07-creating-buy-orders.md

---

sidebar_position: 7
description: Implement the buy order logic.

---

Creating Buy Orders

In this chapter, you implement the creation of buy orders. The logic is very similar to the sell order logic you implemented in the previous chapter.

Modify the Proto Definition

Add the buyer to the proto file definition:

// proto/interchange/dex/packet.proto

message BuyOrderPacketData {
  // ...
  string buyer = 5;
}

Now, use Ignite CLI to build the proto files for the send-buy-order command. You used this command in previous chapters.

ignite generate proto-go --yes

IBC Message Handling in SendBuyOrder

• Check if the pair exists on the order book

• If the token is an IBC token, burn the tokens

• If the token is a native token, lock the tokens

• Save the voucher received on the target chain to later resolve a denom

// x/dex/keeper/msg_server_buy_order.go

package keeper

import (
	"context"
	"errors"

	sdk "github.com/cosmos/cosmos-sdk/types"

	"interchange/x/dex/types"
)

func (k msgServer) SendBuyOrder(goCtx context.Context, msg *types.MsgSendBuyOrder) (*types.MsgSendBuyOrderResponse, error) {
	ctx := sdk.UnwrapSDKContext(goCtx)

	// Cannot send a order if the pair doesn't exist
	pairIndex := types.OrderBookIndex(msg.Port, msg.ChannelID, msg.AmountDenom, msg.PriceDenom)
	_, found := k.GetBuyOrderBook(ctx, pairIndex)
	if !found {
		return &types.MsgSendBuyOrderResponse{}, errors.New("the pair doesn't exist")
	}

	// Lock the token to send
	sender, err := sdk.AccAddressFromBech32(msg.Creator)
	if err != nil {
		return &types.MsgSendBuyOrderResponse{}, err
	}

	// Use SafeBurn to ensure no new native tokens are minted
	if err := k.SafeBurn(ctx, msg.Port, msg.ChannelID, sender, msg.PriceDenom, msg.Amount*msg.Price); err != nil {
		return &types.MsgSendBuyOrderResponse{}, err
	}

	// Save the voucher received on the other chain, to have the ability to resolve it into the original denom
	k.SaveVoucherDenom(ctx, msg.Port, msg.ChannelID, msg.PriceDenom)

	// Construct the packet
	var packet types.BuyOrderPacketData

	packet.Buyer = msg.Creator
	packet.AmountDenom = msg.AmountDenom
	packet.Amount = msg.Amount
	packet.PriceDenom = msg.PriceDenom
	packet.Price = msg.Price

	// Transmit the packet
	err = k.TransmitBuyOrderPacket(
		ctx,
		packet,
		msg.Port,
		msg.ChannelID,
		clienttypes.ZeroHeight(),
		msg.TimeoutTimestamp,
	)
	if err != nil {
		return nil, err
	}

	// Transmit an IBC packet...
	return &types.MsgSendBuyOrderResponse{}, nil
}

On Receiving a Buy Order

• Update the buy order book

• Distribute sold token to the buyer

• Send the sell order to chain A after the fill attempt

// x/dex/keeper/buy_order.go

package keeper

// ...

func (k Keeper) OnRecvBuyOrderPacket(ctx sdk.Context, packet channeltypes.Packet, data types.BuyOrderPacketData) (packetAck types.BuyOrderPacketAck, err error) {
	// validate packet data upon receiving
	if err := data.ValidateBasic(); err != nil {
		return packetAck, err
	}

	// Check if the sell order book exists
	pairIndex := types.OrderBookIndex(packet.SourcePort, packet.SourceChannel, data.AmountDenom, data.PriceDenom)
	book, found := k.GetSellOrderBook(ctx, pairIndex)
	if !found {
		return packetAck, errors.New("the pair doesn't exist")
	}

	// Fill buy order
	remaining, liquidated, purchase, _ := book.FillBuyOrder(types.Order{
		Amount: data.Amount,
		Price:  data.Price,
	})

