RAM购买机制结合多种合约详解

eosio.system.cpp
// native.hpp (newaccount definition is actually in eosio.system.cpp)
(newaccount)(updateauth)(deleteauth)(linkauth)(unlinkauth)(canceldelay)(onerror)
// eosio.system.cpp
(setram)(setparams)(setpriv)(rmvproducer)(bidname)
// delegate_bandwidth.cpp
(buyrambytes)(buyram)(sellram)(delegatebw)(undelegatebw)(refund)
// voting.cpp
(regproducer)(unregprod)(voteproducer)(regproxy)
// producer_pay.cpp
(onblock)(claimrewards)
ram,cpu和net操作相关方法的都定义在delegate_bandwidth.cpp，其中和RAM相关的是buyrambytes(通过指定字节数购买ram),buyram(通过指定货币购买ram)。
delegate_bandwidth.cpp
//根据当前市场的份额，将需要购买的字节数转化为指定的EOS进行购买
void system_contract::buyrambytes( account_name payer, account_name receiver, uint32_t bytes ) {
   //在数据库中查询RAMCORE发行量，默认为100000000000000
   auto itr = _rammarket.find(S(4,RAMCORE));
   auto tmp = *itr;
   auto eosout = tmp.convert( asset(bytes,S(0,RAM)), CORE_SYMBOL );
   //通过转化后，调用buyram使用EOS购买
   buyram( payer, receiver, eosout );
}
解析
RAM的交易机制采用Bancor算法，使每字节的价格保持不变，通过中间代币(RAMCORE)来保证EOS和RAM之间的交易流通性。从上源码看首先获得RAMCORE的发行量，再通过tmp.convert方法RAM->RAMCORE，RAMCORE->EOS(CORE_SYMBOL)再调用buyram进行购买。这里的CORE_SYMBOL不一定是指EOS，查看core_symbol.hpp，发现源码内定义为SYS，也就是说在没有修改的前提下，需要提前发行SYS代币，才能进行RAM购买。
core_symbol.hpp
#define CORE_SYMBOL S(4,SYS)
delegate_bandwidth.cpp
void system_contract::buyram(account_name payer, account_name receiver, asset quant)
{
   //验证权限
   require_auth(payer);
   //不能为0
   eosio_assert(quant.amount > 0, "must purchase a positive amount");
   auto fee = quant;
   //手续费为0.5%，如果amoun为1则手续费为1，如果小于1，则手续费在amoun 0)
   {
         INLINE_ACTION_SENDER(eosio::token, transfer)
         (N(eosio.token), {payer, N(active)},
         {payer, N(eosio.ramfee), fee, std::string("ram fee")});
   }
   int64_t bytes_out;
   //根据ram市场里的EOS和RAM实时汇率计算出能够购买的RAM总量
   const auto &market = _rammarket.get(S(4, RAMCORE), "ram market does not exist");
   _rammarket.modify(market, 0, [&](auto &es) {
         //转化方法请参考下半部分
         bytes_out = es.convert(quant_after_fee, S(0, RAM)).amount;
   });
   //剩余总量大于0判断
   eosio_assert(bytes_out > 0, "must reserve a positive amount");
   //更新全局变量，总共可以使用的内存大小
   _gstate.total_ram_bytes_reserved += uint64_t(bytes_out);
   //更新全局变量，购买RAM冻结总金额
   _gstate.total_ram_stake += quant_after_fee.amount;
   user_resources_table userres(_self, receiver);
   auto res_itr = userres.find(receiver);
   if (res_itr == userres.end())
   {
         //在userres表中添加ram相关记录
         res_itr = userres.emplace(receiver, [&](auto &res) {
               res.owner = receiver;
               res.ram_bytes = bytes_out;
         });
   }
   else
   {
         //在userres表中修改ram相关记录
         userres.modify(res_itr, receiver, [&](auto &res) {
               res.ram_bytes += bytes_out;
         });
   }
   //更新账号的RAM拥有量
   set_resource_limits(res_itr->owner, res_itr->ram_bytes, res_itr->net_weight.amount, res_itr->cpu_weight.amount);
}
相关注释已经在代码中，这里还会用到一个比较重要的内容，那就是代币转化为RAM的公式，此方法请参考下面，这方面还未深入研究会在接下来补充。
exchange_state.cpp
asset exchange_state::convert_to_exchange( connector& c, asset in ) {
   real_type R(supply.amount); //RAM已经售出的总量
   real_type C(c.balance.amount+in.amount);  //RAM总购买金额+本次购买的量
   real_type F(c.weight/1000.0);
   real_type T(in.amount);
   real_type ONE(1.0);
   real_type E = -R * (ONE - std::pow( ONE + T / C, F) );//换算出EOS对应的RAM量
   //print( "E: ", E, "\n");
   int64_t issued = int64_t(E);
   supply.amount += issued;//更新RAM已经售出的总量
   c.balance.amount += in.amount;//更新RAM总购买金额
   return asset( issued, supply.symbol );
}

asset exchange_state::convert( asset from, symbol_type to ) {
   auto sell_symbol  = from.symbol;
   auto ex_symbol = supply.symbol;
   auto base_symbol  = base.balance.symbol;
   auto quote_symbol = quote.balance.symbol;

   //根据币种转化可以购买的RAM量
   if( sell_symbol != ex_symbol ) {
      if( sell_symbol == base_symbol ) {
         from = convert_to_exchange( base, from );
      } else if( sell_symbol == quote_symbol ) {
         from = convert_to_exchange( quote, from );
      } else {
         eosio_assert( false, "invalid sell" );
      }
   } else {
      if( to == base_symbol ) {
         from = convert_from_exchange( base, from );
      } else if( to == quote_symbol ) {
         from = convert_from_exchange( quote, from );
      } else {
         eosio_assert( false, "invalid conversion" );
      }
   }
   if( to != from.symbol )
      return convert( from, to );
   return from;
}
convert_to_exchange的转化公式如下。

ram购买的量是一个绝对值，是根据购买EOS的金额和当前市场内ram的数量计算出来的。一般来说在ram总量不增加的情况下，一样金额的EOS，所能获得的ram会越来越少。所以如果早期你购买了ram，然后过段时间后通过sellram卖掉ram可能还能挣钱。

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