OffsetHelper

The OffsetHelper contract simplifies the carbon offsetting (retirement) process.

The OffsetHelper is a peripheral contract that simplifies the experience of buying and redeeming carbon reference tokens and retiring TCO2 tokens. The contract lives in this repository.

In more exact terms, the OffsetHelper abstracts the process of retiring TCO2, which normally looks like so:

• user exchanges USDC for BCT/NCT tokens at one of the DEXs (Uniswap, Sushiswap, etc. depending on network)

• user interacts with the BCT/NCT token contract to redeem the tokens for TCO2

• user interacts with the TCO2 token contract to retire the TCO2

With the OffsetHelper contract, the user only needs to interact with the OffsetHelper contract, which will take care of the rest of the contract calls in a single transaction.

In these methods, "auto" refers to the fact that these methods use autoRedeem() in order to automatically choose a TCO2 token corresponding to the oldest tokenized carbon project in the specified token pool. There are no fees incurred by the user when using autoRedeem(), i.e., the user receives 1 TCO2 token for each carbon reference token e.g., NCT redeemed.

Testing the OffsetHelper on testnet may lead to an Error: execution reverted as the OffsetHelper depends on available liquidity in some DEX pools, which can not always be guaranteed.

Approvals

The OffsetHelper is a smart contract that calls other smart contracts. As a security measure, users need to approve the OffsetHelper contract address, so the other smart contracts accept interactions routed through this address.

Note: you must approve the pool contract in the TCO2 token that is to be deposited in order for the pool to be able to transfer the TCO2 on your behalf.

Accepted tokens

Chain	Tokens
Celo	cUSD, WETH, USDC
Polygon	USDC, WETH, WMATIC

Functions

autoOffsetExactOutToken

function autoOffsetExactOutToken(address _fromToken, address _poolToken, uint256 _amountToOffset) public returns (address[] tco2s, uint256[] amounts)

Retire carbon credits using the oldest TCO2 tokens available from the specified Toucan token pool by sending ERC20 tokens (cUSD, USDC, WETH, WMATIC).

The view helper function calculateNeededTokenAmount() should be called before using autoOffsetExactOutToken(), to determine how many native tokens (e.g., MATIC) must be sent to the OffsetHelper contract in order to retire the specified amount of TCO2 tokens.

This function:

1. Swaps the ERC20 token sent to the contract for the specified carbon reference token

2. Redeems the reference tokens received for the lowest quality TCO2 tokens available

3. Retires the TCO2 tokens

Note: The user must approve the ERC20 token that will be used for the swap.

Parameters

Name	Type	Description
_fromToken	address	The address of the ERC20 token that the user will use to swap (e.g., cUSD, USDC, WETH, WMATIC)
_poolToken	address	The address of the carbon reference token that the user wants to redeem, e.g., NCT
_amountToOffset	uint256	The amount of TCO2 to retire

Return Values

Name	Type	Description
tco2s	address[]	An array of the TCO2 addresses that were redeemed
amounts	uint256[]	An array of the amounts of each TCO2 that were redeemed

autoOffsetExactInToken

function autoOffsetExactInToken(address _fromToken, address _poolToken, uint256 _amountToSwap) public returns (address[] tco2s, uint256[] amounts)

Retire carbon credits using the oldest TCO2 tokens available from the specified Toucan carbon pool by swapping ERC20 tokens (cUSD, USDC, WETH, WMATIC). All of provided "in" token is consumed for retirement.

The view helper function calculateExpectedPoolTokenForToken() can be used to calculate the expected amount of TCO2s that will be offset using autoOffsetExactInToken().

This function:

1. Swaps the ERC20 token sent to the contract for the specified carbon reference token

2. Redeems the reference token for the lowest quality TCO2 tokens available

3. Retires the TCO2 tokens

Note: The client must approve the ERC20 token that is sent to the contract.

Parameters

Name	Type	Description
_fromToken	address	The address of the ERC20 token that the user sends (e.g., cUSD, USDC, WETH, WMATIC)
_poolToken	address	The address of the carbon reference token to offset
_amountToSwap	uint256	The amount of ERC20 token to swap into carbon reference token. Full amount will be used for retirement.

Return Values

Name	Type	Description
tco2s	address[]	An array of the TCO2 addresses that were redeemed
amounts	uint256[]	An array of the amounts of each TCO2 that were redeemed

autoOffsetExactOutETH

function autoOffsetExactOutETH(address _poolToken, uint256 _amountToOffset) public payable returns (address[] tco2s, uint256[] amounts)

Retire carbon credits using the oldest TCO2 tokens available from the specified Toucan token pool by sending native tokens e.g., MATIC.

