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0x00 摘要

交易(transaction)是比特币甚至所有区块链中最核心的数据结构之一，可以说其他所有的模块都是为交易服务的，包括交易的产生、广播、共识、存储等等，所以我们首先从交易出发，然后逐步延伸到其他的部分。

0x01 COutPoint

/** An outpoint - a combination of a transaction hash and an index n into its vout. * COutPoint主要用在交易的输入CTxIn中，用来确定当前输出的来源， * 包括前一笔交易的hash，以及对应前一笔交易中的第几个输出的序列号。 */
class COutPoint
{
public:
    uint256 hash; // 交易的哈希
    uint32_t n;  // 对应的序列号

    COutPoint() { SetNull(); }
    COutPoint(uint256 hashIn, uint32_t nIn) { hash = hashIn; n = nIn; }

    ADD_SERIALIZE_METHODS;  // 用来序列化数据结构，方便存储和传输

    template <typename Stream, typename Operation>
    inline void SerializationOp(Stream& s, Operation ser_action) {
        READWRITE(hash);
        READWRITE(n);
    }

    void SetNull() { hash.SetNull(); n = (uint32_t) -1; }
    bool IsNull() const { return (hash.IsNull() && n == (uint32_t) -1); }

    //重载小于号
    friend bool operator<(const COutPoint& a, const COutPoint& b)
    {
        int cmp = a.hash.Compare(b.hash);
        return cmp < 0 || (cmp == 0 && a.n < b.n);
    }

    friend bool operator==(const COutPoint& a, const COutPoint& b)
    {
        return (a.hash == b.hash && a.n == b.n);
    }

    friend bool operator!=(const COutPoint& a, const COutPoint& b)
    {
        return !(a == b);
    }

    std::string ToString() const;
};

0x02 CTxIn

/** An input of a transaction. It contains the location of the previous * transaction's output that it claims and a signature that matches the * output's public key. * 交易的输入，包括当前输入对应前一笔交易的输出的位置，以及花费前一笔输出需要的签名脚本 * CScriptWitness是用来支持隔离见证时使用的。 */
class CTxIn
{
public:
    COutPoint prevout;  // 前一笔交易输出的位置
    CScript scriptSig;  // 解锁脚本
    uint32_t nSequence;  // 序列号
    CScriptWitness scriptWitness; //! Only serialized through CTransaction

    /* Setting nSequence to this value for every input in a transaction * disables nLockTime. * 规则1：如果一笔交易中所有的SEQUENCE_FINAL都被赋值了相应的nSequence，那么nLockTime就会被禁用*/
    static const uint32_t SEQUENCE_FINAL = 0xffffffff;

    /* Below flags apply in the context of BIP 68*/
    /* If this flag set, CTxIn::nSequence is NOT interpreted as a * relative lock-time. * 规则2：如果设置了这个变量，那么规则1就失效了*/
    static const uint32_t SEQUENCE_LOCKTIME_DISABLE_FLAG = (1 << 31);

    /* If CTxIn::nSequence encodes a relative lock-time and this flag * is set, the relative lock-time has units of 512 seconds, * otherwise it specifies blocks with a granularity of 1. * 规则3：如果规则1有效并且设置了此变量，那么相对锁定时间就为512秒，否则锁定时间就为1个区块*/
    static const uint32_t SEQUENCE_LOCKTIME_TYPE_FLAG = (1 << 22);

    /* If CTxIn::nSequence encodes a relative lock-time, this mask is * applied to extract that lock-time from the sequence field. * 规则4：如果规则1有效，那么这个变量就用来从nSequence计算对应的锁定时间*/
    static const uint32_t SEQUENCE_LOCKTIME_MASK = 0x0000ffff;

    /* In order to use the same number of bits to encode roughly the * same wall-clock duration, and because blocks are naturally * limited to occur every 600s on average, the minimum granularity * for time-based relative lock-time is fixed at 512 seconds. * Converting from CTxIn::nSequence to seconds is performed by * multiplying by 512 = 2^9, or equivalently shifting up by * 9 bits. */
    static const int SEQUENCE_LOCKTIME_GRANULARITY = 9;