	// Return remaining amount and gains
	packetAck.RemainingAmount = remaining.Amount
	packetAck.Purchase = purchase

	// Before distributing gains, we resolve the denom
	// First we check if the denom received comes from this chain originally
	finalPriceDenom, saved := k.OriginalDenom(ctx, packet.DestinationPort, packet.DestinationChannel, data.PriceDenom)
	if !saved {
		// If it was not from this chain we use voucher as denom
		finalPriceDenom = VoucherDenom(packet.SourcePort, packet.SourceChannel, data.PriceDenom)
	}

	// Dispatch liquidated buy order
	for _, liquidation := range liquidated {
		liquidation := liquidation
		addr, err := sdk.AccAddressFromBech32(liquidation.Creator)
		if err != nil {
			return packetAck, err
		}

		if err := k.SafeMint(
			ctx,
			packet.DestinationPort,
			packet.DestinationChannel,
			addr,
			finalPriceDenom,
			liquidation.Amount*liquidation.Price,
		); err != nil {
			return packetAck, err
		}
	}

	// Save the new order book
	k.SetSellOrderBook(ctx, book)

	return packetAck, nil
}

Implement a FillBuyOrder Function

The FillBuyOrder function tries to fill the sell order with the order book and returns all the side effects:

// x/dex/types/sell_order_book.go

package types

// ...

func (s *SellOrderBook) FillBuyOrder(order Order) (
	remainingBuyOrder Order,
	liquidated []Order,
	purchase int32,
	filled bool,
) {
	var liquidatedList []Order
	totalPurchase := int32(0)
	remainingBuyOrder = order

	// Liquidate as long as there is match
	for {
		var match bool
		var liquidation Order
		remainingBuyOrder, liquidation, purchase, match, filled = s.LiquidateFromBuyOrder(
			remainingBuyOrder,
		)
		if !match {
			break
		}

		// Update gains
		totalPurchase += purchase

		// Update liquidated
		liquidatedList = append(liquidatedList, liquidation)

		if filled {
			break
		}
	}

	return remainingBuyOrder, liquidatedList, totalPurchase, filled
}

Implement a LiquidateFromBuyOrder Function

The LiquidateFromBuyOrder function liquidates the first buy order of the book from the sell order. If no match is found, return false for match:

// x/dex/types/sell_order_book.go

package types

// ...

func (s *SellOrderBook) LiquidateFromBuyOrder(order Order) (
	remainingBuyOrder Order,
	liquidatedSellOrder Order,
	purchase int32,
	match bool,
	filled bool,
) {
	remainingBuyOrder = order

	// No match if no order
	orderCount := len(s.Book.Orders)
	if orderCount == 0 {
		return order, liquidatedSellOrder, purchase, false, false
	}

	// Check if match
	lowestAsk := s.Book.Orders[orderCount-1]
	if order.Price < lowestAsk.Price {
		return order, liquidatedSellOrder, purchase, false, false
	}

	liquidatedSellOrder = *lowestAsk

	// Check if buy order can be entirely filled
	if lowestAsk.Amount >= order.Amount {
		remainingBuyOrder.Amount = 0
		liquidatedSellOrder.Amount = order.Amount
		purchase = order.Amount

		// Remove lowest ask if it has been entirely liquidated
		lowestAsk.Amount -= order.Amount
		if lowestAsk.Amount == 0 {
			s.Book.Orders = s.Book.Orders[:orderCount-1]
		} else {
			s.Book.Orders[orderCount-1] = lowestAsk
		}

		return remainingBuyOrder, liquidatedSellOrder, purchase, true, true
	}

	// Not entirely filled
	purchase = lowestAsk.Amount
	s.Book.Orders = s.Book.Orders[:orderCount-1]
	remainingBuyOrder.Amount -= lowestAsk.Amount

	return remainingBuyOrder, liquidatedSellOrder, purchase, true, false
}

Receiving a Buy Order Acknowledgment

After a buy order acknowledgement is received, chain Mars:

• Stores the remaining sell order in the sell order book.

