Smart Contract API

Create a Pair

createPair

/// SwapFactory.cdc

pub fun createPair(
    token0Vault: @FungibleToken.Vault,
    token1Vault: @FungibleToken.Vault,
    accountCreationFee: @FungibleToken.Vault,
    stableMode: Bool
): Address

Example transaction create_pair_usdc_fusd.cdc

import FlowToken from 0x1654653399040a61
import FungibleToken from 0xf233dcee88fe0abe
import SwapFactory from 0xb063c16cac85dbd1

// Deploy a SwapPair given token{0|1}'s TokenName and contract address.
//`stableMode` specifies whether the pair uses Uniswap-V2 algorithm (stableMode:false) or Solidly-Stableswap algorithm (stableMode:true).
transaction(Token0Name: String, Token0Addr: Address, Token1Name: String, Token1Addr: Address, stableMode: Bool) {
    prepare(userAccount: AuthAccount) {
        let flowVaultRef = userAccount.borrow<&FlowToken.Vault>(from: /storage/flowTokenVault)!
        assert(flowVaultRef.balance >= 0.002, message: "Insufficient balance to create pair, minimum balance requirement: 0.002 flow")
        let accountCreationFeeVault <- flowVaultRef.withdraw(amount: 0.001)
        
        let token0Vault <- getAccount(Token0Addr).contracts.borrow<&FungibleToken>(name: Token0Name)!.createEmptyVault()
        let token1Vault <- getAccount(Token1Addr).contracts.borrow<&FungibleToken>(name: Token1Name)!.createEmptyVault()
        SwapFactory.createPair(token0Vault: <-token0Vault, token1Vault: <-token1Vault, accountCreationFee: <-accountCreationFeeVault, stableMode: stableMode)
    }
}

Get Pair & LpToken Info

Get Pairs' Addresses

/// SwapFactory.cdc && StableSwapFactory.cdc

pub fun getPairAddress(token0Key: String, token1Key: String): Address?
pub fun getAllPairsLength(): Int
pub fun getAllStableSwapPairsLength(): Int
pub fun getSlicedPairs(from: UInt64, to: UInt64): [Address]

Param

Type

Comments

from

UInt64

Start Index

to

UInt64

End Index

Return Value

[Address]

An array of deployed trading pair addresses

Get number of all deployed trading pairs and an array of deployed trading pair addresses.
Example script get_all_pair_addresses.cdc:

import SwapFactory from 0xb063c16cac85dbd1

pub fun main(): [Address] {
  let len = SwapFactory.getAllPairsLength()
  if (len == 0) {
    return []
  } else {
    return SwapFactory.getSlicedPairs(from: 0, to: UInt64.max)
  }
}

getPairInfo

/// SwapFactory.cdc && StableSwapFactory.cdc

pub fun getPairInfo(token0Key: String, token1Key: String): AnyStruct?
pub fun getSlicedPairInfos(from: UInt64, to: UInt64): [AnyStruct]

Param

Type

Comments

token0Key

String

token0's unique identifier, e.g.: A.3c5959b568896393.FUSD

token1Key

String

token1's unique identifier, e.g.: A.b19436aae4d94622.FiatToken

Return Value

AnyStruct?

nil if the pair doesn't exist, otherwise returns detailed pair info, e.g.:

[
    token0Key,
    token1Key,
    token0Balance,
    token1Balance,
    pairAddress,
    lpTokenBalance,
    swapFeeBps,
    isStableswap,
    stableCurveP
]

/// SwapPair.cdc

pub resource PairPublic: SwapInterfaces.PairPublic {
    pub fun getPairInfo(): [AnyStruct] {}
}

Example script to get PairInfo given the pair address:

import SwapInterfaces from 0xb78ef7afa52ff906                                      
import SwapConfig from 0xb78ef7afa52ff906                                          
                                                                                   
pub fun main(pairAddr: Address): [AnyStruct] {                                                                                                                                                            
  let pairPublicRef = getAccount(pairAddr)                                         
    .getCapability<&{SwapInterfaces.PairPublic}>(SwapConfig.PairPublicPath)        
    .borrow()                                                                      
    ?? panic("cannot borrow reference to PairPublic resource")                     
                                                                                   
  return pairPublicRef.getPairInfo()                                               
}

Example script `get_all_pair_infos.cdc`:

import SwapFactory from 0xb063c16cac85dbd1

pub fun main(): [AnyStruct] {
  let len = SwapFactory.getAllPairsLength()
  if (len == 0) {
    return []
  } else {
    return SwapFactory.getSlicedPairInfos(from: 0, to: UInt64.max)
  }
}

LpTokenCollection

Liquidity Provider (LP) of a trading pair will receive corresponding FT (i.e. LpToken) representing LP's pro rata share of that pair. LP may provide liquidity to different trading pairs and receive multiple different LpTokens. All these LpTokens are grouped within the LpTokenCollection resource, with methods exposing LpToken details:

/// SwapFactory.cdc

pub resource LpTokenCollection: SwapInterfaces.LpTokenCollectionPublic {

    pub fun getAllLPTokens(): [Address] {}
    
    pub fun getLpTokenBalance(pairAddr: Address): UFix64 {}
    
    pub fun deposit(pairAddr: Address, lpTokenVault: @FungibleToken.Vault) {}
    
    pub fun withdraw(pairAddr: Address, amount: UFix64): @FungibleToken.Vault {}
    
}

Example script to check all the LPed pairs and liquidity shares of a given account:

