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Routing a Swap

Introduction​

This guide will cover how to use Uniswap's smart order router to compute optimal routes and execute swaps. Rather than trading between a single pool, smart routing may use multiple hops (as many as needed) to ensure that the end result of the swap is the optimal price. It is based on the routing code example, found in the Uniswap code examples repository. To run this example, check out the guide's README and follow the setup instructions.

info

If you need a briefer on the SDK and to learn more about how these guides connect to the examples repository, please visit our background page!

In this example we will trade between WETH and USDC, but you can configure your example to us any two currencies and amount of input currency.

The guide will cover:

  1. Creating a router instance
  2. Creating a route
  3. Swapping using a route

At the end of the guide, we should be able to create a route and and execute a swap between any two currencies tokens using the example's included UI.

For this guide, the following Uniswap packages are used:

The core code of this guide can be found in routing.ts

The config, which we will use in some code snippets in this guides has this structure:

import { Token } from '@uniswap/sdk-core'

interface ExampleConfig {
  env: Environment
  rpc: {
    local: string
    mainnet: string
  }
  wallet: {
    address: string
    privateKey: string
  }
  tokens: {
    in: Token
    amountIn: number
    out: Token
  }
}

export const CurrentConfig: ExampleConfig = {...}

Creating a router instance​

To compute our route, we will use the @uniswap/smart-order-router package, specifically the AlphaRouter class which requires a chainId and a provider. Note that routing is not supported for local forks, so we will use a mainnet provider even when swapping on a local fork:

import { AlphaRouter, ChainId } from '@uniswap/smart-order-router'

const provider = new ethers.providers.JsonRpcProvider(rpcUrl)

const router = new AlphaRouter({
  chainId: ChainId.MAINNET,
  provider,
})

Creating a route​

We will use the SwapRouter02 for our trade. The smart-order-router package provides us with a SwapOptionsSwapRouter02` interface, defining the wallet to use, slippage tolerance, and deadline for the transaction that we need to interact with the contract:

import { SwapOptionsSwapRouter02, SwapType } from '@uniswap/smart-order-router'
import { Percent } from '@uniswap/sdk-core'

const options: SwapOptionsSwapRouter02 = {
  recipient: CurrentConfig.wallet.address,
  slippageTolerance: new Percent(50, 10_000),
  deadline: Math.floor(Date.now() / 1000 + 1800),
  type: SwapType.SWAP_ROUTER_02,
}

Like explained in the previous guide, it is important to set the parameters to sensible values.

Using these options, we can now create a trade (TradeType.EXACT_INPUT or TradeType.EXACT_OUTPUT) with the currency and the input amount to use to get a quote. For this example, we'll use an EXACT_INPUT trade to get a quote outputted in the quote currency.

import { CurrencyAmount, TradeType } from '@uniswap/sdk-core'

const rawTokenAmountIn: JSBI = fromReadableAmount(
      CurrentConfig.currencies.amountIn,
      CurrentConfig.currencies.in.decimals
    )

const route = await router.route(
  CurrencyAmount.fromRawAmount(
    CurrentConfig.currencies.in,
    rawTokenAmountIn
  ),
  CurrentConfig.currencies.out,
  TradeType.EXACT_INPUT,
  options
)

The fromReadableAmount function calculates the amount of tokens in the Token's smallest unit from the full unit and the Token's decimals:

src/libs/conversion.ts
export function fromReadableAmount(amount: number, decimals: number): JSBI {
  const extraDigits = Math.pow(10, countDecimals(amount))
  const adjustedAmount = amount * extraDigits
  return JSBI.divide(
    JSBI.multiply(
      JSBI.BigInt(adjustedAmount),
      JSBI.exponentiate(JSBI.BigInt(10), JSBI.BigInt(decimals))
    ),
    JSBI.BigInt(extraDigits)
  )
}

route and route.methodParameters are optional as the request can fail, for example if no route exists between the two Tokens or because of networking issues. We check if the call was succesful:

if (!route || !route.methodParameters) {
    // Handle failed request
}

Depending on our preferences and reason for the issue we could retry the request or throw an Error.

Swapping using a route​

First, we need to give approval to the SwapRouter smart contract to spend our tokens for us:

import { ethers } from 'ethers'
...

const wallet = new ethers.Wallet(privateKey, provider)
const tokenContract = new ethers.Contract(
    CurrentConfig.tokens.in.address, 
    ERC20ABI, 
    wallet
)
const tokenApproval = await tokenContract.approve(
    V3_SWAP_ROUTER_ADDRESS, 
    ethers.BigNumber.from(rawTokenAmountIn.toString())
)

To be able to spend the tokens of a wallet, a smart contract first needs to get an approval from that wallet. ERC20 tokens have an approve function that accepts the address of the smart contract that we want to allow spending our tokens and the amount the smart contract should be allowed to spend.

We can get the V3_SWAP_ROUTER_ADDRESS for our chain from Github. Keep in mind that different chains might have different deployment addresses for the same contracts. The deployment address for local forks of a network are the same as in the network you forked, so for a fork of mainnet it would be the address for Mainnet.

We need to wait one block for the approval transaction to be included by the blockchain.

Once the approval has been granted, we can now execute the trade using the route's computed calldata, values, and gas values:

const txRes = await wallet.sendTransaction({
  data: route.methodParameters.calldata,
  to: V3_SWAP_ROUTER_ADDRESS,
  value: route.methodParameters.value,
  from: wallet.address,
  maxFeePerGas: MAX_FEE_PER_GAS,
  maxPriorityFeePerGas: MAX_PRIORITY_FEE_PER_GAS,
})

After swapping, you should see the currency balances update in the UI shortly after the block is confirmed.

You can find the full code in routing.ts.

Next Steps​

Now that you're familiar with trading, consider checking out our next guides on pooling liquidity to Uniswap!

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