• Distributes sold marscoin to the buyers.

• Distributes to the seller the price of the amount sold.

• On error, mints back the burned tokens.

// x/dex/keeper/buy_order.go

package keeper

// ...

func (k Keeper) OnAcknowledgementBuyOrderPacket(ctx sdk.Context, packet channeltypes.Packet, data types.BuyOrderPacketData, ack channeltypes.Acknowledgement) error {
	switch dispatchedAck := ack.Response.(type) {
	case *channeltypes.Acknowledgement_Error:
		// In case of error we mint back the native token
		receiver, err := sdk.AccAddressFromBech32(data.Buyer)
		if err != nil {
			return err
		}

		if err := k.SafeMint(
			ctx,
			packet.SourcePort,
			packet.SourceChannel,
			receiver,
			data.PriceDenom,
			data.Amount*data.Price,
		); err != nil {
			return err
		}

		return nil
	case *channeltypes.Acknowledgement_Result:
		// Decode the packet acknowledgment
		var packetAck types.BuyOrderPacketAck

		if err := types.ModuleCdc.UnmarshalJSON(dispatchedAck.Result, &packetAck); err != nil {
			// The counter-party module doesn't implement the correct acknowledgment format
			return errors.New("cannot unmarshal acknowledgment")
		}

		// Get the sell order book
		pairIndex := types.OrderBookIndex(packet.SourcePort, packet.SourceChannel, data.AmountDenom, data.PriceDenom)
		book, found := k.GetBuyOrderBook(ctx, pairIndex)
		if !found {
			panic("buy order book must exist")
		}

		// Append the remaining amount of the order
		if packetAck.RemainingAmount > 0 {
			_, err := book.AppendOrder(
				data.Buyer,
				packetAck.RemainingAmount,
				data.Price,
			)
			if err != nil {
				return err
			}

			// Save the new order book
			k.SetBuyOrderBook(ctx, book)
		}

		// Mint the purchase
		if packetAck.Purchase > 0 {
			receiver, err := sdk.AccAddressFromBech32(data.Buyer)
			if err != nil {
				return err
			}

			finalAmountDenom, saved := k.OriginalDenom(ctx, packet.SourcePort, packet.SourceChannel, data.AmountDenom)
			if !saved {
				// If it was not from this chain we use voucher as denom
				finalAmountDenom = VoucherDenom(packet.DestinationPort, packet.DestinationChannel, data.AmountDenom)
			}

			if err := k.SafeMint(
				ctx,
				packet.SourcePort,
				packet.SourceChannel,
				receiver,
				finalAmountDenom,
				packetAck.Purchase,
			); err != nil {
				return err
			}
		}

		return nil
	default:
		// The counter-party module doesn't implement the correct acknowledgment format
		return errors.New("invalid acknowledgment format")
	}
}

AppendOrder appends an order in the buy order book. Add the following function to the x/dex/types/buy_order_book.go file in the types directory.

// x/dex/types/buy_order_book.go

package types

// ...

func (b *BuyOrderBook) AppendOrder(creator string, amount int32, price int32) (int32, error) {
	return b.Book.appendOrder(creator, amount, price, Increasing)
}

OnTimeout of a Buy Order Packet

If a timeout occurs, mint back the native token:

// x/dex/keeper/buy_order.go

package keeper

// ...

func (k Keeper) OnTimeoutBuyOrderPacket(ctx sdk.Context, packet channeltypes.Packet, data types.BuyOrderPacketData) error {
	// In case of error we mint back the native token
	receiver, err := sdk.AccAddressFromBech32(data.Buyer)
	if err != nil {
		return err
	}

	if err := k.SafeMint(
		ctx,
		packet.SourcePort,
		packet.SourceChannel,
		receiver,
		data.PriceDenom,
		data.Amount*data.Price,
	); err != nil {
		return err
	}

	return nil
}

Summary

Congratulations, you implemented the buy order logic.

Again, it's a good time to save your current state to your local GitHub repository:

git add .
git commit -m "Add Buy Orders"