How do you get the quote for a swap?

[kshitjee]

Asked by sumibisht on Discord (#dev-discussion)

const payload = {
    function: "0xc7efb4076dbe143cbcd98cfaaa929ecfc8f299203dfff63b95ccb6bfe19850fa::router::get_amount_in",
    typeArguments: [parseTypeTag(APTOS_COIN), parseTypeTag(DEVNET_BNB)],
    functionArguments: [new U64(APTOS_COIN_AMOUNT)],
  };
const amount_in = (await aptos.view({ payload }));
console.log(amount_in);

But it says it's not a view function, if it's not a view function then how do people get the quote for swap, and its a public function still I can't call it?

[kshitjee]

It isn't a view function, view functions have the #[view] attribute above them.

Also, you cannot see the return values of functions that are not view. So even if you pay gas and calculate spot price on-chain (swap_utils module), there is no way to access the return values via SDK/API/Indexer based on the design of the given Pancakeswap modules. This is because the information is not stored in a resource, nor is it emitted through an event.

There are multiple ways you can get a quote for a swap, here are just some examples. This answer uses the Pancakeswap modules on testnet as reference.

1. Since you're using Pancakeswap, you can check out their aptos-swap-sdk. This will likely have a method to get a quote.

2. Calculate spot price by using AMM math based on token reserves, you can do this yourself off-chain. This can be more efficient and is often used by DEX aggregators for larger scale applications. You can get the reserves by reading the fields of the TokenPairReserve resource on the swap module (or you can keep track of this information off-chain for speed). This can be done via API, SDK, or indexer.

API endpoint format is below:
http://<API_ENDPOINT>/v1/accounts/<ACCOUNT_ADDRESS>/resource/<RESOURCE_TYPE>

The logic for the math can be found here. Look at get_amount_in and get_amount_out functions. Here's a more detailed explanation of the math.

3. Use the TypeScript SDK to simulate the swap.

Here's a code snippet that demonstrates swap simulation:

// create a new instance of the Aptos SDK
  const config = new AptosConfig({ network: Network.TESTNET }); 
  const aptos = new Aptos(config);

  // generate and fund new account
  const alice: Account = Account.generate();
  await aptos.fundAccount({
    accountAddress: alice.accountAddress,
    amount: 200000000,
  });

    // build the transaction
  const transaction = await aptos.transaction.build.simple({
    sender: alice.accountAddress,
    data: {
      function:
        "0xc7efb4076dbe143cbcd98cfaaa929ecfc8f299203dfff63b95ccb6bfe19850fa::router::swap_exact_input",
      typeArguments: ["0x1::aptos_coin::AptosCoin", "0x16fe2df00ea7dde4a63409201f7f4e536bde7bb7335526a35d05111e68aa322c::TestCoinsV1::USDT"],
      functionArguments: [10000000, 0],
    },
  });

  // simulate the transaction
  const [userTransactionResponse] = await aptos.transaction.simulate.simple({
    signerPublicKey: alice.publicKey,
    transaction,
  });
  console.log(userTransactionResponse.events);

[degenSumi]

Thanks for answering will check it out,
One question though on swap_utils module I see amout_out function however I still don't see any #view attribute?

[kshitjee]

I edited the answer, let me know if it makes sense!

[degenSumi]

Thanks @kshitjee it helped.

[degenSumi]

Hey @kshitjee just a quick question, now how can i send this transaction

const txn = await aptos.transaction.build.simple({
sender: "0xbe5cd08d8fe346b9b55dba64fc09e50dda92a8bd16743652ec8b156821123310",
data: {
function: "0xc7efb4076dbe143cbcd98cfaaa929ecfc8f299203dfff63b95ccb6bfe19850fa::router::swap_exact_input",
typeArguments: [parseTypeTag(APTOS_COIN), parseTypeTag(DEVNET_BNB)],
functionArguments: [new U64(APTOS_COIN_AMOUNT), new U64(5)],
},
});
console.log(txn);

// Now i want to encode this tx and send it over the internet so that anyone else can sign it

const signedTx = aptos.transaction.sign({
signer:signer,
transaction: txn
});

// Now i want to send this signed tx so that any one else can send this
console.log(signedTx.deserialize(), txn);

please let me know as i can't find any functions for it on the sdk