Mint

If you are starting the tutorial from here, please check out this branch and open it in your IDE.

1. Add Mint Functions to Pair Trait

At this stage, we will implement the mint function of the Pair contract.
In the ./logics/traits/pair.rs file add the mint function to the Pair trait. You should also add two internal _mint_fee and _update functions. As those functions modify the state, they should take a &mut self as first argument. When sending transaction (as tx) it return nothing (a tx cannot return a value neither a variant of the Error enum) so in most cases state changes function will return Result<(), PairError>. But if you call the function as dry-run (as a query, it will not modify the state) it can return a value (any value and Error enum as well). That is why the mint message function returns a Balance (and not ()). So before calling mint as tx you can call it as dry-run and gets the liquidity that will be minted. Also add the function to emit mint event that will have to be implemented in the contract:

pub trait Pair {
    ...
    #[ink(message)]
    fn mint(&mut self, to: AccountId) -> Result<Balance, PairError>;

    fn _mint_fee(&mut self, reserve_0: Balance, reserve_1: Balance) -> Result<bool, PairError>;
    
    fn _update(
        &mut self,
        balance_0: Balance,
        balance_1: Balance,
        reserve_0: Balance,
        reserve_1: Balance,
    ) -> Result<(), PairError>;
    
    fn _emit_mint_event(&self, _sender: AccountId, _amount_0: Balance, _amount_1: Balance);

    fn _emit_sync_event(&self, reserve_0: Balance, reserve_1: Balance);
}

2. Mint Fee and Factory Trait

As _update and _mint_fee are child functions of mint, let's start by implementing those. Have a look at _mintFee in Solidity, which takes uint112 _reserve0 and uint112 _reserve1 as arguments, and translates to Balance in ink! that returns a bool, and can make state changes (it can save k_last to storage) so in ink! it should return Result<bool, PairError>.
Let's add it to ./logics/impls/pair/pair.rs:

impl<T: Storage<data::Data> + Storage<psp22::Data>> Pair for T {
    ...
    fn _mint_fee(
        &mut self,
        reserve_0: Balance,
        reserve_1: Balance,
    ) -> Result<bool, PairError> {}
}

In the first line of _mintFee there is a cross-contract call to the Factory contract to obtain the address of the account collecting the fees. To do so we will use Openbrush wrapper around a Factory trait (and demonstrate that the trait only is needed - no implementation). create a file ./logics/traits/factory.rs and add the Factory trait and a fee_to function getter.
Add #[openbrush::trait_definition] to the top of the file:

#[openbrush::trait_definition]
pub trait Factory {
    #[ink(message)]
    fn fee_to(&self) -> AccountId;
}

And then add a wrapper around this trait. Imports what needs to be imported:

use openbrush::traits::AccountId;

#[openbrush::wrapper]
pub type FactoryRef = dyn Factory;
...

Add this file to ./logics/traits/mod.rs:

pub mod pair;
pub mod factory;

In ./logics/impls/pair/pair.rs import this contract FactoryRef:

use crate::traits::factory::FactoryRef;

And in the body of _mint_fee we will obtain the fee_to with a cross-contract call to Factory. When using OpenBrush wrapper around a trait, the first argument of the function should be the contract address you call. So add this line as it is shown below:

    fn _mint_fee(
        &mut self,
        reserve_0: Balance,
        reserve_1: Balance,
    ) -> Result<bool, PairError> {
    let fee_to = FactoryRef::fee_to(&self.data::<data::Data>().factory);
}

The rest of the function body may be somewhat difficult to interpret, so here are a few tips:

For address(0) in Solidity you can use openbrush::traits::ZERO_ADDRESS (which is a const [0; 32]).
For sqrt you can either implement the same function or use integer-sqrt.
When doing Math operations you should handle overflow cases (and return an Error if there is an overflow). You can perform check operations on u128
Use each Error variant only once, so when testing or debugging you will know immediately which line the Error come from.

Then implement line-by-line the same logic as in Uniswap-V2:

    fn _mint_fee(
    &mut self,
    reserve_0: Balance,
    reserve_1: Balance,
) -> Result<bool, PairError> {
    let fee_to = FactoryRef::fee_to(&self.data::<data::Data>().factory);
    let fee_on = fee_to != ZERO_ADDRESS.into();
    let k_last = self.data::<data::Data>().k_last;
    if fee_on {
        if k_last != 0 {
            let root_k = sqrt(
                reserve_0
                    .checked_mul(reserve_1)
                    .ok_or(PairError::MulOverFlow14)?,
            );
            let root_k_last = sqrt(k_last);
            if root_k > root_k_last {
                let total_supply = self.data::<psp22::Data>().supply;
                let numerator = total_supply
                    .checked_mul(
                        root_k
                            .checked_sub(root_k_last)
                            .ok_or(PairError::SubUnderFlow14)?,
                    )
                    .ok_or(PairError::MulOverFlow15)?;
                let denominator = root_k
                    .checked_mul(5)
                    .ok_or(PairError::MulOverFlow15)?
                    .checked_add(root_k_last)
                    .ok_or(PairError::AddOverflow1)?;
                let liquidity = numerator
                    .checked_div(denominator)
                    .ok_or(PairError::DivByZero5)?;
                if liquidity > 0 {
                    self._mint_to(fee_to, liquidity)?;
                }
            }
        }
    } else if k_last != 0 {
        self.data::<data::Data>().k_last = 0;
    }
    Ok(fee_on)
}

