EOS自帶ram購買智能合約，在eosio.system中的exchange_state.cpp，ram的購買核心思想是bancor算法，使用EOS購買RAM時，其實(shí)先從EOS通過bancor轉(zhuǎn)換成中間代幣RAMCORE，再從RAMCORE通過bancor轉(zhuǎn)換成為RAM

EOS==(bancor算法)===》RAMCORE==(bancor算法)===》RAM

這里，RAMCORE是子幣，EOS和RAM是母幣，EOS和ram之間的轉(zhuǎn)換是這2種母幣通過子幣RAMCORE來實(shí)現(xiàn)兌換，如果你看過bancor算法白皮書以及bancor網(wǎng)絡(luò)在以太網(wǎng)上的實(shí)現(xiàn)的話，exchange_state.cpp中的bancor算法在公式上其實(shí)是錯誤的，即使在最新的代碼中我也沒看到有修改過來。EOS和RAM之間的轉(zhuǎn)換有一種快速的轉(zhuǎn)換方法，這里因?yàn)镕系數(shù)一樣的所以中間的2步轉(zhuǎn)換可以抵消掉，如下：

R1 = FP1S
R2 = FP2S

R1：當(dāng)前RAM余量
P1：當(dāng)前S以R1定價的價格
R2：當(dāng)前EOS余量
P2：當(dāng)前S以R2定價的價格
S：當(dāng)前RAMCORE余量

上述因?yàn)镕是相同的，最終 R1/R2 = P1/P2，推出

pr = R2/R1
pe = R1/R2

pr：ram的價格以eos計價
pe：eos的價格以ram計價

ram的價格（以eos計價）pr = R2/R1，可以發(fā)現(xiàn)ram的價格只跟R1和R2相關(guān)，要想提高ram的價格可以降低R1余量或者提高R2余量來實(shí)現(xiàn)，要想降低ram的價格可以通過提高R1余量或者降低R2余量來實(shí)現(xiàn)。下面給出ram購買算法以及添加了快速購買quick_conver的python版實(shí)現(xiàn)

• 下面快速轉(zhuǎn)換quick_conver的推導(dǎo)過程：

1）out1 = ((1 + asset_in/R1)F - 1) * S
2）S = S + out1
3）out2 = (1 - (1 - out1/S)(1/F)) * R2

asset_in：要購買ram的eos數(shù)量
R1：EOS余量
S：RAMCORE余量
out1：購買的RAMCORE數(shù)量
R2：RAM余量
out2：購買的RAM數(shù)量

最終 out2 = (asset_in/(R1+asset_in))*R2

#-*- coding: UTF-8 -*-

class exchange(object):

    def __init__(self, quote_eos, quote_ram, quote_coreram, F):
        self.quote_eos = quote_eos
        self.quote_ram = quote_ram
        self.quote_coreram = quote_coreram
        self.F = F

    def convert_to_exchange(self, asset_in, symbol):
        # Bancor算法  out = ((1 + asset_in/R)**F - 1) * S
        if symbol == 'eos':
            #out = ((1 + asset_in/(self.quote_eos+asset_in))**self.F - 1) * self.quote_coreram  eos代碼中的公式其實(shí)是錯的
            out = ((1 + asset_in/self.quote_eos)**self.F - 1) * self.quote_coreram
            self.quote_eos += asset_in
        elif symbol == 'ram':
            #out = ((1 + asset_in/(self.quote_ram+asset_in))**self.F - 1) * self.quote_coreram  eos代碼中的公式其實(shí)是錯的
            out = ((1 + asset_in/self.quote_ram)**self.F - 1) * self.quote_coreram
            self.quote_ram += asset_in
        self.quote_coreram += out

        return out

    def convert_from_exchange(self, asset_in, symbol):
        # Bancor算法  out = (1 - (1 - asset_in/S)**(1/F)) * R
        if symbol == 'eos':
            #out = ((1 + asset_in/(self.quote_coreram-asset_in))**(1/self.F) - 1) * self.quote_eos  eos代碼中的公式其實(shí)是錯的
            out = (1 - (1 - asset_in/self.quote_coreram)**(1/self.F)) * self.quote_eos
            self.quote_eos -= out
        elif symbol == 'ram':
            #out = ((1 + asset_in/(self.quote_coreram-asset_in))**(1/self.F) - 1) * self.quote_ram  eos代碼中的公式其實(shí)是錯的
            out = (1 - (1 - asset_in/self.quote_coreram)**(1/self.F)) * self.quote_ram
            self.quote_ram -= out
        self.quote_coreram -= asset_in

        return out

    # ram_out = quick_conver(eos_in)  等價于 ram_out = convert_from_exchange(convert_to_exchange(eos_in, 'eos'), 'ram')
    def quick_conver(self, eos_in):
        self.quote_eos += eos_in
        ram = (eos_in/self.quote_eos)*self.quote_ram
        self.quote_ram -= ram

        return ram


def func():
    ec = exchange(1000000., 64*1024*1024*1024., 100000000000., 0.5)
    ram = ec.convert_from_exchange(ec.convert_to_exchange(100, 'eos'), 'ram')
    ram = ec.convert_from_exchange(ec.convert_to_exchange(1000, 'eos'), 'ram')
    print(ram)


def func2():
    ec = exchange(1000000., 64*1024*1024*1024., 100000000000., 0.5)
    ram = ec.quick_conver(100)
    ram = ec.quick_conver(1000)
    print(ram)


if __name__ == '__main__':
    #驗(yàn)證eos到ram的快速轉(zhuǎn)換
    func()
    func2()

輸出結(jié)果：

> python3 et3.py
68637104.66032256
68637104.66032611

可以看到是等價的，所以當(dāng)F值相同時，ram的買賣完全可以通過上述的quick_conver來代替，可惜在exchange_state.cpp中沒有quick_conver的實(shí)現(xiàn)