/// lpinfo.cdc

import SwapConfig from 0xb78ef7afa52ff906
import SwapInterfaces from 0xb78ef7afa52ff906
import SwapFactory from 0xb063c16cac85dbd1

pub fun main(userAddr: Address): {Address: AnyStruct} {
    var lpTokenCollectionPublicPath = SwapConfig.LpTokenCollectionPublicPath
    let lpTokenCollectionCap = getAccount(userAddr).getCapability<&{SwapInterfaces.LpTokenCollectionPublic}>(lpTokenCollectionPublicPath)
    if lpTokenCollectionCap.check() == false {
        return {}
    }
    let lpTokenCollectionRef = lpTokenCollectionCap.borrow()!
    let liquidityPairAddrs = lpTokenCollectionRef.getAllLPTokens()
    var res: {Address: AnyStruct} = {}
    for pairAddr in liquidityPairAddrs {
        let pairRef = getAccount(pairAddr).getCapability<&{SwapInterfaces.PairPublic}>(SwapConfig.PairPublicPath).borrow()
            ?? panic("cannot borrow reference to PairPublic")
        let pairInfo = pairRef.getPairInfo()
        let token0Key = pairInfo[0] as! String
        let token1Key = pairInfo[1] as! String
        let token0Balance = pairInfo[2] as! UFix64
        let token1Balance = pairInfo[3] as! UFix64
        let totalLp = pairInfo[5] as! UFix64
        let lpTokenAmount = lpTokenCollectionRef.getLpTokenBalance(pairAddr: pairAddr)
        res[pairAddr] = {
            "token0": token0Key,
            "token1": token1Key,
            "token0Amount": lpTokenAmount / totalLp * token0Balance,
            "token1Amount": lpTokenAmount / totalLp * token1Balance,
            "shareOfPool": lpTokenAmount / totalLp * 100.0
        }
    }
    return res
}

/// $ flow scripts execute lpinfo.cdc <0xUserAccount> --network mainnet

Swap

A straightforward way to contruct a Swap transaction is to use the SwapRouter, which provides a set of useful methods below.

Calculate output / input amount

getAmountsOut

/// SwapRouter.cdc

pub fun getAmountsOut(
    amountIn: UFix64,
    tokenKeyPath: [String]
): [UFix64]

Param

Type

Comments

amountIn

UFix64

Input token amount for the FT tokenKeyPath[0], e.g. 50.0

tokenKeyPath

[String]

An array of FT identifiers denoting the chained-swap path, e.g.: [A.3c5959b568896393.FUSD, A.1654653399040a61.FlowToken, A.b19436aae4d94622.FiatToken] => denoting the swap path of [FUSD -> Flow -> USDC].

tokenKeyPath.length must be >= 2, pools for each consecutive pair of FTs must exist and have liquidity.

Return Vaule

[UFix64]

Calculated maximum output FT amounts following the given swap path, e.g. [50.0, 10.0, 48.0]

Given the input amount of a FT, with an array of FT identifiers denoting the chained-swap path, calculates all subsequent maximum output token amounts.
Useful for calculating output token amounts before calling Perform chained-swap

getAmountsIn

/// SwapRouter.cdc

pub fun getAmountsIn(
    amountOut: UFix64,
    tokenKeyPath: [String]
): [UFix64]

Param

Type

Comments

amountOut

UFix64

Expected output amount of the FT to receive, e.g. 48.0

tokenKeyPath

[String]

tokenKeyPath.length must be >= 2, pools for each consecutive pair of FTs must exist and have liquidity.

Return Value

[UFix64]

Calculated minimum input FT amounts following the given swap path, e.g. [50.0, 10.0, 48.0]

Given the expected output amount of a FT, with an array of FT identifiers denoting the chained-swap path, calculates the minimum input FT amounts required to buy the given amountOut.
Useful for calculating input token amounts before calling Perform chained-swap

Perform chained-swap

swapExactTokensForTokens

/// SwapRouter.cdc

pub fun swapExactTokensForTokens(
    exactVaultIn: @FungibleToken.Vault,
    amountOutMin: UFix64,
    tokenKeyPath: [String],
    deadline: UFix64
): @FungibleToken.Vault

Param

Type

Comments

exactVaultIn

@FT.Vault

Input FT to sell, its full balance will be used.

amountOutMin

UFix64

The minimum amount of output token that must be received, otherwise the tx will revert.

tokenKeyPath

[String]

tokenKeyPath.length must be >= 2, pools for each consecutive pair of FTs must exist and have liquidity.

deadline

UFix64

Unix timestamp after which the tx will revert.

Return Value

@FT.Vault

Output FT resource

To receive as many output FT as possible for swapping the exact amount of input FT, by following the given swap path.
Example transaction swap_exact_flow_to_usdc.cdc:

import FungibleToken from 0xf233dcee88fe0abe
import SwapRouter from 0xa6850776a94e6551

transaction(
    exactAmountIn: UFix64,
    amountOutMin: UFix64,
    path: [String],
    to: Address,
    deadline: UFix64
) {
    prepare(userAccount: AuthAccount) {
        let tokenInVaultPath = /storage/flowTokenVault
        let tokenOutReceiverPath = /public/USDCVaultReceiver

        let inVaultRef = userAccount.borrow<&FungibleToken.Vault>(from: tokenInVaultPath)
            ?? panic("Could not borrow reference to the owner's in FT.Vault")
        /// Note: Receiver (to) should already have out FT.Vault initialized, otherwise tx reverts.
        let outReceiverRef = getAccount(to).getCapability(tokenOutReceiverPath)
            .borrow<&{FungibleToken.Receiver}>()
            ?? panic("Could not borrow receiver reference to the recipient's out FT.Vault")

