(Part 10) Decentralized Exchange - The Complete Buy Function And Working Exchange!(PT 10)

in utopian-io •  9 days ago



What Will I Learn?

  • Filling orders of the same price
  • Filling order of different price (respecting the limit sell order)
  • Removing a link on the top of the linked list


  • Internet connection.
  • Code editor.
  • Browser.


  • Advanced.

Tutorial Contents


On the previous tutorial, we have implemented the logic for when the user makes a sell order, the smart contract to search on the buy book for orders that fill it. So, if you want to sell a token at a limit price, it will try to find buy orders that are either that price or higher. The sell function iterates through the buy book, from the most expensive order to the least one, filling the sell order. When the sell order finds that the book has no more orders that are at least the required price by the seller, if there are remaining tokens, after filling the order from the compatible sell ones, the order is stored on the sell book.

Now it is time to reverse that and implement that on the opposite side of the book. The buy order will have a similar logic but reversed. The buyer "profits" if he can get cheaper tokens than his limit order, so we will go from the cheapest sell order to the most expensive. Also, here we will consume ETH for the order maker, not tokens.

At the end of the implementation, I will show how everything works together, how the functions behave and do the testings.

As said on the previous tutorial, the focus of this tutorial you are reading is to show the differences from the sell to the buy orders and to demonstrate how to use them and what are the effects. If you want to understand more in depth the logic behind it, the previous tutorial has more details on that.

The function

We are building this inside a function that was already coded into the contract. But so far, it only had the functionality to store a new order in case no order on the sell book matches our new one. This is the function:

function buyToken(address _token, uint _price, uint _amount) public{
//omitted code

Inside it, we had an if and else decision branch. We have already worked on the if, now is time for the else, that means, there are orders that can fill, even if just partially, the new order.

The outer loop

The outer loop iterates through the prices on the linked list of prices:

uint ethAmount = 0;
uint remainingAmount = _amount;
uint buyPrice = loadedToken.minSellPrice;
uint offerPointer;
//outer loop, there will be an inner loop inside this

//calls the function again to store the remaining of the buy order
if (remainingAmount>0){
    buyToken(_token, _price, remainingAmount);

The inner loop

The following chunk of code is indeed the previous outer loop:

offerPointer = loadedToken.sellBook[buyPrice].offerPointer;
while(offerPointer <= loadedToken.sellBook[buyPrice].offerLength && remainingAmount >0){
    uint volumeAtPointer = loadedToken.sellBook[buyPrice].offers[offerPointer].amount;

//if the stored order will be filled completely
//if there will be something remaining not he stored sell order

//if the queue at this price gets empty
//if the buy book is empty
//stores that the buy book is empty
//removes the current price as it is now empty of orders
//moves the pointer up
//updates the minimum buy price

Notice two key differences from the sell function:

  • We are going from the cheapest sell order to the least expensive
  • We remove the lowest price, not the highest

Because it is of the interest of the buyer to get the lowest price.

If the stored order will be filled completely

This is the first if statement, and here is the logic of what to do in case the stored sell order is completely filled by the new buy order (means, the sell is lower or equal to the buy):

//the eth moved on this operation
//requires that the buyer has the amount
//requires the buyer won't underflow
//deducts the balance
//gives the tokens to the seller
//cleans the sell order at the pointer
//adds the balance to the seller
//moves the pointer
//updates how much the buyer still wants to buy

Notice above the security against the race conditions. We deduct the ETH balance from the buyer before doing any transaction. So if he tries to spam with many calls to this function, he won't have enough balance to exploit a possibly not updated balance.

If there will be something remaining not he stored sell order

//requires that we aren't doing this accidentally
//the remaining from the buy is the smallest, so this defined the amount of ETH
//requires the buyer won't underflow
//deducts the ETH from the buyer
//deducts the sell order
//gives the ETH to the seller
//gives the tokens to the buyer
//records that the order is completely filled

In case the order is not completely filled and all loops have ended, if there is something left that the buyer still has to buy, the function will call itself at the condition previously written:

if (remainingAmount>0){
    buyToken(_token, _price, remainingAmount);

And this will store a new buy order with the remaining.


To start the tests a simple exchange a very basic token were deployed on a virtual testing environment:
Captura de Tela 20190210 às 20.38.42.png

It is worth mentioning that, at this point, the contract is so complex that the deployment cost 4266979 gas.
Captura de Tela 20190210 às 20.39.46.png

According to a gas price calculator, that means a regular deployment would cost around 1.5 dollars, and a priority deployment would cost around 23 dollars of miner fees!

Captura de Tela 20190210 às 20.42.37.png

I have placed a few orders on both the buy and sell book. Let us do some tests to see them filling:
Captura de Tela 20190210 às 20.47.48.png

First, I sell 2 tokens at the price of 9:
Captura de Tela 20190210 às 20.49.04.png

As you see, this is what happened:

  1. the order at 10 was completely filled
  2. the order at 9 was partially filled

Now, I have placed an order to buy 10 tokens at the price of 25:
Captura de Tela 20190210 às 20.50.54.png

What happened is that all the orders up to 25 were filled, and the volume left was stored in a buy order at the price of 25.

So far, we pretty much have a working exchange, the contract can:

  • Manage ETH deposits
  • Manage ETH withdrawals
  • Manage Token deposits
  • Manage Token withdrawals
  • Execute trades
  • Store orders


A real contract would take anywhere from weeks to months to execute all the security testings for all known exploits. So, do not use the contract from this class to release a real exchange. This is for educational purposes only.


You might have noticed it was very expensive to deploy this contract, from 1.5 dollars to over 20 dollars for the miner fees. And the calls to the contract are also very expensive, ranging from 2 cents (for very basic trades) up to 10 dollars and maybe more, depending on the size of the books!

A decentralized exchange is a mean of overcoming censorship and government control, but it has its price. It is not for people to transact small amounts, considering our exchange does not take fees and all the costs are just for the miners, people that trade huge volumes would benefit, because, even though a trade can cost 10 dollars, if someone uses the decentralized exchanger to do a million dollars trade, the fee would be negligible and much better than a centralized exchange!

Also, the new pattern that appears to be accepted as the best one, is to use the decentralized exchange as a settlement layer. Exchanges are investing in building their interfaces on a centralized server, and do a few trades on the decentralized version for settlement of assets, to cut down costs.


Latest tutorial of the series:

First tutorial of the series:


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Thank you for your contribution.

  • Another cool concept introduced today. Building DEX is definitely a nice thing to attempt.
  • I loved how you are now adding actual screenshots and examples, very helpful.
  • Also your security warning was well in place. Very important for anyone attempting to build a DEX.
  • There were few typos in your post. One header i could barely understand.

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Thank you for your review, @mcfarhat! Keep up the good work!

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