Note: While this method refers to "ETH", it actually refers to the blockchain's native token, and will use WMATIC on Polygon.

The view helper function calculateNeededETHAmount() should be called before using autoOffsetExactOutETH(), to determine how much native tokens e.g., MATIC must be sent to the OffsetHelper contract in order to retire the specified amount of carbon.

This function:

1. Swaps the native token e.g. MATIC sent to the contract for the specified pool token

2. Redeems the pool token for the poorest quality TCO2 tokens available

3. Retires the TCO2 tokens

If the user sends too much native tokens, the leftover amount will be sent back to the user. This function is only available on Polygon, not on Celo.

Parameters

Name	Type	Description
_poolToken	address	The address of the pool token to offset, e.g., NCT
_amountToOffset	uint256	The amount of TCO2 to retire

Return Values

Name	Type	Description
tco2s	address[]	An array of the TCO2 addresses that were redeemed
amounts	uint256[]	An array of the amounts of each TCO2 that were redeemed

autoOffsetExactInETH

function autoOffsetExactInETH(address _poolToken) public payable returns (address[] tco2s, uint256[] amounts)

Retire carbon credits using the oldest TCO2 tokens available from the specified Toucan token pool by sending native tokens e.g., MATIC. All provided native tokens is consumed for offsetting.

The view helper function calculateExpectedPoolTokenForETH() can be used to calculate the expected amount of TCO2s that will be offset using autoOffsetExactInETH().

This function:

1. Swaps the native token e.g. MATIC sent to the contract for the specified pool token

2. Redeems the pool token for the poorest quality TCO2 tokens available

3. Retires the TCO2 tokens

This function is only available on Polygon, not on Celo.

Parameters

Name	Type	Description
_poolToken	address	The address of the pool token to offset

Return Values

Name	Type	Description
tco2s	address[]	An array of the TCO2 addresses that were redeemed
amounts	uint256[]	An array of the amounts of each TCO2 that were redeemed

autoOffsetPoolToken

function autoOffsetPoolToken(address _poolToken, uint256 _amountToOffset) public returns (address[] tco2s, uint256[] amounts)

Retire carbon credits using the oldest TCO2 tokens available by sending Toucan carbon reference tokens, e.g., NCT.

This function:

1. Redeems the reference tokens for the poorest quality TCO2 tokens available

2. Retires the TCO2 tokens

Note: The user must approve the carbon reference token to be swapped.

Parameters

Name	Type	Description
_poolToken	address	The address of the pool token to offset, e.g., NCT
_amountToOffset	uint256	The amount of TCO2 to retire

Return Values

Name	Type	Description
tco2s	address[]	An array of the TCO2 addresses that were redeemed
amounts	uint256[]	An array of the amounts of each TCO2 that were redeemed

swapExactOutToken

function swapExactOutToken(address _fromToken, address _poolToken, uint256 _toAmount) public

Swap eligible ERC20 tokens for Toucan carbon reference tokens (BCT/NCT) on a DEX.

Note: The user must approve the carbon reference token to be swapped.

Parameters

Name	Type	Description
_fromToken	address	The address of the ERC20 token used for the swap
_poolToken	address	The address of the carbon reference token to swap for, e.g., NCT
_toAmount	uint256	The required amount of the Toucan reference token (NCT/BCT)

swapExactInToken

function swapExactInToken(address _fromToken, address _poolToken, uint256 _fromAmount) public returns (uint256 amountOut)

Swap eligible ERC20 tokens for Toucan carbon reference tokens (BCT/NCT) on a DEX. All provided ERC20 tokens will be swapped.

Note: The user must approve the carbon reference token to be swapped.

Parameters

Name	Type	Description
_fromToken	address	The address of the ERC20 token used for the swap
_poolToken	address	The address of the carbon reference token to swap for, e.g., NCT
_fromAmount	uint256	The amount of ERC20 token to swap

Return Values

Name	Type	Description
amountOut	uint256	Resulting amount of Toucan carbon reference tokens that were acquired for the swapped ERC20 tokens.

swapExactOutETH

function swapExactOutETH(address _poolToken, uint256 _toAmount) public payable

Swap native tokens (e.g., MATIC) for Toucan carbon reference tokens (BCT/NCT) on a DEX. Remaining native tokens that were not consumed by the swap are returned.

Parameters

Name	Type	Description
_poolToken	address	The address of the carbon reference token to swap for, e.g., NCT
_toAmount	uint256	The required amount of the Toucan carbon reference token (NCT/BCT)

swapExactInETH

function swapExactInETH(address _poolToken) public payable returns (uint256 amountOut)

Swap native tokens (e.g., MATIC) for Toucan carbon reference tokens (BCT/NCT) on a DEX. All provided native tokens will be swapped.