        let exactVaultIn <- inVaultRef.withdraw(amount: exactAmountIn)
        let vaultOut <- SwapRouter.swapExactTokensForTokens(
            exactVaultIn: <-exactVaultIn,
            amountOutMin: amountOutMin,
            tokenKeyPath: path,
            deadline: deadline
        )
        outReceiverRef.deposit(from: <-vaultOut)
    }
}

swapTokensForExactTokens

/// SwapRouter.cdc

pub fun swapTokensForExactTokens(
    vaultInMax: @FungibleToken.Vault,
    exactAmountOut: UFix64,
    tokenKeyPath: [String],
    deadline: UFix64
): @[FungibleToken.Vault] {

Param

Type

Comments

vaultInMax

@FT.Vault

Input FT to sell, whose balance is the maximum amount can be used before the tx reverts. Any remaining input token will be put in returnValue[1].

exactAmountOut

UFix64

The exact amount of output FT expected to receive.

tokenKeyPath

[String]

tokenKeyPath.length must be >= 2, pools for each consecutive pair of FTs must exist and have liquidity.

deadline

UFix64

Unix timestamp after which the tx will revert.

Return Value

@[FT.Vault]

2-element array of FT resources. * returnValue[0]: output token resource, whose balance will be exactAmountOut. * returnValue[1]: Any remaining input token resource.

To receive the exact amount of output FT for swapping as few input FT as possible, by following the given swap path.
Example transaction swap_flow_to_exact_usdc.cdc:

import FungibleToken from 0xf233dcee88fe0abe
import SwapRouter from 0xa6850776a94e6551

transaction(
    amountInMax: UFix64,
    exactAmountOut: UFix64,
    path: [String],
    to: Address,
    deadline: UFix64
) {
    prepare(userAccount: AuthAccount) {
        let tokenInVaultPath = /storage/flowTokenVault
        let tokenOutReceiverPath = /public/USDCVaultReceiver
        
        let inVaultRef = userAccount.borrow<&FungibleToken.Vault>(from: tokenInVaultPath)
            ?? panic("Could not borrow reference to the owner's in FT.Vault")
        /// Note: Receiver (to) should already have out FT.Vault initialized, otherwise tx reverts.
        let outReceiverRef = getAccount(to).getCapability(tokenOutReceiverPath)
            .borrow<&{FungibleToken.Receiver}>()
            ?? panic("Could not borrow receiver reference to the recipient's out FT.Vault")
            
        let vaultInMax <- inVaultRef.withdraw(amount: amountInMax)
        let swapResVault <- SwapRouter.swapTokensForExactTokens(
            vaultInMax: <-vaultInMax,
            exactAmountOut: exactAmountOut,
            tokenKeyPath: path,
            deadline: deadline
        )
        let vaultOut <- swapResVault.removeFirst()
        let vaultInLeft <- swapResVault.removeLast()
        destroy swapResVault
        
        outReceiverRef.deposit(from: <-vaultOut)
        /// Deposit any remaining input FT back
        inVaultRef.deposit(from: <-vaultInLeft)
    }
}

Note: Using flow-cli tool in the commandline environment, or fcl-js library as long with tx.cdc / script.cdc to interact with smart contracts directly.
*SwapRouter provides an easy way to perform chained swaps among non-stableswap pairs, but it won't work acrossing stableswap pairs: as for any two given tokens, there could be two SwapPair for them, the existing SwapRouter interfaces cannot easily support it. To perform chained swap among non-stableswap pairs and stableswap pairs, one needs to take an address array of pairs and understand SwapPair's raw apis (read below).

Helper functions and Raw apis

Helper functions in SwapConfig

/// SwapConfig.cdc

// For a non-stableswap pair (under the standard constant product formula x * y = k)
// Given pair reserves, swapFeeRateBps(0 ~ 10000), and the exact input amount of an asset, returns the maximum output amount of the other asset.
pub fun getAmountOutVolatile(amountIn: UFix64, reserveIn: UFix64, reserveOut: UFix64, swapFeeRateBps: UInt64): UFix64

// For a non-stableswap pair, given pair reserves, swapFeeRateBps, and the exact output amount of an asset wanted, returns the required (minimum) input amount of the other asset
pub fun getAmountInVolatile(amountOut: UFix64, reserveIn: UFix64, reserveOut: UFix64, swapFeeRateBps: UInt64): UFix64

// For a stableswap pair (under the solidly-stableswap formula: x^3 * y + x * y^3 = k)
// Given pair reserves, swapFeeRateBps, stableswap p-value(usually 1.0), and the exact input amount of an asset, returns the maximum output amount of the other asset
pub fun getAmountOutStable(amountIn: UFix64, reserveIn: UFix64, reserveOut: UFix64, p: UFix64, swapFeeRateBps: UInt64): UFix64

// For a stableswap pair, given pair reserves, swapFeeRateBps, stableswap p-value, and the exact input amount of an asset, returns the required (minimum) input amount of the other asset
pub fun getAmountInStable(amountOut: UFix64, reserveIn: UFix64, reserveOut: UFix64, p: UFix64, swapFeeRateBps: UInt64): UFix64

// Can be used to compute the spot price of a non-stableswap pair.
pub fun quote(amountA: UFix64, reserveA: UFix64, reserveB: UFix64): UFix64

