# Matching

Econia's matching engine is atomic, which means that taker orders either fill against maker orders during the transaction in which they were placed, or do not fill at all. This fully-autonomous process eliminates the need for a so-called "crank" sometimes required by other on-chain order books, which rely on third parties to trigger the matching process via external transactions. In practice, this means that Econia matching operations can be stacked back-to-back within a single transaction, for maximum composability.

Moreover, Econia's matching engine routes assets directly between counterparties, rather than updating units of account within a central vault. This peer-to-peer approach reduces the size of an economic attack target (the vault) by distributing assets among constituents, and eliminates transaction collisions against the matching engine that would otherwise result from user deposits or withdrawals against a global resource. Econia's matching engine is additionally parallelized across markets, such that APT/USDC orders and wBTC/USDC orders, for example, can fill concurrently.

Econia's matching engine operates purely on integers, thus eliminating potential arithmetic errors incurred by floating point operations. This efficient computational strategy is essentially implemented as a loop over head of the AVL queue for a market, as seen in match().

## Functions​

Econia's market module documentation contains a comprehensive breakdown of the functions associated with Econia's matching engine, with a dependency chart that is duplicated here:

Assorted wrappers allow for orders to be placed by a signing user, by a custodian, in a public fun or in a public entry fun, etc.

## Limit orders​

Econia supports limit orders with a side, size, price, and restriction, per place_limit_order(). Notably, this include a size in lots and an integer price.

Depending on the price and restriction, a limit order will first match across the spread as a taker, then will post as a maker order.

### Restrictions​

Econia supports the following limit order restriction flags:

FlagMeaning
NO_RESTRICTIONOptionally fill as a taker, then post to the book as a maker
FILL_OR_ABORTAbort if any size posts as a maker (only fill)
POST_OR_ABORTAbort if any size fills as a taker (only post)
IMMEDIATE_OR_CANCELFill as a taker as much as possible, then cancel any remaining size

Econia supports a special type of limit order known as a "passive advance limit order", which allows a user to place a passive limit order that advances a specified amount into the spread. As described in place_limit_order_passive_advance(), a user can specify a passive advance amount either in ticks or as a percent of the maximum possible advance into the spread, resulting in a passive advance price that is then input to place_limit_order() as a post-or-abort order.

For example, consider placing a passive advance limit order on a market where the maximum bid price is 100 and the minimum ask price is 106:

BidTicks0100
BidTicks1101
BidPercent100105
BidPercent80104

Passive advance limit orders enable the following scenarios, without any knowledge of the maximum bid or minimum ask price:

• Providing liquidity at the best price offered by the market (0% advance).
• Providing liquidity at the least-aggressive price required to take price-time priority (1 tick advance).
• Providing liquidity at the most-aggressive price while ensuring passivity (100% advance).
• Providing liquidity at the mid price (50% advance).

In the presence of miner extractable value (MEV)-style front-running, any passive advance limit order is essentially converted to a 100% passive advance: continuing the above example, if someone places a 0% passive advance bid, a front-runner simply needs to place a bid at price 105, execute the legitimate transaction, then cancel their malicious bid, to ensure that the passive advance limit order posts at a price of 105 instead of 100. Notably, however, even if the legitimate order holder has their bid filled at a price of 105, they will still end up buying at a better price than if they were to place a market order (which would fill against the minimum ask at a price of 106). Hence passive advance orders can, in the presence of MEV, be used to effectively eliminate paying taker fees, if a user is willing to risk that their order might not fill.

## Taker-only orders​

Per place_market_order(), Econia supports taker-only market orders that fill from a user's market account. Econia also supports taker-only swaps per assorted wrappers for swap(), which fill from coins or generic assets held outside of a market account.

Like limit orders, taker-only orders specify an integer price, but instead of specifying a size in lots, taker-only orders indicate a minimum and maximum amount of base and quote asset to trade. This is like saying "I am willing to buy up to 36 oranges (maximum base) but no fewer than 12 (minimum base), and I am willing to spend up to $3.50 (maximum quote) but no more than$1.50 per dozen (price)".

## Self match behavior​

Econia's matching engine supports configurable self match behavior, with a self match defined as a hypothetical fill where taker and maker assets are derived from the same market account. Hence since limit orders and market orders can both fill as a taker, they require one of the following self match behavior flags:

FlagBehavior during self match
ABORTAbort
CANCEL_BOTHCancel maker and taker orders
CANCEL_MAKERCancel maker order only
CANCEL_TAKERCancel taker order only

Note that self matching only applies within a market account: one custodian cannot adversarially cancel orders placed by a different custodian for the same user and market.

## Fee assessment​

Econia's matching engine charges taker fees, which are assessed at the end of the matching process and are denominated in the quote coin for a given market. Calls to the matching engine, however, specify quote trade amounts that denote the net change in a taker's quote coin holdings due to matching and fees, or in other words, the minimum or maximum amount of quote coins that a taker is willing to trade away or receive from a trade. As such, match() calculates a maximum amount of quote coins to match on an order for the given direction (BUY or SELL) per calculate_max_quote_match(), matches accordingly, then assesses fees per assess_taker_fees().

For example, consider a hypothetical market with a 5% fee taker fee. A user places a taker buy for a maximum quote trade amount of 105 quote coins, meaning that they are willing to spend up to 105 quote coins. At most the matching engine can match 100 quote coins, with the remaining 5 coins set aside to cover taker fees. Here, the maximum quote asset trade amount is passed in (105 quote coins) and a reserve is essentially set aside to pay taker fees. After matching and fees, the taker thus experiences a net change of up to 105 quote coins, even though only as many as 100 were matched.

For taker sells, conversely, no quote coins are passed in. Rather, the base asset is passed in and matched, then a portion of quote proceeds are deducted for fees. For example, consider a hypothetical market with a 4% taker fee, where the taker specifies a maximum quote trade amount of 100 quote coins. Here, the matching engine matches up to 104 quote coins then assesses a 4% fee, deducted from the quote coins received in the trade. After matching and fees, the taker thus experiences a net change of up to 100 quote coins, even though as many as 104 were matched.

## Units and flags​

Both limit orders and taker-only orders accept an integer price, but rely on different asset units and flags:

For limit orders:

For limit orders, which can match as a maker or as a taker, the specified size is matched regardless of fees, whereas for taker-only orders, maximum quote trade amounts are adjusted for taker fees per above.

Per issue 56, in the interest of developer ease, side and direction flag polarities are equivalent, such that ASK == SELL and BID == BUY.