Exchange tokens with different decimal

Question

I'm in the process of developing a smart contract that allows users to purchase tokens. However, I'm encountering an issue with the price calculation:

1. The token I'm selling (let's call it "SellToken") has 18 decimals.
2. The token being used for purchase (e.g., USDC) has 6 decimals.

Here's the relevant function from my smart contract:

function purchaseTokens(address inputToken, uint256 lockDuration, uint256 amount, uint256 price) external {
    require(acceptedTokens[inputToken], "Token not accepted");
    require(price == priceByLockDuration[lockDuration], "Invalid price");

    IERC20 paymentToken = IERC20(inputToken);

    uint256 totalPrice = price.mul(amount);
    console.log('totalPrice', totalPrice);
    console.log('amount', amount);
    require(paymentToken.balanceOf(msg.sender) >= totalPrice, "Insufficient balance");
    require(totalTokensForSale.sub(totalTokensSold) >= amount, "Insufficient sale tokens for sale");
    require(paymentToken.transferFrom(msg.sender, address(this), totalPrice), "Payment transfer failed");

    Purchase memory newPurchase;
    newPurchase.amount = amount;
    newPurchase.unlockTime = block.timestamp.add(lockDuration);
    newPurchase.claimed = false;

    purchases[msg.sender].push(newPurchase);
    purchaseCount[msg.sender] = purchaseCount[msg.sender].add(1);
    totalTokensSold = totalTokensSold.add(amount);
}

When I set the price as follows:

const price = ethers.parseUnits('0.1', USDCdecimals)
const amount = 1

tokenDistribution.purchaseTokens(
  USDCtokenAddress,
  unlockPeriod,
  amount,
  price,
)

The result is that when a user spends 0.1 USDC, they only receive 0.000000000000000001 SellToken.

How can I adjust the price calculation so that spending 0.1 USDC will give the user 1 SellToken?

Answer 1

It is the good old Cross-Multiplication trick.

For example let's say 1 usdc = 10 tokens, then for 0.23 usdc you have 10 tokens * 0.23 usdc / 1 usdc = 2.3 tokens.

For convinience you could store the price as a fraction.

uint256 priceNumerator = 10*10**18;  // in tokens units
uint256 priceDenominator = 1*10**6;  // in USDC units

If you paid amountUSDC in USDC then

uint256 amountTokens = priceNumerator * amountUSDC / priceDenominator;

Going back to the example you paid 0.23 USDC, that will be amountUSDC = 230000.

amountTokens = 10 *10**18 * 230000 / 10**6;

After doing the math you get amountTokens = 2.3 * 10**18, o 2.3 tokens as expected.