// Can be used to compute the spot price of a stableswap pair.
pub fun quoteStable(amountA: UFix64, reserveA: UFix64, reserveB: UFix64, p: UFix64): UFix64

// Calculates the latest cumulative price of the given SwapPair, using the last cumulative record and current spot price.
// Usually a helper function in building dex-based TWAP oracle.
pub fun getCurrentCumulativePrices(pairAddr: Address): [UInt256; 3]

Raw apis in SwapPair

/// Each SwapPair.cdc

// The default rate is: 30 bps (0.3%) for non-stableswap pairs, and 4 bps (0.04%) for stableswap pairs.
pub fun getSwapFeeBps(): UInt64
pub fun isStableSwap(): Bool
// 1.0 for all non-stableswap pairs and most stableswap pairs. For now the only stableswap pair with a non-1.0 p-value is the `stFlow<>Flow` pair.
pub fun getStableCurveP(): UFix64

pub fun getAmountIn(amountOut: UFix64, tokenOutKey: String): UFix64
pub fun getAmountOut(amountIn: UFix64, tokenInKey: String): UFix64

// DEX-based TWAP oracle related helpful data, see twap oracle related section below for more examples.
pub fun getPrice0CumulativeLastScaled(): UInt256
pub fun getPrice1CumulativeLastScaled(): UInt256
pub fun getBlockTimestampLast(): UFix64

// Raw swap function, one can swap with pair contract directly and build any of the chained routes.
// If `exactAmountOut` is not-nil then a safety check ensures swapped output will not be smaller than the given value.
pub fun swap(vaultIn: @FungibleToken.Vault, exactAmountOut: UFix64?): @FungibleToken.Vault

// Request a flashloan from the current SwapPair, the default interest rate is 5 bps (0.05%). 
// See flashloan related section below for explanations and examples.
pub fun flashloan(executorCap: Capability<&{SwapInterfaces.FlashLoanExecutor}>, requestedTokenVaultType: Type, requestedAmount: UFix64, params: {String: AnyStruct})

Add & Remove Liquidity

AddLiquidity

/// SwapPair.cdc

pub fun addLiquidity(
    tokenAVault: @FungibleToken.Vault,
    tokenBVault: @FungibleToken.Vault
): @FungibleToken.Vault

It's not recommended to use the low-level addLiquidity method directly, unless you're the 1st LP to set the initial price.
Use below example transaction to do slippage check and add liquidity at the ideal ratio:

import FungibleToken from 0xFungibleToken
import SwapFactory from 0xSwapFactory
import StableSwapFactory from 0xStableSwapFactory
import SwapInterfaces from 0xSwapInterfaces
import SwapConfig from 0xSwapConfig
import SwapError from 0xSwapError

transaction(
    token0Key: String,
    token1Key: String,
    token0InDesired: UFix64,
    token1InDesired: UFix64,
    token0InMin: UFix64,
    token1InMin: UFix64,
    deadline: UFix64,
    token0VaultPath: StoragePath,
    token1VaultPath: StoragePath,
    stableMode: Bool
) {
    prepare(userAccount: AuthAccount) {
        assert(deadline >= getCurrentBlock().timestamp, message:
            SwapError.ErrorEncode(
                msg: "AddLiquidity: expired ".concat(deadline.toString()).concat(" < ").concat(getCurrentBlock().timestamp.toString()),
                err: SwapError.ErrorCode.EXPIRED
            )
        )
        let pairAddr = (stableMode)? 
            StableSwapFactory.getPairAddress(token0Key: token0Key, token1Key: token1Key) ?? panic("AddLiquidity: nonexistent stable pair ".concat(token0Key).concat(" <-> ").concat(token1Key).concat(", create stable pair first"))
            :
            SwapFactory.getPairAddress(token0Key: token0Key, token1Key: token1Key) ?? panic("AddLiquidity: nonexistent pair ".concat(token0Key).concat(" <-> ").concat(token1Key).concat(", create pair first"))
        let pairPublicRef = getAccount(pairAddr).getCapability<&{SwapInterfaces.PairPublic}>(SwapConfig.PairPublicPath).borrow()!
        /*
            pairInfo = [
                SwapPair.token0Key,
                SwapPair.token1Key,
                SwapPair.token0Vault.balance,
                SwapPair.token1Vault.balance,
                SwapPair.account.address,
                SwapPair.totalSupply
                ...
            ]
        */
        let pairInfo = pairPublicRef.getPairInfo()
        var token0In = 0.0
        var token1In = 0.0
        var token0Reserve = 0.0
        var token1Reserve = 0.0
        if token0Key == (pairInfo[0] as! String) {
            token0Reserve = (pairInfo[2] as! UFix64)
            token1Reserve = (pairInfo[3] as! UFix64)
        } else {
            token0Reserve = (pairInfo[3] as! UFix64)
            token1Reserve = (pairInfo[2] as! UFix64)
        }
        if token0Reserve == 0.0 && token1Reserve == 0.0 {
            token0In = token0InDesired
            token1In = token1InDesired
        } else {
            var amount1Optimal = SwapConfig.quote(amountA: token0InDesired, reserveA: token0Reserve, reserveB: token1Reserve)
            if (amount1Optimal <= token1InDesired) {
                assert(amount1Optimal >= token1InMin, message:
                    SwapError.ErrorEncode(
                        msg: "SLIPPAGE_OFFSET_TOO_LARGE expect min".concat(token1InMin.toString()).concat(" got ").concat(amount1Optimal.toString()),
                        err: SwapError.ErrorCode.SLIPPAGE_OFFSET_TOO_LARGE
                    )
                )
                token0In = token0InDesired
                token1In = amount1Optimal
            } else {
                var amount0Optimal = SwapConfig.quote(amountA: token1InDesired, reserveA: token1Reserve, reserveB: token0Reserve)
                assert(amount0Optimal <= token0InDesired)
                assert(amount0Optimal >= token0InMin, message:
                    SwapError.ErrorEncode(
                        msg: "SLIPPAGE_OFFSET_TOO_LARGE expect min".concat(token0InMin.toString()).concat(" got ").concat(amount0Optimal.toString()),
                        err: SwapError.ErrorCode.SLIPPAGE_OFFSET_TOO_LARGE
                    )
                )
                token0In = amount0Optimal
                token1In = token1InDesired
            }
        }
        
