fix: Add rounding to Vault invariants (#6217) (#6955)

Co-authored-by: Vito Tumas <5780819+Tapanito@users.noreply.github.com>
Co-authored-by: Ed Hennis <ed@ripple.com>

src/test/app/Invariants_test.cpp

@@ -34,6 +34,7 @@
 #include <xrpl/protocol/Protocol.h>
 #include <xrpl/protocol/SField.h>
 #include <xrpl/protocol/SOTemplate.h>
+#include <xrpl/protocol/STAmount.h>
 #include <xrpl/protocol/STArray.h>
 #include <xrpl/protocol/STLedgerEntry.h>
 #include <xrpl/protocol/STObject.h>
@@ -46,6 +47,7 @@
 #include <xrpl/protocol/XRPAmount.h>
 #include <xrpl/protocol/jss.h>
 #include <xrpl/tx/ApplyContext.h>
+#include <xrpl/tx/invariants/VaultInvariant.h>
 
 #include <algorithm>
 #include <array>
@@ -4081,6 +4083,128 @@ class Invariants_test : public beast::unit_test::suite
         }
     }
 
+    void
+    testVaultComputeCoarsestScale()
+    {
+        using namespace jtx;
+
+        Account const issuer{"issuer"};
+        PrettyAsset const vaultAsset = issuer["IOU"];
+
+        struct TestCase
+        {
+            std::string name;
+            std::int32_t expectedMinScale;
+            std::vector<ValidVault::DeltaInfo> values;
+        };
+
+        NumberMantissaScaleGuard const g{MantissaRange::large};
+
+        auto makeDelta = [&vaultAsset](Number const& n) -> ValidVault::DeltaInfo {
+            return {n, scale(n, vaultAsset.raw())};
+        };
+
+        auto const testCases = std::vector<TestCase>{
+            {
+                .name = "No values",
+                .expectedMinScale = 0,
+                .values = {},
+            },
+            {
+                .name = "Mixed integer and Number values",
+                .expectedMinScale = -15,
+                .values = {makeDelta(1), makeDelta(-1), makeDelta(Number{10, -1})},
+            },
+            {
+                .name = "Mixed scales",
+                .expectedMinScale = -17,
+                .values =
+                    {makeDelta(Number{1, -2}), makeDelta(Number{5, -3}), makeDelta(Number{3, -2})},
+            },
+            {
+                .name = "Equal scales",
+                .expectedMinScale = -16,
+                .values =
+                    {makeDelta(Number{1, -1}), makeDelta(Number{5, -1}), makeDelta(Number{1, -1})},
+            },
+            {
+                .name = "Mixed mantissa sizes",
+                .expectedMinScale = -12,
+                .values =
+                    {makeDelta(Number{1}),
+                     makeDelta(Number{1234, -3}),
+                     makeDelta(Number{12345, -6}),
+                     makeDelta(Number{123, 1})},
+            },
+        };
+
+        for (auto const& tc : testCases)
+        {
+            testcase("vault computeCoarsestScale: " + tc.name);
+
+            auto const actualScale = ValidVault::computeCoarsestScale(tc.values);
+
+            BEAST_EXPECTS(
+                actualScale == tc.expectedMinScale,
+                "expected: " + std::to_string(tc.expectedMinScale) +
+                    ", actual: " + std::to_string(actualScale));
+            for (auto const& num : tc.values)
+            {
+                // None of these scales are far enough apart that rounding the
+                // values would lose information, so check that the rounded
+                // value matches the original.
+                auto const actualRounded = roundToAsset(vaultAsset, num.delta, actualScale);
+                BEAST_EXPECTS(
+                    actualRounded == num.delta,
+                    "number " + to_string(num.delta) + " rounded to scale " +
+                        std::to_string(actualScale) + " is " + to_string(actualRounded));
+            }
+        }
+
+        auto const testCases2 = std::vector<TestCase>{
+            {
+                .name = "False equivalence",
+                .expectedMinScale = -15,
+                .values =
+                    {
+                        makeDelta(Number{1234567890123456789, -18}),
+                        makeDelta(Number{12345, -4}),
+                        makeDelta(Number{1}),
+                    },
+            },
+        };
+
+        // Unlike the first set of test cases, the values in these test could
+        // look equivalent if using the wrong scale.
+        for (auto const& tc : testCases2)
+        {
+            testcase("vault computeCoarsestScale: " + tc.name);
+
+            auto const actualScale = ValidVault::computeCoarsestScale(tc.values);
+
+            BEAST_EXPECTS(
+                actualScale == tc.expectedMinScale,
+                "expected: " + std::to_string(tc.expectedMinScale) +
+                    ", actual: " + std::to_string(actualScale));
+            std::optional<Number> first;
+            Number firstRounded;
+            for (auto const& num : tc.values)
+            {
+                if (!first)
+                {
+                    first = num.delta;
+                    firstRounded = roundToAsset(vaultAsset, num.delta, actualScale);
+                    continue;
+                }
+                auto const numRounded = roundToAsset(vaultAsset, num.delta, actualScale);
+                BEAST_EXPECTS(
+                    numRounded != firstRounded,
+                    "at a scale of " + std::to_string(actualScale) + " " + to_string(num.delta) +
+                        " == " + to_string(*first));
+            }
+        }
+    }
+
 public:
     void
     run() override
@@ -4111,6 +4235,7 @@ public:
         testValidLoanBroker();
         testVault();
         testMPT();
+        testVaultComputeCoarsestScale();
     }
 };