[amount] Add support for negative fee rates

src/amount.h

@@ -11,6 +11,7 @@
 #include <stdlib.h>
 #include <string>
 
+/** Amount in satoshis (Can be negative) */
 typedef int64_t CAmount;
 
 static const CAmount COIN = 100000000;
@@ -30,22 +31,28 @@ extern const std::string CURRENCY_UNIT;
 static const CAmount MAX_MONEY = std::numeric_limits<int64_t>::max();
 inline bool MoneyRange(const CAmount& nValue) { return (nValue >= 0 && nValue <= MAX_MONEY); }
 
-/** Type-safe wrapper class for fee rates
- * (how much to pay based on transaction size)
+/**
+ * Fee rate in satoshis per kilobyte: CAmount / kB
  */
 class CFeeRate
 {
 private:
     CAmount nSatoshisPerK; // unit is satoshis-per-1,000-bytes
 public:
+    /** Fee rate of 0 satoshis per kB */
     CFeeRate() : nSatoshisPerK(0) { }
     explicit CFeeRate(const CAmount& _nSatoshisPerK): nSatoshisPerK(_nSatoshisPerK) { }
-    CFeeRate(const CAmount& nFeePaid, size_t nSize);
+    /** Constructor for a fee rate in satoshis per kB. The size in bytes must not exceed (2^63 - 1)*/
+    CFeeRate(const CAmount& nFeePaid, size_t nBytes);
     CFeeRate(const CFeeRate& other) { nSatoshisPerK = other.nSatoshisPerK; }
-
-    CAmount GetFee(size_t size) const; // unit returned is satoshis
-    CAmount GetFeePerK() const { return GetFee(1000); } // satoshis-per-1000-bytes
-
+    /**
+     * Return the fee in satoshis for the given size in bytes.
+     */
+    CAmount GetFee(size_t nBytes) const;
+    /**
+     * Return the fee in satoshis for a size of 1000 bytes
+     */
+    CAmount GetFeePerK() const { return GetFee(1000); }
     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; }