        let token0Vault <- userAccount.borrow<&FungibleToken.Vault>(from: token0VaultPath)!.withdraw(amount: token0In)
        let token1Vault <- userAccount.borrow<&FungibleToken.Vault>(from: token1VaultPath)!.withdraw(amount: token1In)
        let lpTokenVault <- pairPublicRef.addLiquidity(
            tokenAVault: <- token0Vault,
            tokenBVault: <- token1Vault
        )
        
        let lpTokenCollectionStoragePath = SwapConfig.LpTokenCollectionStoragePath
        let lpTokenCollectionPublicPath = SwapConfig.LpTokenCollectionPublicPath
        var lpTokenCollectionRef = userAccount.borrow<&SwapFactory.LpTokenCollection>(from: lpTokenCollectionStoragePath)
        if lpTokenCollectionRef == nil {
            destroy <- userAccount.load<@AnyResource>(from: lpTokenCollectionStoragePath)
            userAccount.save(<-SwapFactory.createEmptyLpTokenCollection(), to: lpTokenCollectionStoragePath)
            userAccount.link<&{SwapInterfaces.LpTokenCollectionPublic}>(lpTokenCollectionPublicPath, target: lpTokenCollectionStoragePath)
            lpTokenCollectionRef = userAccount.borrow<&SwapFactory.LpTokenCollection>(from: lpTokenCollectionStoragePath)
        }
        lpTokenCollectionRef!.deposit(pairAddr: pairAddr, lpTokenVault: <- lpTokenVault)
    }
}

RemoveLiquidity

/// SwapPair.cdc

pub fun removeLiquidity(
    lpTokenVault: @FungibleToken.Vault
) : @[FungibleToken.Vault] {

Example transaction below to burn lpToken and deposit back removed two-sided liquidities:

import FungibleToken from 0xFungibleToken
import SwapFactory from 0xSwapFactory
import StableSwapFactory from 0xStableSwapFactory
import SwapInterfaces from 0xSwapInterfaces
import SwapConfig from 0xSwapConfig
import SwapError from 0xSwapError

transaction(
    token0Key: String,
    token1Key: String,
    lpTokenAmount: UFix64,
    token0OutMin: UFix64,
    token1OutMin: UFix64,
    deadline: UFix64,
    token0VaultPath: StoragePath,
    token1VaultPath: StoragePath,
    stableMode: Bool
) {
    prepare(userAccount: AuthAccount) {
        assert(deadline >= getCurrentBlock().timestamp, message:
            SwapError.ErrorEncode(
                msg: "RemoveLiquidity: expired ".concat(deadline.toString()).concat(" < ").concat(getCurrentBlock().timestamp.toString()),
                err: SwapError.ErrorCode.EXPIRED
            )
        )
        let pairAddr = (stableMode)? 
            StableSwapFactory.getPairAddress(token0Key: token0Key, token1Key: token1Key) ?? panic("AddLiquidity: nonexistent stable pair ".concat(token0Key).concat(" <-> ").concat(token1Key).concat(", create stable pair first"))
            :
            SwapFactory.getPairAddress(token0Key: token0Key, token1Key: token1Key) ?? panic("AddLiquidity: nonexistent pair ".concat(token0Key).concat(" <-> ").concat(token1Key).concat(", create pair first"))
        
        let lpTokenCollectionRef = userAccount.borrow<&SwapFactory.LpTokenCollection>(from: SwapConfig.LpTokenCollectionStoragePath)
            ?? panic("RemoveLiquidity: cannot borrow reference to LpTokenCollection")

        let lpTokenRemove <- lpTokenCollectionRef.withdraw(pairAddr: pairAddr, amount: lpTokenAmount)
        let tokens <- getAccount(pairAddr).getCapability<&{SwapInterfaces.PairPublic}>(SwapConfig.PairPublicPath).borrow()!.removeLiquidity(lpTokenVault: <-lpTokenRemove)
        let token0Vault <- tokens[0].withdraw(amount: tokens[0].balance)
        let token1Vault <- tokens[1].withdraw(amount: tokens[1].balance)
        destroy tokens

        assert(token0Vault.balance >= token0OutMin && token1Vault.balance >= token1OutMin, message:
            SwapError.ErrorEncode(
                msg: "RemoveLiquidity: INSUFFICIENT_REMOVE_LIQUIDITY_OUT_AMOUNT",
                err: SwapError.ErrorCode.SLIPPAGE_OFFSET_TOO_LARGE
            )
        )

        /// Here does not detect whether the local receiver vault exsit.
        let localVault0Ref = userAccount.borrow<&FungibleToken.Vault>(from: token0VaultPath)!
        let localVault1Ref = userAccount.borrow<&FungibleToken.Vault>(from: token1VaultPath)!
        if token0Vault.isInstance(localVault0Ref.getType()) {
            localVault0Ref.deposit(from: <-token0Vault)
            localVault1Ref.deposit(from: <-token1Vault)
        } else {
            localVault0Ref.deposit(from: <-token1Vault)
            localVault1Ref.deposit(from: <-token0Vault)
        
        }
    }
}

Build a TWAP Oracle

DEX-based TWAP (time-weighted-average-price) oracles can be built using the last cumulative prices recorded in each SwapPair.
However, to correctly use & integrate the twap-oracle into your projects, you must ensure the sampling of the cumulative price data are kept up to date. As long as your oracle is up to date, you can depend on it to produce average prices.

