Enforce levelization in libxrpl with CMake (#5111)

Adds two CMake functions:

* add_module(library subdirectory): Declares an OBJECT "library" (a CMake abstraction for a collection of object files) with sources from the given subdirectory of the given library, representing a module. Isolates the module's headers by creating a subdirectory in the build directory, e.g. .build/tmp123, that contains just a symlink, e.g. .build/tmp123/basics, to the module's header directory, e.g. include/xrpl/basics, in the source directory, and putting .build/tmp123 (but not include/xrpl) on the include path of the module sources. This prevents the module sources from including headers not explicitly linked to the module in CMake with target_link_libraries.
* target_link_modules(library scope modules...): Links the library target to each of the module targets, and removes their sources from its source list (so they are not compiled and linked twice).

Uses these functions to separate and explicitly link modules in libxrpl:

    Level 01: beast
    Level 02: basics
    Level 03: json, crypto
    Level 04: protocol
    Level 05: resource, server

include/xrpl/protocol/FeeUnits.h

@@ -0,0 +1,569 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of rippled: https://github.com/ripple/rippled
+    Copyright (c) 2019 Ripple Labs Inc.
+
+    Permission to use, copy, modify, and/or distribute this software for any
+    purpose  with  or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL ,  DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+
+#ifndef BASICS_FEES_H_INCLUDED
+#define BASICS_FEES_H_INCLUDED
+
+#include <xrpl/basics/safe_cast.h>
+#include <xrpl/beast/utility/Zero.h>
+#include <xrpl/beast/utility/instrumentation.h>
+#include <xrpl/json/json_value.h>
+#include <boost/multiprecision/cpp_int.hpp>
+#include <boost/operators.hpp>
+
+#include <cmath>
+#include <ios>
+#include <iosfwd>
+#include <limits>
+#include <optional>
+#include <sstream>
+#include <string>
+#include <utility>
+
+namespace ripple {
+
+namespace feeunit {
+
+/** "drops" are the smallest divisible amount of XRP. This is what most
+    of the code uses. */
+struct dropTag;
+/** "fee units" calculations are a not-really-unitless value that is used
+    to express the cost of a given transaction vs. a reference transaction.
+    They are primarily used by the Transactor classes. */
+struct feeunitTag;
+/** "fee levels" are used by the transaction queue to compare the relative
+    cost of transactions that require different levels of effort to process.
+    See also: src/ripple/app/misc/FeeEscalation.md#fee-level */
+struct feelevelTag;
+/** unitless values are plain scalars wrapped in a TaggedFee. They are
+    used for calculations in this header. */
+struct unitlessTag;
+
+template <class T>
+using enable_if_unit_t = typename std::enable_if_t<
+    std::is_class_v<T> && std::is_object_v<typename T::unit_type> &&
+    std::is_object_v<typename T::value_type>>;
+
+/** `is_usable_unit_v` is checked to ensure that only values with
+    known valid type tags can be used (sometimes transparently) in
+    non-fee contexts. At the time of implementation, this includes
+    all known tags, but more may be added in the future, and they
+    should not be added automatically unless determined to be
+    appropriate.
+*/
+template <class T, class = enable_if_unit_t<T>>
+constexpr bool is_usable_unit_v =
+    std::is_same_v<typename T::unit_type, feeunitTag> ||
+    std::is_same_v<typename T::unit_type, feelevelTag> ||
+    std::is_same_v<typename T::unit_type, unitlessTag> ||
+    std::is_same_v<typename T::unit_type, dropTag>;
+
+template <class UnitTag, class T>
+class TaggedFee : private boost::totally_ordered<TaggedFee<UnitTag, T>>,
+                  private boost::additive<TaggedFee<UnitTag, T>>,
+                  private boost::equality_comparable<TaggedFee<UnitTag, T>, T>,
+                  private boost::dividable<TaggedFee<UnitTag, T>, T>,
+                  private boost::modable<TaggedFee<UnitTag, T>, T>,
+                  private boost::unit_steppable<TaggedFee<UnitTag, T>>
+{
+public:
+    using unit_type = UnitTag;
+    using value_type = T;
+
+private:
+    value_type fee_;
+
+protected:
+    template <class Other>
+    static constexpr bool is_compatible_v =
+        std::is_arithmetic_v<Other> && std::is_arithmetic_v<value_type> &&
+        std::is_convertible_v<Other, value_type>;
+
+    template <class OtherFee, class = enable_if_unit_t<OtherFee>>