3. Update

The update function will update the oracle price of the tokens with time-weighted average prices (TWAPs). Please check the Uniswap V2 implementation.
To implement this in ink!:

ink! contracts should never panic!. The reason being that a panic! will give the user no information about the Error (it only returns CalleeTrapped). Every potential business/logical error should be returned in a predictable way using Result<T, Error>.
To handle time use Self::env().block_timestamp() that is the time in milliseconds since the Unix epoch.
In Solidity, float point division is not supported, it uses Q number UQ112x112 for more precision. We will use div for our example (note that is the DEX template we use U256 for more precision).
To store values in storage (but first verify, then save), modify the value of the Storage field (as the function takes &mut self it can modify Storage struct fields)

You can then implement update:

    fn _update(
    &mut self,
    balance_0: Balance,
    balance_1: Balance,
    reserve_0: Balance,
    reserve_1: Balance,
) -> Result<(), PairError> {
    if balance_0 == u128::MAX || balance_1 == u128::MAX {
        return Err(PairError::Overflow)
    }
    let now = Self::env().block_timestamp();
    let time_elapsed = now - self.data::<data::Data>().block_timestamp_last;
    if time_elapsed > 0 && reserve_0 != 0 && reserve_1 != 0 {
        let price_cumulative_last_0 = (reserve_1 / reserve_0)
            .checked_mul(time_elapsed as u128)
            .ok_or(PairError::MulOverFlow4)?;
        let price_cumulative_last_1 = (reserve_0 / reserve_1)
            .checked_mul(time_elapsed as u128)
            .ok_or(PairError::MulOverFlow4)?;
        self.data::<data::Data>().price_0_cumulative_last += price_cumulative_last_0;
        self.data::<data::Data>().price_1_cumulative_last += price_cumulative_last_1;
    }
    self.data::<data::Data>().reserve_0 = balance_0;
    self.data::<data::Data>().reserve_1 = balance_1;
    self.data::<data::Data>().block_timestamp_last = now;

    self._emit_sync_event(reserve_0, reserve_1);
    Ok(())
}

4. Mint

Now that all child functions have been added, we can add mint. First, add the function definition in the impl block of ./logics/impls/pair/pair.rs :

fn mint(&mut self, to: AccountId) -> Result<Balance, PairError> {}

On line 112 of Pair.sol there is a cross-contract call to the ERC20 to obtain the balance of the contract uint balance0 = IERC20(token0).balanceOf(address(this));.
To implement this cross-contract call we will use PSP22Ref from Openbrush. To obtain the address of the contract, use Self::env().account_id(). Read more about how to find all the ink_env getters in this doc.

First, add the psp22::Data Trait bound to the generic impl block:

impl<T: Storage<data::Data> + Storage<psp22::Data>> Pair for T { 
    ...
}

In the body of mint:

use openbrush::contracts::traits::psp22::PSP22Ref;
...
fn mint(&mut self, to: AccountId) -> Result<Balance, PairError> {
    let reserves = self.get_reserves();
    let contract = Self::env().account_id();
    let balance_0 = PSP22Ref::balance_of(&self.data::<data::Data>().token_0, contract);
    let balance_1 = PSP22Ref::balance_of(&self.data::<data::Data>().token_1, contract);
    ...
}

Now, as the call to PSP22Ref returns Result<Balance, PSP22Error> we should implement the From trait for our PairError (to not have to map_err for every calls). We will do so in the file .logics/traits/pair.rs where we defined PairError. Add a field that takes an PSP22Error, and implement the From Trait for it (also add all the error fields used in the implementation):

use openbrush::contracts::psp22::PSP22Error;
...
#[derive(Debug, PartialEq, Eq, scale::Encode, scale::Decode)]
#[cfg_attr(feature = "std", derive(scale_info::TypeInfo))]
pub enum PairError {
    PSP22Error(PSP22Error),
    InsufficientLiquidityMinted,
    Overflow,
    SubUnderFlow1,
    SubUnderFlow2,
    SubUnderFlow3,
    SubUnderFlow14,
    MulOverFlow1,
    MulOverFlow2,
    MulOverFlow3,
    MulOverFlow4,
    MulOverFlow5,
    MulOverFlow14,
    MulOverFlow15,
    DivByZero1,
    DivByZero2,
    DivByZero5,
    AddOverflow1,
}

impl From<PSP22Error> for PairError {
    fn from(error: PSP22Error) -> Self {
        PairError::PSP22Error(error)
    }
}