Check below 2 examples for the sampling (update()) and twap-data consuming (twap()).

Fixed-window TWAP oracle example:

import StableSwapFactory from 0xStableSwapFactoryAddr
import SwapFactory from 0xSwapFactoryAddr
import SwapInterfaces from 0xSwapInterfacesAddr
import SwapConfig from 0xSwapConfigAddr

/// Fixed window oracle
///
/// Calculate the average price for the entire period based on the on-chain dex swap pair.
/// note that the price average is only guaranteed to be over at least 1 period, but may be over a longer period
///
pub contract FixedWindowOracleExample {
    /// Window period of the average in seconds
    pub let PERIOD: UInt64
    /// A.contractAddr.contractName: A.11111111.FlowToken, A.2222222.FUSD
    pub let token0Key: String
    pub let token1Key: String
    pub let isStableswap: Bool
    /// pair address in dex
    pub let pairAddr: Address

    /// Average price for each PERIOD, updated once per PERIOD (updating interval could be longer than 1 PERIOD)
    pub var price0Average: UFix64
    pub var price1Average: UFix64

    /// Cumulative price/timestamp for the last update
    pub var price0CumulativeLastScaled: UInt256
    pub var price1CumulativeLastScaled: UInt256
    pub var blockTimestampLast: UFix64


    /// Sampling: update the accumulated price if it exceeds the period.
    pub fun update() {
        let now = getCurrentBlock().timestamp
        let timeElapsed = now - self.blockTimestampLast
        assert(timeElapsed >= UFix64(self.PERIOD), message: "PERIOD_NOT_ELAPSED ".concat(timeElapsed.toString().concat("s")))

        let res = SwapConfig.getCurrentCumulativePrices(pairAddr: self.pairAddr)
        let currentPrice0CumulativeScaled = res[0]
        let currentPrice1CumulativeScaled = res[1]
        let timeElapsedScaled = SwapConfig.UFix64ToScaledUInt256(timeElapsed)
        let price0AverageScaled = SwapConfig.underflowSubtractUInt256(currentPrice0CumulativeScaled, self.price0CumulativeLastScaled) * SwapConfig.scaleFactor / timeElapsedScaled
        let price1AverageScaled = SwapConfig.underflowSubtractUInt256(currentPrice1CumulativeScaled, self.price1CumulativeLastScaled) * SwapConfig.scaleFactor / timeElapsedScaled

        self.price0Average = SwapConfig.ScaledUInt256ToUFix64(price0AverageScaled)
        self.price1Average = SwapConfig.ScaledUInt256ToUFix64(price1AverageScaled)

        self.price0CumulativeLastScaled = currentPrice0CumulativeScaled
        self.price1CumulativeLastScaled = currentPrice1CumulativeScaled
        self.blockTimestampLast = now
    }

    /// Queries twap price data
    /// Returns 0.0 for data n/a or invalid input token
    pub fun twap(tokenKey: String): UFix64 {
        if (tokenKey == self.token0Key) {
            return self.price0Average
        } else if (tokenKey == self.token1Key) {
            return self.price1Average
        } else {
            return 0.0
        }
    }

    /// @Param - token{A|B}Key: e.g. A.f8d6e0586b0a20c7.FUSD
    /// @Param - isStableswap: whether the twap is for stableswap pair or not
    /// @Param - period: average period (in seconds)
    init(tokenAKey: String, tokenBKey: String, isStableswap: Bool, period: UInt64) {
        self.PERIOD = period
        self.isStableswap = isStableswap
        self.pairAddr = isStableswap ?
            StableSwapFactory.getPairAddress(token0Key: tokenAKey, token1Key: tokenBKey) ?? panic("non-existent stableswap-pair") :
            SwapFactory.getPairAddress(token0Key: tokenAKey, token1Key: tokenBKey) ?? panic("non-existent pair")

        let pairPublicRef = getAccount(self.pairAddr).getCapability<&{SwapInterfaces.PairPublic}>(SwapConfig.PairPublicPath).borrow()
            ?? panic("cannot borrow reference to PairPublic")
        let pairInfo = pairPublicRef.getPairInfo()
        self.token0Key = pairInfo[0] as! String
        self.token1Key = pairInfo[1] as! String
        let reserve0 = pairInfo[2] as! UFix64
        let reserve1 = pairInfo[3] as! UFix64
        assert(reserve0 * reserve1 != 0.0, message: "There's no liquidity in the pair")

        self.price0CumulativeLastScaled = pairPublicRef.getPrice0CumulativeLastScaled()
        self.price1CumulativeLastScaled = pairPublicRef.getPrice1CumulativeLastScaled()
        self.blockTimestampLast = pairPublicRef.getBlockTimestampLast()
        self.price0Average = 0.0
        self.price1Average = 0.0
    }
}