+    static constexpr bool is_compatiblefee_v =
+        is_compatible_v<typename OtherFee::value_type> &&
+        std::is_same_v<UnitTag, typename OtherFee::unit_type>;
+
+    template <class Other>
+    using enable_if_compatible_t =
+        typename std::enable_if_t<is_compatible_v<Other>>;
+
+    template <class OtherFee>
+    using enable_if_compatiblefee_t =
+        typename std::enable_if_t<is_compatiblefee_v<OtherFee>>;
+
+public:
+    TaggedFee() = default;
+    constexpr TaggedFee(TaggedFee const& other) = default;
+    constexpr TaggedFee&
+    operator=(TaggedFee const& other) = default;
+
+    constexpr explicit TaggedFee(beast::Zero) : fee_(0)
+    {
+    }
+
+    constexpr TaggedFee&
+    operator=(beast::Zero)
+    {
+        fee_ = 0;
+        return *this;
+    }
+
+    constexpr explicit TaggedFee(value_type fee) : fee_(fee)
+    {
+    }
+
+    TaggedFee&
+    operator=(value_type fee)
+    {
+        fee_ = fee;
+        return *this;
+    }
+
+    /** Instances with the same unit, and a type that is
+        "safe" to convert to this one can be converted
+        implicitly */
+    template <
+        class Other,
+        class = std::enable_if_t<
+            is_compatible_v<Other> &&
+            is_safetocasttovalue_v<value_type, Other>>>
+    constexpr TaggedFee(TaggedFee<unit_type, Other> const& fee)
+        : TaggedFee(safe_cast<value_type>(fee.fee()))
+    {
+    }
+
+    constexpr TaggedFee
+    operator*(value_type const& rhs) const
+    {
+        return TaggedFee{fee_ * rhs};
+    }
+
+    friend constexpr TaggedFee
+    operator*(value_type lhs, TaggedFee const& rhs)
+    {
+        // multiplication is commutative
+        return rhs * lhs;
+    }
+
+    constexpr value_type
+    operator/(TaggedFee const& rhs) const
+    {
+        return fee_ / rhs.fee_;
+    }
+
+    TaggedFee&
+    operator+=(TaggedFee const& other)
+    {
+        fee_ += other.fee();
+        return *this;
+    }
+
+    TaggedFee&
+    operator-=(TaggedFee const& other)
+    {
+        fee_ -= other.fee();
+        return *this;
+    }
+
+    TaggedFee&
+    operator++()
+    {
+        ++fee_;
+        return *this;
+    }
+
+    TaggedFee&
+    operator--()
+    {
+        --fee_;
+        return *this;
+    }
+
+    TaggedFee&
+    operator*=(value_type const& rhs)
+    {
+        fee_ *= rhs;
+        return *this;
+    }
+
+    TaggedFee&
+    operator/=(value_type const& rhs)
+    {
+        fee_ /= rhs;
+        return *this;
+    }
+
+    template <class transparent = value_type>
+    std::enable_if_t<std::is_integral_v<transparent>, TaggedFee&>
+    operator%=(value_type const& rhs)
+    {
+        fee_ %= rhs;
+        return *this;
+    }
+
+    TaggedFee
+    operator-() const
+    {
+        static_assert(
+            std::is_signed_v<T>, "- operator illegal on unsigned fee types");
+        return TaggedFee{-fee_};
+    }
+
+    bool
+    operator==(TaggedFee const& other) const
+    {
+        return fee_ == other.fee_;
+    }
+
+    template <class Other, class = enable_if_compatible_t<Other>>
+    bool
+    operator==(TaggedFee<unit_type, Other> const& other) const
+    {
+        return fee_ == other.fee();
+    }
+
+    bool
+    operator==(value_type other) const
+    {
+        return fee_ == other;
+    }
+
+    template <class Other, class = enable_if_compatible_t<Other>>
+    bool
+    operator!=(TaggedFee<unit_type, Other> const& other) const
+    {
+        return !operator==(other);
+    }
+
+    bool
+    operator<(TaggedFee const& other) const
+    {
+        return fee_ < other.fee_;
+    }
+
+    /** Returns true if the amount is not zero */
+    explicit constexpr
+    operator bool() const noexcept
+    {
+        return fee_ != 0;
+    }
+
+    /** Return the sign of the amount */
+    constexpr int
+    signum() const noexcept
+    {
+        return (fee_ < 0) ? -1 : (fee_ ? 1 : 0);
+    }
+
+    /** Returns the number of drops */
+    constexpr value_type
+    fee() const
+    {
+        return fee_;
+    }
+
+    template <class Other>
+    constexpr double
+    decimalFromReference(TaggedFee<unit_type, Other> reference) const
+    {
+        return static_cast<double>(fee_) / reference.fee();
+    }
+
+    // `is_usable_unit_v` is checked to ensure that only values with
+    // known valid type tags can be converted to JSON. At the time
+    // of implementation, that includes all known tags, but more may
+    // be added in the future.
