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Off-chain external call preparation for Solana

Instructions should be serialized one by one, final calldata is a concatenation of separately serialized instructions.
For the info about the rewards, substitutions, and expenses check the previous section.
Solana's TransactionInstructions could be serialized into calldata format using @debridge-finance/debridge-external-call npm package:
import * as wasm from "@debridge-finance/debridge-external-call";
import { PublicKey, TransactionInstruction } from "@solana/web3.js";
/**
* Substitutes amount at offset with `walletBalance(accounts[account_index]) - subtraction`
*/
type AmountSubstitution = {
/**
* big or little endian
*/
is_big_endian: boolean;
/**
* At what offset substitution should be done
*/
offset: number;
/**
* index of account in TransactionInstruction.keys to get balance for
*/
account_index: number;
/**
* Amount to deduct from wallet balance
*/
subtraction: number;
};
/**
* Since we don't know submissionAuth at the moment of calldata preparation we can prepare substitution to replace
* account at `index` with actual ATA(submissionAuth, tokenMint) during execution
*/
type WalletSubstitution = {
/**
* Token mint to calculate ATA for
*/
token_mint: string;
/**
* Account at this index will be replaced with ATA(submissionAuth, tokenMint) during execution
*/
index: number;
};
/**
* Structure required by wasm module
*/
interface IExtIx {
keys: {
pubkey: string;
isSigner: boolean;
isWritable: boolean;
}[];
data: Buffer;
programId: string;
}
function ixToIExtIx(ix: TransactionInstruction): IExtIx {
return {
keys: ix.keys.map((meta) => ({
pubkey: meta.pubkey.toBase58(),
isSigner: meta.isSigner,
isWritable: meta.isWritable,
})),
programId: ix.programId.toBase58(),
data: ix.data,
};
}
function serialize(
instruction: TransactionInstruction,
substitutions?: {
amountSubstitutions?: AmountSubstitution[];
walletSubstitutions?: WalletSubstitution[];
},
expense?: bigint,
reward?: bigint,
isInMandatoryBlock: boolean = false,
) {
const ixWrapper = new wasm.ExternalInstructionWrapper(
reward,
expense,
isInMandatoryBlock,
substitutions?.amountSubstitutions ?? [],
substitutions?.walletSubstitutions ?? [],
ixToIExtIx(instruction),
);
return ixWrapper.serialize();
}
const ix1: TransactionInstruction;
const ix2: TransactionInstruction;
const serializedIx1 = serialize(ix1, undefined, 1000n);
const serializedIx2 = serialize(ix2, undefined, 2000n);
const calldata = Buffer.concat([serializedIx1, serializedIx2 /** rest serialized instructions if any */]);