Sliding-window TWAP oracle example

import StableSwapFactory from 0xStableSwapFactoryAddr
import SwapFactory from 0xSwapFactoryAddr
import SwapInterfaces from 0xSwapInterfacesAddr
import SwapConfig from 0xSwapConfigAddr

/// Sliding window oracle
///
pub contract SlidingWindowOracleExample {
    /// The amount of time (in seconds) the moving average should be computed, e.g. 24 hours
    pub let windowSize: UInt64
    /// The number of observation data stored for windowSize.
    /// As granularity increases from 2, more frequent updates are needed, but moving averages become more precise.
    /// twap data is computed over intervals with sizes in the range: [windowSize - (windowSize / granularity) * 2, windowSize]
    pub let granularity: UInt64
    /// The amount of time once an update() is needed, periodSize * granularity == windowSize.
    pub let periodSize: UInt64
    /// A.contractAddr.contractName: A.11111111.FlowToken, A.2222222.FUSD
    pub let token0Key: String
    pub let token1Key: String
    pub let isStableswap: Bool
    /// pair address in dex
    pub let pairAddr: Address
    /// An array of price observation data of the pair
    access(self) let pairObservations: [Observation]

    pub struct Observation {
        pub let timestamp: UFix64
        pub let price0CumulativeScaled: UInt256
        pub let price1CumulativeScaled: UInt256

        init(t: UFix64, p0Scaled: UInt256, p1Scaled: UInt256) {
            self.timestamp = t
            self.price0CumulativeScaled = p0Scaled
            self.price1CumulativeScaled = p1Scaled
        }
    }


    /// Returns the index of the observation corresponding to the given timestamp
    pub fun observationIndexOf(timestamp: UFix64): UInt64 {
        return UInt64(timestamp) / self.periodSize % self.granularity
    }

    /// Returns the index of the earliest observation of a windowSize (relative to the given timestamp)
    pub fun firstObservationIndexInWindow(timestamp: UFix64): UInt64 {
        let idx = self.observationIndexOf(timestamp: timestamp)
        return (idx + 1) % self.granularity
    }

    /// Sampling: update the cumulative price for the observation at the current timestamp.
    /// Each observation is updated at most once per periodSize.
    pub fun update() {
        let now = getCurrentBlock().timestamp
        let idx = self.observationIndexOf(timestamp: now)
        let ob = self.pairObservations[idx]
        let timeElapsed = now - ob.timestamp

        if (timeElapsed > UFix64(self.periodSize)) {
            let timeElapsedScaled = SwapConfig.UFix64ToScaledUInt256(timeElapsed)
            let res = SwapConfig.getCurrentCumulativePrices(pairAddr: self.pairAddr)
            let currentPrice0CumulativeScaled = res[0]
            let currentPrice1CumulativeScaled = res[1]
            self.pairObservations[idx] = Observation(t: now, p0Scaled: currentPrice0CumulativeScaled, p1Scaled: currentPrice1CumulativeScaled)
        }
    }

    /// Queries twap price data of the time range [now - [windowSize, windowSize - 2 * periodSize], now]
    /// Returns 0.0 for data n/a or invalid input token
    pub fun twap(tokenKey: String): UFix64 {
        let now = getCurrentBlock().timestamp
        let first_ob_idx = self.firstObservationIndexInWindow(timestamp: now)
        let first_ob = self.pairObservations[first_ob_idx]
        let timeElapsed = now - first_ob.timestamp

        assert(UInt64(timeElapsed) <= self.windowSize, message: "missing historical observations, more update() needed")
        assert(UInt64(timeElapsed) >= self.windowSize - self.periodSize * 2, message: "should never happen")

        let res = SwapConfig.getCurrentCumulativePrices(pairAddr: self.pairAddr)
        let currentPrice0CumulativeScaled = res[0]
        let currentPrice1CumulativeScaled = res[1]
        let timeElapsedScaled = SwapConfig.UFix64ToScaledUInt256(timeElapsed)

        if (tokenKey == self.token0Key) {
            let price0AverageScaled = SwapConfig.underflowSubtractUInt256(currentPrice0CumulativeScaled, first_ob.price0CumulativeScaled) * SwapConfig.scaleFactor / timeElapsedScaled
            return SwapConfig.ScaledUInt256ToUFix64(price0AverageScaled)
        } else if (tokenKey == self.token1Key) {
            let price1AverageScaled = SwapConfig.underflowSubtractUInt256(currentPrice1CumulativeScaled, first_ob.price1CumulativeScaled) * SwapConfig.scaleFactor / timeElapsedScaled
            return SwapConfig.ScaledUInt256ToUFix64(price1AverageScaled)
        } else {
            return 0.0
        }
    }

