTransaction(
+  connection,
+  transaction,
+  [payer, mintKeypair], // Signers
+);
+
+console.log(
+  "\nCreate Mint Account:",
+  `https://solana.fm/tx/${transactionSignature}?cluster=devnet-solana`,
+);
+```
+
+Run the script by clicking the `Run` button. You can then inspect the
+transaction details on SolanaFM.
+
+## Update Interest Rate
+
+The designated Rate Authority can then update the interest rate on the Mint
+Account at any time.
+
+```javascript
+// New interest rate in basis points
+const updateRate = 0;
+// Update interest rate on Mint Account
+transactionSignature = await updateRateInterestBearingMint(
+  connection,
+  payer, // Transaction fee payer
+  mint, // Mint Account Address
+  rateAuthority, // Designated Rate Authority
+  updateRate, // New interest rate
+  undefined, // Additional signers
+  undefined, // Confirmation options
+  TOKEN_2022_PROGRAM_ID, // Token Extension Program ID
+);
+
+console.log(
+  "\nUpdate Rate:",
+  `https://solana.fm/tx/${transactionSignature}?cluster=devnet-solana`,
+);
+```
+
+## Fetch Interest Config State
+
+Next, let's check the updated interest rate by fetching the Mint Account data.
+
+```javascript
+// Fetch Mint Account data
+const mintAccount = await getMint(
+  connection,
+  mint, // Mint Account Address
+  undefined, // Optional commitment
+  TOKEN_2022_PROGRAM_ID, // Token Extension Program ID
+);
+
+// Get Interest Config for Mint Account
+const interestBearingMintConfig = await getInterestBearingMintConfigState(
+  mintAccount, // Mint Account data
+);
+
+console.log(
+  "\nMint Config:",
+  JSON.stringify(interestBearingMintConfig, null, 2),
+);
+```
+
+## Calculate Accrued Interest
+
+Lastly, let's calculate the interest accrued for a given amount. Note that this
+calculation can be performed independently for any amount, without the need for
+minting tokens.
+
+```javascript
+// Wait 1 second
+sleep(1000);
+
+// Amount to convert
+const amount = 100;
+// Convert amount to UI amount with accrued interest
+const uiAmount = await amountToUiAmount(
+  connection, // Connection to the Solana cluster
+  payer, // Account that will transfer lamports for the transaction
+  mint, // Address of the Mint account
+  amount, // Amount to be converted
+  TOKEN_2022_PROGRAM_ID, // Token Extension Program ID
+);
+
+console.log("\nAmount with Accrued Interest:", uiAmount);
+```
+
+Run the script by clicking the `Run` button. You can then inspect the
+transaction details on SolanaFM and view the data logged in the Playground
+terminal.
+
+<Callout type="caution">
+
+Interest is continuously compounded based on the timestamp in the network. Due
+to drift that may occur in the network timestamp, the accumulated interest could
+be lower than the expected value. Thankfully, this is rare.
+
+</Callout>
+
+You should see an output similar to snippet below, where the decimal values
+indicate the interest that has accumulated:
+
+```
+Mint Config: {
+  "rateAuthority": "3z9vL1zjN6qyAFHhHQdWYRTFAcy69pJydkZmSFBKHg1R",
+  "initializationTimestamp": 1702321738,
+  "preUpdateAverageRate": 32767,
+  "lastUpdateTimestamp": 1702321740,
+  "currentRate": 0
+}
+
+Amount with Accrued Interest: 1.000000207670422
+```
+
+## Conclusion
+
+The `InterestBearingConfig` extension introduces a simple mechanism for tokens
+to increase or decrease in value over time. By seamlessly integrating tools
+commonly found in traditional finance, this innovation broadens Solana's
+capabilities, bridging the gap between conventional financial instruments and
+the world of blockchain.