    CTxIn()
    {
        nSequence = SEQUENCE_FINAL;
    }

    // 禁用隐式转换，构造函数必须明确使用当前形式
    explicit CTxIn(COutPoint prevoutIn, CScript scriptSigIn=CScript(), uint32_t nSequenceIn=SEQUENCE_FINAL);
    CTxIn(uint256 hashPrevTx, uint32_t nOut, CScript scriptSigIn=CScript(), uint32_t nSequenceIn=SEQUENCE_FINAL);

    ADD_SERIALIZE_METHODS;

    template <typename Stream, typename Operation>
    inline void SerializationOp(Stream& s, Operation ser_action) {
        READWRITE(prevout);
        READWRITE(*(CScriptBase*)(&scriptSig));
        READWRITE(nSequence);
    }

    friend bool operator==(const CTxIn& a, const CTxIn& b)
    {
        return (a.prevout   == b.prevout &&
                a.scriptSig == b.scriptSig &&
                a.nSequence == b.nSequence);
    }

    friend bool operator!=(const CTxIn& a, const CTxIn& b)
    {
        return !(a == b);
    }

    std::string ToString() const;
};

0x03 CTxOut

/** An output of a transaction. It contains the public key that the next input must be able to sign with to claim it. * 交易的输出，包含金额和锁定脚本 */
class CTxOut
{
public:
    CAmount nValue;  // 输出金额
    CScript scriptPubKey;  // 锁定脚本

    CTxOut()
    {
        SetNull();
    }

    CTxOut(const CAmount& nValueIn, CScript scriptPubKeyIn);

    ADD_SERIALIZE_METHODS;

    template <typename Stream, typename Operation>
    inline void SerializationOp(Stream& s, Operation ser_action) {
        READWRITE(nValue);
        READWRITE(*(CScriptBase*)(&scriptPubKey));
    }

    void SetNull()
    {
        nValue = -1;
        scriptPubKey.clear();
    }

    bool IsNull() const
    {
        return (nValue == -1);
    }

    // 获取dust阈值，一笔交易如果交易费小于dust阈值，就会被认为是dust tx， 
    // 此函数在最新版本中已转移到src/policy/policy.h中
    CAmount GetDustThreshold(const CFeeRate &minRelayTxFee) const
    {
        // "Dust" is defined in terms of CTransaction::minRelayTxFee,
        // which has units satoshis-per-kilobyte.
        // If you'd pay more than 1/3 in fees
        // to spend something, then we consider it dust.
        // A typical spendable non-segwit txout is 34 bytes big, and will
        // need a CTxIn of at least 148 bytes to spend:
        // so dust is a spendable txout less than
        // 546*minRelayTxFee/1000 (in satoshis).
        // A typical spendable segwit txout is 31 bytes big, and will
        // need a CTxIn of at least 67 bytes to spend:
        // so dust is a spendable txout less than
        // 294*minRelayTxFee/1000 (in satoshis).
        /** "Dust"是根据CTransaction中的minRelayTxFee来定义的，单位是satoshis/千字节， * 如果在一笔交易中交易费占了1/3以上，那么我们就认为该交易是"Dust"交易。 * 因此dust交易的金额小于 546*minRelayTxFee/1000; * 而在支持隔离见证的交易中，txout通常大小为31字节，CTxIn大小至少为67字节， * 此时dust交易的金额则一般小于 294*minRelayTxFee/1000. */
        if (scriptPubKey.IsUnspendable())  // 判断脚本格式是否正确
            return 0;

        size_t nSize = GetSerializeSize(*this, SER_DISK, 0);
        int witnessversion = 0;
        std::vector<unsigned char> witnessprogram;