Parameters

Name	Type	Description
_poolToken	address	The address of the carbon reference token to swap for, e.g., NCT

Return Values

Name	Type	Description
amountOut	uint256	Resulting amount of Toucan carbon reference token that got acquired for the swapped native tokens

autoRedeem

function autoRedeem(address _fromToken, uint256 _amount) public returns (address[] tco2s, uint256[] amounts)

Auto-redeems the specified amount of NCT / BCT for default TCO2 tokens.

Note: The user must approve the carbon reference token to be redeemed.

Parameters

Name	Type	Description
_fromToken	address	The address of the carbon pool to redeem from
_amount	uint256	Amount to redeem

Return Values

Name	Type	Description
tco2s	address[]	An array of the TCO2 addresses that were redeemed
amounts	uint256[]	An array of the amounts of each TCO2 that were redeemed

autoRetire

function autoRetire(address[] _tco2s, uint256[] _amounts) public

Retire the specified TCO2 tokens.

The account sending the transaction needs to hold enough of each of the TCO2 tokens specified for this function to execute.

Parameters

Name	Type	Description
_tco2s	address[]	The addresses of the TCO2s to retire
_amounts	uint256[]	The amounts to retire from each of the corresponding TCO2 addresses

calculateNeededTokenAmount

function calculateNeededTokenAmount(address _fromToken, address _poolToken, uint256 _toAmount) public view returns (uint256 amountIn)

Returns how much of the specified ERC20 token is required in order to swap for the desired amount of a Toucan carbon reference token such as NCT.

Parameters

Name	Type	Description
_fromToken	address	The address of the ERC20 token used for the swap
_poolToken	address	The address of the pool token to swap for, e.g., NCT
_toAmount	uint256	The desired amount of pool token to receive

Return Values

Name	Type	Description
amountIn	uint256	The amount of the ERC20 token required in order to swap for the specified amount of the pool token

calculateExpectedPoolTokenForToken

function calculateExpectedPoolTokenForToken(address _fromToken, address _poolToken, uint256 _fromAmount) public view returns (uint256 amountOut)

Calculates the expected amount of Toucan carbon reference token that can be acquired by swapping the provided amount of ERC20 token.

Parameters

Name	Type	Description
_fromToken	address	The address of the ERC20 token used for the swap
_poolToken	address	The address of the carbon reference token to swap for, such as NCT
_fromAmount	uint256	The amount of ERC20 token to swap

Return Values

Name	Type	Description
amountOut	uint256	The expected amount of carbon reference token that can be acquired

calculateNeededETHAmount

function calculateNeededETHAmount(address _poolToken, uint256 _toAmount) public view returns (uint256 amountIn)

Return how much native tokens e.g, MATIC is required in order to swap for the desired amount of a Toucan carbon reference token, e.g., NCT.

Parameters

Name	Type	Description
_poolToken	address	The address of the carbon reference token to swap for, e.g., NCT
_toAmount	uint256	The desired amount of carbon reference token to receive

Return Values

Name	Type	Description
amountIn	uint256	The amount of native tokens required in order to swap for the specified amount of the carbon reference token

calculateExpectedPoolTokenForETH

function calculateExpectedPoolTokenForETH(address _poolToken, uint256 _fromTokenAmount) public view returns (uint256 amountOut)

Calculates the expected amount of Toucan carbon reference tokens that can be acquired by swapping the provided amount of native tokens e.g., MATIC.

Parameters

Name	Type	Description
_poolToken	address	The address of the carbon reference token to swap for, e.g., NCT
_fromTokenAmount	uint256	The amount of native tokens to swap

Return Values

Name	Type	Description
amountOut	uint256	The expected amount of carbon reference token that can be acquired

isPoolAddressEligible

function isPoolAddressEligible(address _poolToken) public view returns (bool _isEligible)

Checks if an address identifies a Toucan carbon pool.

Parameters

Name	Type	Description
_poolToken	address	The address of the carbon reference token to test

Return Values

Name	Type	Description
_isEligible	bool	Returns a bool if the address identifies a Toucan carbon pool

isERC20AddressEligible

function isERC20AddressEligible(address _erc20Address) public view returns (address[] _path)

Checks if ERC20 Token is supported for swapping. Check the accepted tokens.

Parameters

Name	Type	Description
_erc20Address	address	The address of the ERC20 token that the user sends (e.g., cUSD, cUSD, USDC, WETH, WMATIC)

Return Values

Name	Type	Description
_path	address[]	Returns the path of the token to be exchanged