feat: Add local rent exemption calculator (#1065)

* Add local rent exemption calculator

Closes #777

* Tweak docstring

---------

Co-authored-by: Callum <callum.mcintyre@anza.xyz>

Author: rajgoesout

.changeset/pink-dragons-happen.md

@@ -0,0 +1,5 @@
+---
+'@solana/kit': minor
+---
+
+Add local rent exemption calculator

packages/kit/src/__tests__/get-minimum-balance-for-rent-exemption-test.ts

@@ -0,0 +1,15 @@
+import type { Lamports } from '@solana/rpc-types';
+
+import { getMinimumBalanceForRentExemption } from '../get-minimum-balance-for-rent-exemption';
+
+describe('getMinimumBalanceForRentExemption', () => {
+    it.each`
+        space     | lamports
+        ${0n}     | ${890_880n}
+        ${100n}   | ${890_880n + 100n * 3_480n * 2n}
+        ${1_024n} | ${890_880n + 1_024n * 3_480n * 2n}
+    `('calculates the correct rent for an account with $space bytes of space allocated', ({ space, lamports }) => {
+        expect.assertions(1);
+        expect(getMinimumBalanceForRentExemption(space)).toBe(lamports as unknown as Lamports);
+    });
+});

packages/kit/src/get-minimum-balance-for-rent-exemption.ts

@@ -0,0 +1,26 @@
+import type { Lamports } from '@solana/rpc-types';
+
+/**
+ * Calculates the minimum {@link Lamports | lamports} required to make an account rent exempt for a
+ * given data size, without performing an RPC call.
+ *
+ * Values are sourced from the on-chain rent parameters in the Solana runtime:
+ * https://github.com/anza-xyz/solana-sdk/blob/c07f692e41d757057c8700211a9300cdcd6d33b1/rent/src/lib.rs#L93-L97
+ *
+ * Note that this logic may change, or be incorrect depending on the cluster you are connected to.
+ * You can always use the RPC method `getMinimumBalanceForRentExemption` to get the current value.
+ *
+ * @param space The number of bytes of account data.
+ */
+export function getMinimumBalanceForRentExemption(space: bigint): Lamports {
+    const RENT = {
+        ACCOUNT_STORAGE_OVERHEAD: 128n,
+        DEFAULT_EXEMPTION_THRESHOLD: 2n,
+        DEFAULT_LAMPORTS_PER_BYTE_YEAR: 3_480n,
+    } as const;
+    const requiredLamports =
+        (RENT.ACCOUNT_STORAGE_OVERHEAD + space) *
+        RENT.DEFAULT_LAMPORTS_PER_BYTE_YEAR *
+        RENT.DEFAULT_EXEMPTION_THRESHOLD;
+    return requiredLamports as Lamports;
+}

packages/kit/src/index.ts

@@ -27,6 +27,7 @@ export * from '@solana/transactions';
 export * from './airdrop';
 export * from './decompile-transaction-message-fetching-lookup-tables';
 export * from './fetch-lookup-tables';
+export * from './get-minimum-balance-for-rent-exemption';
 export * from './send-and-confirm-durable-nonce-transaction';
 export * from './send-and-confirm-transaction';
 export * from './send-transaction-without-confirming';