Move mulDiv and lowestTerms to STAmount.cpp

src/ripple/protocol/impl/STAmount.cpp

@@ -1257,4 +1257,24 @@ divRound (STAmount const& num, STAmount const& den,
     return STAmount (issue, amount, offset, resultNegative);
 }
 
+// compute (value)*(mul)/(div) - avoid overflow but keep precision
+std::uint64_t
+mulDiv(std::uint64_t value, std::uint64_t mul, std::uint64_t div)
+{
+    lowestTerms(value, div);
+    lowestTerms(mul, div);
+
+    if (value < mul)
+        std::swap(value, mul);
+    const auto max = std::numeric_limits<std::uint64_t>::max();
+    if (value > max / mul)
+    {
+        value /= div;
+        if (value > max / mul)
+            throw std::overflow_error("mulDiv");
+        return value * mul;
+    }
+    return value * mul / div;
+}
+
 } // ripple