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+# Creates a decentralized market between any two subcurrencies
+
+# Here, the first subcurrency is the base asset and the second
+# subcurrency is the asset priced against the base asset. Hence,
+# "buying" refers to trading the first for the second, and
+# "selling" refers to trading the second for the first
+
+# storage 0: buy orders
+# storage 1: sell orders
+# storage 1000: first subcurrency
+# storage 1001: last first subcurrency txid
+# storage 2000: second subcurrency
+# storage 2001: last second subcurrency txid
+# storage 3000: current epoch
+# storage 4000: price
+# storage 4001: volume
+
+init:
+ # Heap for buy orders
+ contract.storage[0] = create('heap.se')
+ # Heap for sell orders
+ contract.storage[1] = create('heap.se')
+code:
+ # Initialize with [ first_subcurrency, second_subcurrency ]
+ if !contract.storage[1000]:
+ contract.storage[1000] = msg.data[0] # First subcurrency
+ contract.storage[1001] = -1
+ contract.storage[2000] = msg.data[1] # Second subcurrency
+ contract.storage[2001] = -1
+ contract.storage[3000] = block.number / 1000
+ stop
+ first_subcurrency = contract.storage[1000]
+ second_subcurrency = contract.storage[2000]
+ buy_heap = contract.storage[0]
+ sell_heap = contract.storage[1]
+ # This contract operates in "epochs" of 100 blocks
+ # At the end of each epoch, we process all orders
+ # simultaneously, independent of order. This algorithm
+ # prevents front-running, and generates a profit from
+ # the spread. The profit is permanently kept in the
+ # market (ie. destroyed), making both subcurrencies
+ # more valuable
+
+ # Epoch transition code
+ if contract.storage[3000] < block.number / 100:
+ done = 0
+ volume = 0
+ while !done:
+ # Grab the top buy and sell order from each heap
+ topbuy = call(buy_heap, 1)
+ topsell = call(sell_heap, 1)
+ # An order is recorded in the heap as:
+ # Buys: (2^48 - 1 - price) * 2^208 + units of first currency * 2^160 + from
+ # Sells: price * 2^208 + units of second currency * 2^160 + from
+ buyprice = -(topbuy / 2^208)
+ buyfcvalue = (topbuy / 2^160) % 2^48
+ buyer = topbuy % 2^160
+ sellprice = topsell / 2^208
+ sellscvalue = (topsell / 2^160) % 2^48
+ seller = topsell % 2^160
+ # Heap empty, or no more matching orders
+ if not topbuy or not topsell or buyprice < sellprice:
+ done = 1
+ else:
+ # Add to volume counter
+ volume += buyfcvalue
+ # Calculate how much of the second currency the buyer gets, and
+ # how much of the first currency the seller gets
+ sellfcvalue = sellscvalue / buyprice
+ buyscvalue = buyfcvalue * sellprice
+ # Send the currency units along
+ call(second_subcurrency, [buyer, buyscvalue], 2)
+ call(first_subcurrency, [seller, sellfcvalue], 2)
+ if volume:
+ contract.storage[4000] = (buyprice + sellprice) / 2
+ contract.storage[4001] = volume
+ contract.storage[3000] = block.number / 100
+ # Make buy order [0, price]
+ if msg.data[0] == 0:
+ # We ask the first subcurrency contract what the last transaction was. The
+ # way to make a buy order is to send the amount of first currency units that
+ # you wish to buy with, and then immediately call this contract. For security
+ # it makes sense to set up a tx which sends both messages in sequence atomically
+ data = call(first_subcurrency, [], 0, 4)
+ from = data[0]
+ to = data[1]
+ value = data[2]
+ txid = data[3]
+ price = msg.data[1]
+ if txid > contract.storage[1001] and to == contract.address:
+ contract.storage[1001] = txid
+ # Adds the order to the heap
+ call(buy_heap, [0, -price * 2^208 + (value % 2^48) * 2^160 + from], 2)
+ # Make sell order [1, price]
+ elif msg.data[0] == 1:
+ # Same mechanics as buying
+ data = call(second_subcurrency, [], 0, 4)
+ from = data[0]
+ to = data[1]
+ value = data[2]
+ txid = data[3]
+ price = msg.data[1]
+ if txid > contract.storage[2001] and to == contract.address:
+ contract.storage[2001] = txid
+ call(sell_heap, [0, price * 2^208 + (value % 2^48) * 2^160 + from], 2)
+ # Ask for price
+ elif msg.data[0] == 2:
+ return(contract.storage[4000])
+ # Ask for volume
+ elif msg.data[0] == 3:
+ return(contract.storage[1000])
+ # Ask for first currency
+ elif msg.data[0] == 4:
+ return(contract.storage[2000])
+ # Ask for second currency
+ elif msg.data[0] == 5:
+ return(contract.storage[4001])