For the MINIMUM_LIQUIDTY constant, please add:

pub const MINIMUM_LIQUIDITY: u128 = 1000;

For the min function, add the same implementation below the impl block:

    default fn _emit_mint_event(&self, _sender: AccountId, _amount_0: Balance, _amount_1: Balance) {}
    
    default fn _emit_sync_event(&self, _reserve_0: Balance, _reserve_1: Balance) {}
}

fn min(x: u128, y: u128) -> u128 {
    if x < y {
        return x
    }
    y
}

For sqrt function add the same implementation below the min function:

fn sqrt(y: u128) -> u128 {
    let mut z = 1;
    if y > 3 {
        z = y;
        let mut x = y / 2 + 1;
        while x < z {
            z = x;
            x = (y / x + x) / 2;
        }
    }
    z
}

If you handle all overflows (that requires most of the lines of the function body):

First obtain the values for reserves, balances and liquidity.
mint_fee
Mint liquidity to to
Update reserves.
Emit an event.

    fn mint(&mut self, to: AccountId) -> Result<Balance, PairError> {
        let reserves = self.get_reserves();
        let contract = Self::env().account_id();
        let balance_0 = PSP22Ref::balance_of(&self.data::<data::Data>().token_0, contract);
        let balance_1 = PSP22Ref::balance_of(&self.data::<data::Data>().token_1, contract);
        let amount_0 = balance_0
            .checked_sub(reserves.0)
            .ok_or(PairError::SubUnderFlow1)?;
        let amount_1 = balance_1
            .checked_sub(reserves.1)
            .ok_or(PairError::SubUnderFlow2)?;

        let fee_on = self._mint_fee(reserves.0, reserves.1)?;
        let total_supply = self.data::<psp22::Data>().supply;

        let liquidity;
        if total_supply == 0 {
            let liq = amount_0
                .checked_mul(amount_1)
                .ok_or(PairError::MulOverFlow1)?;
            liquidity = sqrt(liq)
                .checked_sub(MINIMUM_LIQUIDITY)
                .ok_or(PairError::SubUnderFlow3)?;
            self._mint_to(ZERO_ADDRESS.into(), MINIMUM_LIQUIDITY)?;
        } else {
            let liquidity_1 = amount_0
                .checked_mul(total_supply)
                .ok_or(PairError::MulOverFlow2)?
                .checked_div(reserves.0)
                .ok_or(PairError::DivByZero1)?;
            let liquidity_2 = amount_1
                .checked_mul(total_supply)
                .ok_or(PairError::MulOverFlow3)?
                .checked_div(reserves.1)
                .ok_or(PairError::DivByZero2)?;
            liquidity = min(liquidity_1, liquidity_2);
        }

        if liquidity == 0 {
            return Err(PairError::InsufficientLiquidityMinted)
        }

        self._mint_to(to, liquidity)?;

        self._update(balance_0, balance_1, reserves.0, reserves.1)?;

        if fee_on {
            let k = reserves
                .0
                .checked_mul(reserves.1)
                .ok_or(PairError::MulOverFlow5)?;
            self.data::<data::Data>().k_last = k;
        }

        self._emit_mint_event(Self::env().caller(), amount_0, amount_1);

        Ok(liquidity)
    }

Add the empty implementation of _emit_mint_event and _emit_sync_event in the Pair impl. It should have the default keyword as we will override those functions in the Pair contract.

    default fn _emit_mint_event(&self, _sender: AccountId, _amount_0: Balance, _amount_1: Balance) {}

    default fn _emit_sync_event(&self, _reserve_0: Balance, _reserve_1: Balance) {}

The default keyword needs to have the attribute min_specialization added in ./logics/lib.rs :

#![cfg_attr(not(feature = "std"), no_std)]
#![feature(min_specialization)]

5. Implement Event

in the contracts ./cotnracts/pair/lib.rs file, add the Event struct and override the implementation of emit event:

...
#[ink(event)]
pub struct Mint {
    #[ink(topic)]
    pub sender: AccountId,
    pub amount_0: Balance,
    pub amount_1: Balance,
}
...
impl Pair for PairContract {
    fn _emit_mint_event(&self, sender: AccountId, amount_0: Balance, amount_1: Balance) {
        self.env().emit_event(Mint {
            sender,
            amount_0,
            amount_1,
        })
    }
}

Don't forget to add overflow-checks = false in your pair Cargo.toml:

[profile.dev]
overflow-checks = false

[profile.release]
overflow-checks = false

And that's it! In these examples we have created a wrapper around a Trait that performs cross-contract calls, which is an advanced Rust & ink! implementation. Check your Pair contract with (run in contract folder):

cargo contract build

It should now look like this branch.

Mint

1. Add Mint Functions to Pair Trait​

2. Mint Fee and Factory Trait​

3. Update​

4. Mint​

5. Implement Event​

1. Add Mint Functions to Pair Trait

2. Mint Fee and Factory Trait

3. Update

4. Mint

5. Implement Event