+    std::enable_if_t<is_usable_unit_v<TaggedFee>, Json::Value>
+    jsonClipped() const
+    {
+        if constexpr (std::is_integral_v<value_type>)
+        {
+            using jsontype = std::conditional_t<
+                std::is_signed_v<value_type>,
+                Json::Int,
+                Json::UInt>;
+
+            constexpr auto min = std::numeric_limits<jsontype>::min();
+            constexpr auto max = std::numeric_limits<jsontype>::max();
+
+            if (fee_ < min)
+                return min;
+            if (fee_ > max)
+                return max;
+            return static_cast<jsontype>(fee_);
+        }
+        else
+        {
+            return fee_;
+        }
+    }
+
+    /** Returns the underlying value. Code SHOULD NOT call this
+        function unless the type has been abstracted away,
+        e.g. in a templated function.
+    */
+    constexpr value_type
+    value() const
+    {
+        return fee_;
+    }
+
+    friend std::istream&
+    operator>>(std::istream& s, TaggedFee& val)
+    {
+        s >> val.fee_;
+        return s;
+    }
+};
+
+// Output Fees as just their numeric value.
+template <class Char, class Traits, class UnitTag, class T>
+std::basic_ostream<Char, Traits>&
+operator<<(std::basic_ostream<Char, Traits>& os, const TaggedFee<UnitTag, T>& q)
+{
+    return os << q.value();
+}
+
+template <class UnitTag, class T>
+std::string
+to_string(TaggedFee<UnitTag, T> const& amount)
+{
+    return std::to_string(amount.fee());
+}
+
+template <class Source, class = enable_if_unit_t<Source>>
+constexpr bool can_muldiv_source_v =
+    std::is_convertible_v<typename Source::value_type, std::uint64_t>;
+
+template <class Dest, class = enable_if_unit_t<Dest>>
+constexpr bool can_muldiv_dest_v =
+    can_muldiv_source_v<Dest> &&  // Dest is also a source
+    std::is_convertible_v<std::uint64_t, typename Dest::value_type> &&
+    sizeof(typename Dest::value_type) >= sizeof(std::uint64_t);
+
+template <
+    class Source1,
+    class Source2,
+    class = enable_if_unit_t<Source1>,
+    class = enable_if_unit_t<Source2>>
+constexpr bool can_muldiv_sources_v =
+    can_muldiv_source_v<Source1> && can_muldiv_source_v<Source2> &&
+    std::is_same_v<typename Source1::unit_type, typename Source2::unit_type>;
+
+template <
+    class Source1,
+    class Source2,
+    class Dest,
+    class = enable_if_unit_t<Source1>,
+    class = enable_if_unit_t<Source2>,
+    class = enable_if_unit_t<Dest>>
+constexpr bool can_muldiv_v =
+    can_muldiv_sources_v<Source1, Source2> && can_muldiv_dest_v<Dest>;
+// Source and Dest can be the same by default
+
+template <
+    class Source1,
+    class Source2,
+    class Dest,
+    class = enable_if_unit_t<Source1>,
+    class = enable_if_unit_t<Source2>,
+    class = enable_if_unit_t<Dest>>
+constexpr bool can_muldiv_commute_v = can_muldiv_v<Source1, Source2, Dest> &&
+    !std::is_same_v<typename Source1::unit_type, typename Dest::unit_type>;
+
+template <class T>
+using enable_muldiv_source_t =
+    typename std::enable_if_t<can_muldiv_source_v<T>>;
+
+template <class T>
+using enable_muldiv_dest_t = typename std::enable_if_t<can_muldiv_dest_v<T>>;
+
+template <class Source1, class Source2>
+using enable_muldiv_sources_t =
+    typename std::enable_if_t<can_muldiv_sources_v<Source1, Source2>>;
+
+template <class Source1, class Source2, class Dest>
+using enable_muldiv_t =
+    typename std::enable_if_t<can_muldiv_v<Source1, Source2, Dest>>;
+
+template <class Source1, class Source2, class Dest>
+using enable_muldiv_commute_t =
+    typename std::enable_if_t<can_muldiv_commute_v<Source1, Source2, Dest>>;
+
+template <class T>
+TaggedFee<unitlessTag, T>
+scalar(T value)
+{
+    return TaggedFee<unitlessTag, T>{value};
+}
+
+template <
+    class Source1,
+    class Source2,
+    class Dest,
+    class = enable_muldiv_t<Source1, Source2, Dest>>
+std::optional<Dest>
+mulDivU(Source1 value, Dest mul, Source2 div)
+{
+    // Fees can never be negative in any context.
+    if (value.value() < 0 || mul.value() < 0 || div.value() < 0)
+    {
+        // split the asserts so if one hits, the user can tell which
+        // without a debugger.
