[fold] add precision loss

include/xrpl/protocol/STAmount.h

@@ -684,6 +684,48 @@ isXRP(STAmount const& amount)
     return amount.native();
 }
 
+/** returns true if adding or subtracting results in less than or equal to
+ * 0.01% precision loss **/
+inline bool
+isAddable(STAmount const& amt1, STAmount const& amt2)
+{
+    // special case: adding anything to zero is always fine
+    if (amt1 == beast::zero || amt2 == beast::zero)
+        return true;
+
+    // special case: adding two xrp amounts together.
+    // this is just an overflow check
+    if (isXRP(amt1) && isXRP(amt2))
+    {
+        XRPAmount A = (amt1.signum() == -1 ? -(amt1.xrp()) : amt1.xrp());
+        XRPAmount B = (amt2.signum() == -1 ? -(amt2.xrp()) : amt2.xrp());
+
+        XRPAmount finalAmt = A + B;
+        return (finalAmt >= A && finalAmt >= B);
+    }
+
+    static const STAmount one{IOUAmount{1, 0}, noIssue()};
+    static const STAmount maxLoss{IOUAmount{1, -4}, noIssue()};
+
+    STAmount A = amt1;
+    STAmount B = amt2;
+
+    if (isXRP(A))
+        A = STAmount{IOUAmount{A.xrp().drops(), -6}, noIssue()};
+
+    if (isXRP(B))
+        B = STAmount{IOUAmount{B.xrp().drops(), -6}, noIssue()};
+
+    A.setIssue(noIssue());
+    B.setIssue(noIssue());
+
+    STAmount lhs = divide((A - B) + B, A, noIssue()) - one;
+    STAmount rhs = divide((B - A) + A, B, noIssue()) - one;
+
+    return ((rhs.negative() ? -rhs : rhs) + (lhs.negative() ? -lhs : lhs)) <=
+        maxLoss;
+}
+
 // Since `canonicalize` does not have access to a ledger, this is needed to put
 // the low-level routine stAmountCanonicalize on an amendment switch. Only
 // transactions need to use this switchover. Outside of a transaction it's safe

include/xrpl/protocol/TER.h

@@ -344,6 +344,7 @@ enum TECcodes : TERUnderlyingType {
     tecARRAY_TOO_LARGE = 191,
     tecLOCKED = 192,
     tecBAD_CREDENTIALS = 193,
+    tecPRECISION_LOSS = 194,
 };
 
 //------------------------------------------------------------------------------

src/libxrpl/protocol/TER.cpp

@@ -117,6 +117,7 @@ transResults()
         MAKE_ERROR(tecARRAY_TOO_LARGE,               "Array is too large."),
         MAKE_ERROR(tecLOCKED,                        "Fund is locked."),
         MAKE_ERROR(tecBAD_CREDENTIALS,               "Bad credentials."),
+        MAKE_ERROR(tecPRECISION_LOSS,                "The amounts used by the transaction cannot interact."),
 
         MAKE_ERROR(tefALREADY,                     "The exact transaction was already in this ledger."),
         MAKE_ERROR(tefBAD_ADD_AUTH,                "Not authorized to add account."),