        // 判断是否支持隔离见证
        if (scriptPubKey.IsWitnessProgram(witnessversion, witnessprogram)) {
            // sum the sizes of the parts of a transaction input
            // with 75% segwit discount applied to the script size.
            nSize += (32 + 4 + 1 + (107 / WITNESS_SCALE_FACTOR) + 4);
        } else {
            nSize += (32 + 4 + 1 + 107 + 4); // the 148 mentioned above
        }

        return 3 * minRelayTxFee.GetFee(nSize);
    }

    bool IsDust(const CFeeRate &minRelayTxFee) const
    {
        return (nValue < GetDustThreshold(minRelayTxFee));
    }

    friend bool operator==(const CTxOut& a, const CTxOut& b)
    {
        return (a.nValue       == b.nValue &&
                a.scriptPubKey == b.scriptPubKey);
    }

    friend bool operator!=(const CTxOut& a, const CTxOut& b)
    {
        return !(a == b);
    }

    std::string ToString() const;
};

0x04 CTransaction

/** The basic transaction that is broadcasted on the network and contained in blocks. * A transaction can contain multiple inputs and outputs. * 下面就是在网络中广播然后被打包进区块的最基本的交易的结构，一个交易可能包含多个交易输入和输出。 */
class CTransaction
{
public:
    // Default transaction version. 默认交易版本
    static const int32_t CURRENT_VERSION=2;

    // Changing the default transaction version requires a two step process: first
    // adapting relay policy by bumping MAX_STANDARD_VERSION, and then later date
    // bumping the default CURRENT_VERSION at which point both CURRENT_VERSION and
    // MAX_STANDARD_VERSION will be equal.
    static const int32_t MAX_STANDARD_VERSION=2;

    // The local variables are made const to prevent unintended modification
    // without updating the cached hash value. However, CTransaction is not
    // actually immutable; deserialization and assignment are implemented,
    // and bypass the constness. This is safe, as they update the entire
    // structure, including the hash.
    /** 下面这些变量都被定义为常量类型，从而避免无意识的修改了交易而没有更新缓存的hash值； * 但还是可以通过重新构造一个交易然后赋值给当前交易来进行修改，这样就更新了交易的所有内容 */
    const int32_t nVersion;  // 版本
    const std::vector<CTxIn> vin; // 交易输入
    const std::vector<CTxOut> vout; // 交易输出
    const uint32_t nLockTime; // 锁定时间

private:
    /** Memory only. */
    const uint256 hash;

    uint256 ComputeHash() const;

public:
    /** Construct a CTransaction that qualifies as IsNull() */
    CTransaction();

    /** Convert a CMutableTransaction into a CTransaction. */
    CTransaction(const CMutableTransaction &tx);
    CTransaction(CMutableTransaction &&tx);

    template <typename Stream>
    inline void Serialize(Stream& s) const {
        SerializeTransaction(*this, s);
    }

    /** This deserializing constructor is provided instead of an Unserialize method. * Unserialize is not possible, since it would require overwriting const fields. */
    template <typename Stream>
    CTransaction(deserialize_type, Stream& s) : CTransaction(CMutableTransaction(deserialize, s)) {}

    bool IsNull() const {
        return vin.empty() && vout.empty();
    }

    const uint256& GetHash() const {
        return hash;
    }

    // Compute a hash that includes both transaction and witness data
    uint256 GetWitnessHash() const;

    // Return sum of txouts.
    CAmount GetValueOut() const; // 返回交易输出金额之和
    // GetValueIn() is a method on CCoinsViewCache, because
    // inputs must be known to compute value in.

    /** * Get the total transaction size in bytes, including witness data. * "Total Size" defined in BIP141 and BIP144. * @return Total transaction size in bytes */
    unsigned int GetTotalSize() const; // 返回交易大小

    bool IsCoinBase() const  // 判断是否是coinbase交易
    {
        return (vin.size() == 1 && vin[0].prevout.IsNull());
    }

    friend bool operator==(const CTransaction& a, const CTransaction& b)
    {
        return a.hash == b.hash;
    }

    friend bool operator!=(const CTransaction& a, const CTransaction& b)
    {
        return a.hash != b.hash;
    }

    std::string ToString() const;

    bool HasWitness() const
    {
        for (size_t i = 0; i < vin.size(); i++) {
            if (!vin[i].scriptWitness.IsNull()) {
                return true;
            }
        }
        return false;
    }
};

除了这个结构以外，还定义了一个CMutableTransaction其中的变量内容和CTransaction相同，但是都是可以直接修改的，但是最后广播和网络中传输的类型都是CTransaction。

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