Repository: bitcoin/bitcoin
File: src/policy/feerate.h (C/C++)

Code before:

const std::string CURRENCY_UNIT = "BTC"; // One formatted unit
const std::string CURRENCY_ATOM = "sat"; // One indivisible minimum value unit

/* Used to determine type of fee estimation requested */
enum class FeeEstimateMode {
    UNSET,        //!< Use default settings based on other criteria
    ECONOMICAL,   //!< Force estimateSmartFee to use non-conservative estimates
    CONSERVATIVE, //!< Force estimateSmartFee to use conservative estimates
    BTC_KVB,      //!< Use BTC/kvB fee rate unit
    SAT_VB,       //!< Use sat/vB fee rate unit
};

/**
 * Fee rate in satoshis per kilovirtualbyte: CAmount / kvB
 */
class CFeeRate
{
private:
    /** Fee rate in sat/kvB (satoshis per 1000 virtualbytes) */
    CAmount nSatoshisPerK;

public:
    /** Fee rate of 0 satoshis per kvB */
    CFeeRate() : nSatoshisPerK(0) { }
    template<std::integral I> // Disallow silent float -> int conversion
    explicit CFeeRate(const I _nSatoshisPerK): nSatoshisPerK(_nSatoshisPerK) {
    }

    /**
     * Construct a fee rate from a fee in satoshis and a vsize in vB.
     *
     * param@[in]   nFeePaid    The fee paid by a transaction, in satoshis
     * param@[in]   num_bytes   The vsize of a transaction, in vbytes
     */
    CFeeRate(const CAmount& nFeePaid, uint32_t num_bytes);

    /**
     * Return the fee in satoshis for the given vsize in vbytes.
     * If the calculated fee would have fractional satoshis, then the
     * returned fee will always be rounded up to the nearest satoshi.
     */
    CAmount GetFee(uint32_t num_bytes) const;

    /**
     * Return the fee in satoshis for a vsize of 1000 vbytes
     */
    CAmount GetFeePerK() const { return nSatoshisPerK; }
    friend bool operator<(const CFeeRate& a, const CFeeRate& b) { return a.nSatoshisPerK < b.nSatoshisPerK; }
    friend bool operator>(const CFeeRate& a, const CFeeRate& b) { return a.nSatoshisPerK > b.nSatoshisPerK; }
    friend bool operator==(const CFeeRate& a, const CFeeRate& b) { return a.nSatoshisPerK == b.nSatoshisPerK; }

Review comment (suggestion):
nit: I think it should be clear that when using this constructor we also have 0 virtual bytes, I think it is possible to have 0 fee rate per n virtual bytes I think.
```suggestion
    /** Fee rate of 0 satoshis per 0 vB */
```

Code after:
#include <type_traits>

const std::string CURRENCY_UNIT = "BTC"; // One formatted unit
const std::string CURRENCY_ATOM = "sat"; // One indivisible minimum value unit

/* Used to determine type of fee estimation requested */
enum class FeeEstimateMode {
    UNSET,        //!< Use default settings based on other criteria
    ECONOMICAL,   //!< Force estimateSmartFee to use non-conservative estimates
    CONSERVATIVE, //!< Force estimateSmartFee to use conservative estimates
    BTC_KVB,      //!< Use BTC/kvB fee rate unit
    SAT_VB,       //!< Use sat/vB fee rate unit
};

/**
 * Fee rate in satoshis per virtualbyte: CAmount / vB
 * the feerate is represented internally as FeeFrac
 */
class CFeeRate
{
private:
    /** Fee rate in sats/vB (satoshis per N virtualbytes) */
    FeePerVSize m_feerate;

public:
    /** Fee rate of 0 satoshis per 0 vB */
    CFeeRate() = default;
    template<std::integral I> // Disallow silent float -> int conversion
    explicit CFeeRate(const I m_feerate_kvb) : m_feerate(FeePerVSize(m_feerate_kvb, 1000)) {}

    /**
     * Construct a fee rate from a fee in satoshis and a vsize in vB.
     *
     * param@[in]   nFeePaid    The fee paid by a transaction, in satoshis
     * param@[in]   virtual_bytes   The vsize of a transaction, in vbytes
     *
     * Passing any virtual_bytes less than or equal to 0 will result in 0 fee rate per 0 size.
     */
    CFeeRate(const CAmount& nFeePaid, int32_t virtual_bytes);

    /**
     * Return the fee in satoshis for the given vsize in vbytes.
     * If the calculated fee would have fractional satoshis, then the
     * returned fee will always be rounded up to the nearest satoshi.
     */
    CAmount GetFee(int32_t virtual_bytes) const;

    /**
     * Return the fee in satoshis for a vsize of 1000 vbytes
     */
    CAmount GetFeePerK() const { return CAmount(m_feerate.EvaluateFeeDown(1000)); }