Fill in payment computation shortages (#5941)

- Ensures a consistent fixed payment amount for the entire life of the
  loan, except the final payment, which is guaranteed to be the same or
  smaller.
- Convert some Loan structs to compute values that had need manual
  updates to stay consistent.
- Fail the transaction in `LoanPay` if it violates the Vault `assetsAvailable <=
  assetsTotal` invariant.
- Use constexpr to check that min mantissa value for Number and STAmount
  is a power of 10, and compute the max in terms of the min.
- Improve unit tests:
  - Use BrokerParameters and Loan Parameters instead of semi-global
    class values
  - In tests, check that the expected number of loan payments are made.
  - Add LoanBatch manual test to generate a set number of random loans,
    set them up, and pay them off.
  - Add LoanArbitrary manual test to run a single test with specific
    (hard-coded for now) parameters.
  - Add Number support to XRP_t.

include/xrpl/basics/Number.h

@@ -32,6 +32,15 @@ class Number;
 std::string
 to_string(Number const& amount);
 
+template <typename T>
+constexpr bool
+isPowerOfTen(T value)
+{
+    while (value >= 10 && value % 10 == 0)
+        value /= 10;
+    return value == 1;
+}
+
 class Number
 {
     using rep = std::int64_t;
@@ -41,7 +50,9 @@ class Number
 public:
     // The range for the mantissa when normalized
     constexpr static std::int64_t minMantissa = 1'000'000'000'000'000LL;
-    constexpr static std::int64_t maxMantissa = 9'999'999'999'999'999LL;
+    static_assert(isPowerOfTen(minMantissa));
+    constexpr static std::int64_t maxMantissa = minMantissa * 10 - 1;
+    static_assert(maxMantissa == 9'999'999'999'999'999LL);
 
     // The range for the exponent when normalized
     constexpr static int minExponent = -32768;
@@ -58,8 +69,6 @@ public:
     explicit Number(rep mantissa, int exponent);
     explicit constexpr Number(rep mantissa, int exponent, unchecked) noexcept;
 
-    static Number const zero;
-
     constexpr rep
     mantissa() const noexcept;
     constexpr int
@@ -153,22 +162,7 @@ public:
     }
 
     Number
-    truncate() const noexcept
-    {
-        if (exponent_ >= 0 || mantissa_ == 0)
-            return *this;
-
-        Number ret = *this;
-        while (ret.exponent_ < 0 && ret.mantissa_ != 0)
-        {
-            ret.exponent_ += 1;
-            ret.mantissa_ /= rep(10);
-        }
-        // We are guaranteed that normalize() will never throw an exception
-        // because exponent is either negative or zero at this point.
-        ret.normalize();
-        return ret;
-    }
+    truncate() const noexcept;
 
     friend constexpr bool
     operator>(Number const& x, Number const& y) noexcept
@@ -213,6 +207,8 @@ private:
     class Guard;
 };
 
+constexpr static Number numZero{};
+
 inline constexpr Number::Number(rep mantissa, int exponent, unchecked) noexcept
     : mantissa_{mantissa}, exponent_{exponent}
 {

include/xrpl/protocol/Protocol.h

@@ -102,7 +102,9 @@ std::uint16_t constexpr maxTransferFee = 50000;
  * Example: 50% is 0.50 * bipsPerUnity = 5,000 bps.
  */
 Bips32 constexpr bipsPerUnity(100 * 100);
+static_assert(bipsPerUnity == Bips32{10'000});
 TenthBips32 constexpr tenthBipsPerUnity(bipsPerUnity.value() * 10);
+static_assert(tenthBipsPerUnity == TenthBips32(100'000));
 
 constexpr Bips32
 percentageToBips(std::uint32_t percentage)

include/xrpl/protocol/STAmount.h

@@ -66,16 +66,18 @@ public:
     static int const cMaxOffset = 80;
 
     // Maximum native value supported by the code
-    static std::uint64_t const cMinValue = 1'000'000'000'000'000ull;
-    static std::uint64_t const cMaxValue = 9'999'999'999'999'999ull;
-    static std::uint64_t const cMaxNative = 9'000'000'000'000'000'000ull;
+    constexpr static std::uint64_t cMinValue = 1'000'000'000'000'000ull;
+    static_assert(isPowerOfTen(cMinValue));
+    constexpr static std::uint64_t cMaxValue = cMinValue * 10 - 1;
+    static_assert(cMaxValue == 9'999'999'999'999'999ull);
+    constexpr static std::uint64_t cMaxNative = 9'000'000'000'000'000'000ull;
 
     // Max native value on network.
-    static std::uint64_t const cMaxNativeN = 100'000'000'000'000'000ull;
-    static std::uint64_t const cIssuedCurrency = 0x8'000'000'000'000'000ull;
-    static std::uint64_t const cPositive = 0x4'000'000'000'000'000ull;
-    static std::uint64_t const cMPToken = 0x2'000'000'000'000'000ull;
-    static std::uint64_t const cValueMask = ~(cPositive | cMPToken);
+    constexpr static std::uint64_t cMaxNativeN = 100'000'000'000'000'000ull;
+    constexpr static std::uint64_t cIssuedCurrency = 0x8'000'000'000'000'000ull;
+    constexpr static std::uint64_t cPositive = 0x4'000'000'000'000'000ull;
+    constexpr static std::uint64_t cMPToken = 0x2'000'000'000'000'000ull;
+    constexpr static std::uint64_t cValueMask = ~(cPositive | cMPToken);
 
     static std::uint64_t const uRateOne;