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                                          XIN Feature Currencies
                                     Infinity Foundation May, 2017, v1


Content:

 1. Currency Overview
 2. Currency Properties
 3. Currency Usage Examples


1. Currency Overview

 The currency entity is the basic building block of the XIN Currencies System. A currency has a unique 
 name and code and uniqueness is guaranteed by the protocol, currencies can be deleted and their code 
 can be reused under certain conditions. The total currency supply is divisible into currency units. 
 Like assets, currency units supports decimal positions implemented as a client side feature. The 
 maximum number of currency units which can be issued per currency is similar to XIN 10^9 * 10^8. 
 The actual maximum units supply is set by the currency issuer. The currency issuer is the account 
 which issues the currency and pays the issuance fee. The issuer is responsible for setting the 
 currency properties and in some configurations has additional control over the currency usage. 
 Like asset balance, currency units can be transferred between accounts.

2. Currency Properties

 The currency entity supports several properties. Properties can be mixed and matched in various ways 
 to compose the currency type. The currency type then controls the inner workings of the currency. 
 The list of available currency properties is as follows:

 - EXCHANGEABLE 
 The currency can be exchanged with XIN. Holders of the currency can publish an exchange offer 
 specifying the buy and sell rate of the currency, in much the same way as banks or currency exchanges 
 publish their exchange rates. Each account can publish only a single exchange offer at any given time. 
 Exchange offers have an expiry block after which they are no longer in effect. Buyers and sellers can 
 issue exchange requests to match published exchange offers. Unlike asset bid/ask orders, exchange requests 
 are not saved, they are either executed immediately (fully or partially) or not executed at all. A match 
 of exchange offer with a buy or sell exchange request creates an exchange entity which represents 
 the transfer of currency units, in return, to XIN balance and causes the relevant account balances 
 to update. Issuing an exchange offer reduces the XIN and currency balance of the offering account 
 temporarily until the offer expires. Exchange offers also specify a limit on the number of exchanged 
 units which can be larger than the number of units offered. When a buy exchange request matches an 
 exchange offer the number of units offered for sell is reduced and the number of units offered for 
 buy is increased as long as the limit has not been reached. Once the exchange limit of an exchange 
 offer has been reached, this exchange offer can no longer be used.
 
 - CONTROLLABLE 
 Currency property suitable for currencies which need to track an external entity. It imposes the 
 following limitations on the currency: (1) Currency can be transferred only to/from the issuer 
 account (2) only the issuer account can publish exchange offers. The issuer account can issue a 
 large (practically infinite) supply of units in advance, then transfer units to accounts, or offer 
 to exchange units, to reflect actual transactions which take place in an external system. The large 
 supply of units in the issuer account can be used to mimic the effect of creating units out of 
 nowhere to support features such as creating new units and interest payments.
 
 - RESERVABLE 
 Currency units are not issued immediately. Instead the currency issuer sets a block height 
 by which the currency is to be issued and a limit of XIN per unit needed in order to issue the currency. 
 Currency ‘founders’ then spend their XIN to reserve their currency stake. If the amount of XIN per unit 
 needed in order to issue the currency is not reserved before reaching the block height the issuance is 
 cancelled and funds are returned minus fees. If the required reserve is allocated, the currency is issued 
 and units are split between founders according to their proportional stake of invested XIN. In case of 
 rounding, leftovers are sent to the issuer account.
 
 - CLAIMABLE 
 Currency units of reservable currency can later be claimed at the same XIN per unit rate reached 
 when reserving the currency. The ability to claim a currency at a certain rate imposes some practical 
 limits on the rates at which users would want to exchange it. However claimable currency can also be 
 exchanged if only for the purpose of exchanging the whole currency supply, so that the currency can 
 be deleted.
 
 - Currency Exchange
 For exchangeable currency, each currency holder account, can publish a single exchange offer specifying 
 the buy rate and sell rate vs XIN and the number of units she is willing to exchange (which cannot 
 exceed her available currency units and XIN balance). Users can observe all currency exchange offers 
 (intuitively similar to fiat exchange offices) and try to match them with buy/sell exchange requests. 
 An exchange offer has an expiration height, as well as a limit on the total number of units which can 
 be exchanged. When units are bought from an exchange offer the number of units to sell increases 
 automatically and vise versa. The publisher can also limit the total transaction volume of currency 
 units traded for a specific exchange offer.
 
Store of Value
 
 The combination of RESERVABLE and CLAIMABLE properties can be used to allocate initial value for a 
 currency by locking XIN. Once the currency is activated the reserved XIN are locked and the only 
 way to release them is to claim back the currency units in return for XIN. This provides the currency 
 with a value based on the locked XIN balance. 
 
3. Currency Usage Examples
 
 Special interest token, internal company token, micro-finance, charity, game token, coupons, 
 vouchers, commodity pegging, crowdfunding, crosschain exchange.
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