+        ASSERT(
+            value.value() >= 0,
+            "ripple::feeunit::mulDivU : minimum value input");
+        ASSERT(
+            mul.value() >= 0, "ripple::feeunit::mulDivU : minimum mul input");
+        ASSERT(
+            div.value() >= 0, "ripple::feeunit::mulDivU : minimum div input");
+        return std::nullopt;
+    }
+
+    using desttype = typename Dest::value_type;
+    constexpr auto max = std::numeric_limits<desttype>::max();
+
+    // Shortcuts, since these happen a lot in the real world
+    if (value == div)
+        return mul;
+    if (mul.value() == div.value())
+    {
+        if (value.value() > max)
+            return std::nullopt;
+        return Dest{static_cast<desttype>(value.value())};
+    }
+
+    using namespace boost::multiprecision;
+
+    uint128_t product;
+    product = multiply(
+        product,
+        static_cast<std::uint64_t>(value.value()),
+        static_cast<std::uint64_t>(mul.value()));
+
+    auto quotient = product / div.value();
+
+    if (quotient > max)
+        return std::nullopt;
+
+    return Dest{static_cast<desttype>(quotient)};
+}
+
+}  // namespace feeunit
+
+template <class T>
+using FeeLevel = feeunit::TaggedFee<feeunit::feelevelTag, T>;
+using FeeLevel64 = FeeLevel<std::uint64_t>;
+using FeeLevelDouble = FeeLevel<double>;
+
+template <
+    class Source1,
+    class Source2,
+    class Dest,
+    class = feeunit::enable_muldiv_t<Source1, Source2, Dest>>
+std::optional<Dest>
+mulDiv(Source1 value, Dest mul, Source2 div)
+{
+    return feeunit::mulDivU(value, mul, div);
+}
+
+template <
+    class Source1,
+    class Source2,
+    class Dest,
+    class = feeunit::enable_muldiv_commute_t<Source1, Source2, Dest>>
+std::optional<Dest>
+mulDiv(Dest value, Source1 mul, Source2 div)
+{
+    // Multiplication is commutative
+    return feeunit::mulDivU(mul, value, div);
+}
+
+template <class Dest, class = feeunit::enable_muldiv_dest_t<Dest>>
+std::optional<Dest>
+mulDiv(std::uint64_t value, Dest mul, std::uint64_t div)
+{
+    // Give the scalars a non-tag so the
+    // unit-handling version gets called.
+    return feeunit::mulDivU(feeunit::scalar(value), mul, feeunit::scalar(div));
+}
+
+template <class Dest, class = feeunit::enable_muldiv_dest_t<Dest>>
+std::optional<Dest>
+mulDiv(Dest value, std::uint64_t mul, std::uint64_t div)
+{
+    // Multiplication is commutative
+    return mulDiv(mul, value, div);
+}
+
+template <
+    class Source1,
+    class Source2,
+    class = feeunit::enable_muldiv_sources_t<Source1, Source2>>
+std::optional<std::uint64_t>
+mulDiv(Source1 value, std::uint64_t mul, Source2 div)
+{
+    // Give the scalars a dimensionless unit so the
+    // unit-handling version gets called.
+    auto unitresult = feeunit::mulDivU(value, feeunit::scalar(mul), div);
+
+    if (!unitresult)
+        return std::nullopt;
+
+    return unitresult->value();
+}
+
+template <
+    class Source1,
+    class Source2,
+    class = feeunit::enable_muldiv_sources_t<Source1, Source2>>
+std::optional<std::uint64_t>
+mulDiv(std::uint64_t value, Source1 mul, Source2 div)
+{
+    // Multiplication is commutative
+    return mulDiv(mul, value, div);
+}
+
+template <class Dest, class Src>
+constexpr std::enable_if_t<
+    std::is_same_v<typename Dest::unit_type, typename Src::unit_type> &&
+        std::is_integral_v<typename Dest::value_type> &&
+        std::is_integral_v<typename Src::value_type>,
+    Dest>
+safe_cast(Src s) noexcept
+{
+    // Dest may not have an explicit value constructor
+    return Dest{safe_cast<typename Dest::value_type>(s.value())};
+}
+
+template <class Dest, class Src>
+constexpr std::enable_if_t<
+    std::is_same_v<typename Dest::unit_type, typename Src::unit_type> &&
+        std::is_integral_v<typename Dest::value_type> &&
+        std::is_integral_v<typename Src::value_type>,
+    Dest>
+unsafe_cast(Src s) noexcept
+{
+    // Dest may not have an explicit value constructor
+    return Dest{unsafe_cast<typename Dest::value_type>(s.value())};
+}
+
+}  // namespace ripple
+
+#endif  // BASICS_FEES_H_INCLUDED