BSIP: 0033
    Title: Maker Orders With Better Prices Take Precedence
    Author: Abit More <https://github.com/abitmore>
    Status: Draft
    Type: Protocol
    Created: 2018-02-17
    Discussion: https://github.com/bitshares/bitshares-core/issues/625,
                https://github.com/bitshares/bitshares-core/issues/453
    Replaces: -
    Worker: To be done

# Abstract

Currently, in BitShares, under certian circumstances, a taker order may be
matched with a maker order which is not "on the top" of the order book.
This behavior is unexpected and irritating for users.

This BSIP proposes a principle: always match taker orders with maker orders
with the best prices (aka on the top of the order book) first.

# Motivation

As expected, when matching taker limit orders with maker limit orders, the maker
limit orders with better prices will always be matched first.

For example, if trader A has a limit order selling 100 BTS at
0.1 USD per BTS, trader B has a limit order selling 100 BTS at 0.09 USD per BTS,
which means B's limit order has a better price for the opposite side to buy.
Now if trader C placed an order that buys 10 BTS at 0.105 USD per BTS, B's
limit order will take precedence, A's limit order won't be matched.

However, when there are (maker) margin call orders in the market which have met
the requirements that to be matched (able to be margin called), they always
take precedence over the (maker) limit orders on the same side, no matter
whether the limit orders provided better price.

For example, if trader A's margin call order is selling 100 BTS at no less than
0.1 USD per BTS, trader B has a limit order selling 100 BTS at 0.09 USD per BTS,
which means B's limit order has a better price for the opposite side to buy.
Now if trader C placed an order that buys 10 BTS at 0.105 USD per BTS, A's
margin call order will take precedence, B's limit order won't be matched. That
means C is forced to "buy high" when have chance to "buy low", which is
unexpected.

Users have been confused by this behavior, as discussed in [bitshares-core
issue #625](https://github.com/bitshares/bitshares-core/issues/625) and other
threads.

Another scenario is described in [Bitshares-core
issue #453](https://github.com/bitshares/bitshares-core/issues/453)
that sometimes a taker margin call order may be matched with a maker limit order
which is not on the top of the order book. This can be seen as a bug.

# Rationale

Always matching taker order (e.g. a buy) with a maker order which offered best
price (aka lowest ask), is a simpler rule which most users would understand
easily.

There is a parameter in price feed named MSSR, which stands for "maximum short
squeeze ratio". Maker price of margin call orders is MSSP, which stands for
"maximum short squeeze price", is calculated as `feed_price / MSSR`.
Note: `feed_price` here is in terms of debt/collateral, aka "how much debt per
collateral".

That said, the price that margin call orders are offering is MSSP. The prices
those limit orders are offering are the limit prices.

When placing a limit (e.g. buy) order with a price beyond the lowest sell,
the order is expected to "walk the book", matching each order on the opposite
side in turn, at that order's price, until the new limit order is completely
filled, or there is no more sell order matching its price.

To meet the expectation,
* firstly, we need to match the limit buy order with the limit sell orders
  whose prices are lower than MSSP and prices can match the new order;
* then, if the new limit buy order hasn't been completely filled, match it with
  the margin calls if MSSP can match the new order's price;
* then, if the new limit buy order still hasn't been completely filled, match it
  with the rest sell orders until it's completely filled or no more sell order
  matching its price


# Specifications

## Matching a taker limit order

New limit order is being processed in `apply_order(...)` function of `database`
class.

Currently, in the function, firstly call orders will be checked and matched.
After that, limit orders on the opposite side will be checked and matched.

Need to change the logic to:
1. firstly, sort the limit orders on the opposite by `price`, best price first,
   end at MSSP; check them one by one, calls `match(...)` function until the
   return value is not `2` which means the new order is completely filled;
2. if reach the end (MSSP), which means the new order is still unfilled,
   call `check_call_orders(..)` function or an equivalent;
3. check if `new_order_object` still exist, if yes, redo step one but set the
   maximum possible price of the market as end price.

## Matching taker margin call orders

For [bitshares-core
issue #453](https://github.com/bitshares/bitshares-core/issues/453),
in `check_call_orders(...)` function of `database` class,
iterator `limit_itr` will move forward when variable `filled_limit` is `true`.
`filled_limit` will be set to `true` when a limit order get completely filled.
However, since `filled_limit` is declared out of the `while` block,
it doesn't get reset to `false` after `limit_itr` moved forward. That means
after the first limit order get completedly filled, `filled_limit` will always
be `true`, so `limit_itr` will always move forward no matter whether *current*
limit order got completedly filled, so a taker call order may match
with a limit order that is not on the top of the order book.

To fix this, need to change the code to make sure `limit_itr` always references
the limit order on the top of the order book when dereferencing.

# Discussion

[to be added if any]

# Summary for Shareholders

[to be added if any]

# Copyright

This document is placed in the public domain.

# See Also

* https://github.com/bitshares/bitshares-core/issues/625
* https://github.com/bitshares/bitshares-core/issues/453
* https://bitsharestalk.org/index.php?topic=25926.0