Ion Takehome

• Because takehomes are not paid for hours worked, this task is designed to help us gauge the participants’ skills and understanding, rather than to produce work that will be used by the team.
• To respect the participant’s work, we will not be taking this work or modifying it for the team’s interest in any way.

Problem

Here's the scenario:

1. I want to purchase 1 WBTC by paying the necessary amount of WETH.
2. But I do not yet have any WETH that I can use to pay for this swap.
3. Fortunately, I have a friend who will transfer me the necessary WETH amount, if I give them just 0.5 of WBTC.
4. This is a great deal, since I'm not paying for anything and I get half of WBTC.
5. One issue is that I don't have access to that 0.5 WBTC, before I make the swap itself.
   • So we need a way to access the token that I'm buying first, send a part of that token to the friend, receive the WETH amount necessary to purchase the WBTC amount, then pay the Uniswap pool.
6. Callbacks in Uniswap's swap function helps us achieve this.
   • Concretely,
     1. I can receive the 1 WBTC amount that I'm buying before this callback function is called.
     2. In the callback, I can transfer 0.5 WBTC to my friend, then receive some amount of WETH from my friend.
     3. Then at the end of the callback, I can use this WETH that I received from my friend to pay for the cost of the swap. (If I don't pay the pool, then Uniswap will revert the swap);
     4. We call this a flashswap since we gain access to the output asset first, take control flow, and have the pool guarantee that the call succeeds if and only if the input asset is paid for.
7. Now here's the real problem: How can I implement a multi-hop flashswap?
   • A single-hop flashswap between WETH (sell) <> WBTC (buy) means I get access to WBTC, insert a callback in the middle, then pays the WETH back to the pool.
   • A multi-hop flashswap between WETH (sell) <> DAI <> USDC <> WBTC (buy) means I get access to WBTC, do a callback, then pay WETH.
   • In both cases, I am buying WBTC and paying WETH and taking control flow in between. But importantly, the second bullet point conducts the swap across multiple pools.
   • Your job is to implement this second bullet point, a multi-hop flashswap that allows you to access to the output token (WBTC) before paying the input token (WETH), while routing the swap across multiple pools

Task

• There are three pools.
  1. WBTC <> USDC
  2. USDC <> DAI
  3. DAI <> WETH
• TODO
  • See all the TODO items in Flashswap.sol and Flashswap.t.sol
  • Implement the exactOutput and the uniswapV3SwapCallback functions for the multi-hop flashswap.
  • Implement the Caller.flashSwapCallback function.
  • Implement the _path in test_ExactOutput_ThreePools.
  • Run the test_ExactOutput_ThreePools test and make it pass!
• test_ExactOutput_ThreePool
  • This test is meant to validate the following process:
    1. Flashswap WETH (sell) <> WBTC (buy) using the three pools in the path.
    2. After you receive WBTC, trade with the Friend contract in the callback. (The Friend will take 0.5 WBTC and give you any requested amount of WETH).
    3. Use the WETH from the friend to pay the swap.
    4. Congratulations! You just finished a multi-hop flashswap.
  • NOTE You must set up the .env file for the fork test. See .env.example.
• It is strongly recommended that you review the FlashSwap.t.sol and Flashswap.sol for the TODO items as you begin.

Resources

1. Multi-hop Swap

• SwapRouter.sol contract implements a multi-hop swap that takes in the path that encodes a list of pools that the swap should go through.
• Your solution is also a multi-hop swap and may look very similar to this contract.

2. Path

• Uniswap swap paths can be encoded in bytes and decoded using the Path.sol library.
• Example of using the path is also in SwapRouter.sol.

3. You may need to review other resources related to UniV3 if not yet familiar.

Note

1. Recommendations

• Start by diagramming out the flow for callback control flow to ensure understanding of the requirements.

2. NOTE

• If the task is challenging, please do not hestitate to reach out with any questions if there are blockers.
  • We would much rather have open discussions around the problem to work it out together as part of the takehome process than to have the participant be discouraged and not complete the task. We are looking for a glimpse into your thought process as you tacke non-trivial problems.
• You are also encouraged to reference any resources or tools online if it helps your process.

Usage

Build

$ forge build

Test

$ forge test

Format

$ forge fmt

Gas Snapshots

$ forge snapshot

Anvil

$ anvil

Deploy

$ forge script script/Counter.s.sol:CounterScript --rpc-url <your_rpc_url> --private-key <your_private_key>

Cast

$ cast <subcommand>

Help

$ forge --help
$ anvil --help
$ cast --help