    /// @Param - token{A|B}Key: e.g. A.f8d6e0586b0a20c7.FUSD
    /// @Param - isStableswap: whether the twap is for stableswap pair or not
    /// @Param - windowSize: The amount of time (in seconds) the moving average should be computed, e.g.: 24 hours (86400)
    /// @Param - granularity: The number of observation data stored for windowSize, e.g.: 24. The more granularity, the more precise the moving average, but with the cost of more frequent updates are needed.
    init(tokenAKey: String, tokenBKey: String, isStableswap: Bool, windowSize: UInt64, granularity: UInt64) {
        pre {
            granularity > 1 && granularity <= windowSize: "invalid granularity"
            windowSize / granularity * granularity == windowSize: "windowSize not-divisible by granularity"
        }
        post {
            UInt64(self.pairObservations.length) == granularity: "pairObservations array not initialized"
        }

        self.windowSize = windowSize
        self.granularity = granularity
        self.periodSize = windowSize / granularity
        self.isStableswap = isStableswap
        self.pairAddr = isStableswap ?
            StableSwapFactory.getPairAddress(token0Key: tokenAKey, token1Key: tokenBKey) ?? panic("non-existent stableswap-pair") :
            SwapFactory.getPairAddress(token0Key: tokenAKey, token1Key: tokenBKey) ?? panic("non-existent pair")

        let pairPublicRef = getAccount(self.pairAddr).getCapability<&{SwapInterfaces.PairPublic}>(SwapConfig.PairPublicPath).borrow()
            ?? panic("cannot borrow reference to PairPublic")
        let pairInfo = pairPublicRef.getPairInfo()
        self.token0Key = pairInfo[0] as! String
        self.token1Key = pairInfo[1] as! String
        let reserve0 = pairInfo[2] as! UFix64
        let reserve1 = pairInfo[3] as! UFix64
        assert(reserve0 * reserve1 != 0.0, message: "There's no liquidity in the pair")

        self.pairObservations = []
        var i: UInt64 = 0
        while (i < granularity) {
            self.pairObservations.append(Observation(t: 0.0, p0Scaled: 0, p1Scaled: 0))
            i = i + 1
        }
    }
}

Flashloan

Flashloan Interfaces

/// interface in SwapInterfaces.cdc
/// Before using the flashloan you need to first implement this interface and plugin customized logic (see example below)

pub resource interface FlashLoanExecutor {
    /// @params: User-definited extra data passed to executor for further auth/check/decode
    pub fun executeAndRepay(loanedToken: @FungibleToken.Vault, params: {String: AnyStruct}): @FungibleToken.Vault
}

Example usage

// Example_FlashloanLiquidation.cdc

import FungibleToken from 0xFungibleTokenAddr
import SwapConfig from 0xSwapConfigAddr
import SwapFactory from 0xSwapFactoryAddr
import SwapInterfaces from 0xSwapInterfacesAddr

pub contract Example_FlashloanLiquidation {
    // Specific address to receive flashloan-liquidation profits, used as auth purposes
    pub let profitReceiver: Address

    /// Implement the flashloan interface
    pub resource FlashloanExecutor: SwapInterfaces.FlashLoanExecutor {
        pub fun executeAndRepay(loanedToken: @FungibleToken.Vault, params: {String: AnyStruct}): @FungibleToken.Vault {
            pre {
                params.containsKey("profitReceiver") && ((params["profitReceiver"]! as! Address) == Example_FlashloanLiquidation.profitReceiver): "not-authorized caller"
            }

            /* 
                Do magic - custom logic goes here. E.g.:
                 - 0. Flashloan request $USDC from FUSD/USDC pool (in `do_flashloan.transaction.cdc`)
                 - 1. Liquidate underwater borrower by repaying borrowed $USDC and grab borrower's collateralized $Flow
                 - 2. Swap $Flow -> $USDC through IncrementSwap (cannot use flashloan-ed pool then) or BloctoSwap
                 - 3. Repay {flashloan-ed $USDC + fees} back to FUSD/USDC pool and keep remaining $USDC as profit
            */

            // TODO: plugin detailed example here
            let amountIn = loanedToken.balance

            // TODO: plugin detailed example here
            /// amountOut = amountIn x (1 + fee%)
            let amountOut = amountIn * (1.0 + UFix64(SwapFactory.getFlashloanRateBps()) / 10000.0) + SwapConfig.ufix64NonZeroMin

            // TODO: modify this
            return <-loanedToken
        }
    }

    init(profitReceiver: Address) {
        self.profitReceiver = profitReceiver

        // Set up FlashLoanExecutor resource
        let pathStr = "swap_flashloan_executor_path"
        let executorPrivatePath = PrivatePath(identifier: pathStr)!
        let executorStoragePath = StoragePath(identifier: pathStr)!
        destroy <-self.account.load<@AnyResource>(from: executorStoragePath)
        self.account.save(<- create FlashloanExecutor(), to: executorStoragePath)
        self.account.link<&{SwapInterfaces.FlashLoanExecutor}>(executorPrivatePath, target: executorStoragePath)
    }
}

/// do_flashloan.transaction.cdc - tx that triggers flashloan from a SwapPair

import FungibleToken from 0xFungibleTokenAddr
import SwapConfig from 0xSwapConfigAddr
import SwapFactory from 0xSwapFactoryAddr
import SwapInterfaces from 0xSwapInterfacesAddr

/*
    E.g.: Flashloan request only $USDC from FUSD/USDC pool
*/
transaction(pairAddr: Address, requestedVaultType: Type, requestedAmount: UFix64) {
    prepare(signer: AuthAccount) {
        let pairRef = getAccount(pairAddr).getCapability<&{SwapInterfaces.PairPublic}>(SwapConfig.PairPublicPath).borrow()
            ?? panic("cannot borrow reference to PairPublic")

        // TODO: add additional args? and generalize this transaction
        let args: {String: AnyStruct} = {
            "profitReceiver": signer.address
        }
        let executorCap = signer.getCapability<&{SwapInterfaces.FlashLoanExecutor}>(/private/swap_flashloan_executor_path)
        pairRef.flashloan(executorCap: executorCap, requestedTokenVaultType: requestedVaultType, requestedAmount: requestedAmount, params: args)
    }
}

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