mulDiv returns 0 if numerator is 0 and denominator isn't:

* Otherwise overflow checks result in divide by 0.
* If denominator is 0, let the divide by 0 exception throw.
* Move mulDiv out of STAmount

src/ripple/basics/impl/mulDiv.cpp

@@ -0,0 +1,65 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of rippled: https://github.com/ripple/rippled
+    Copyright (c) 2012-2015 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.
+*/
+//==============================================================================
+
+#include <BeastConfig.h>
+#include <ripple/basics/mulDiv.h>
+#include <ripple/basics/contract.h>
+#include <limits>
+#include <stdexcept>
+#include <utility>
+
+namespace ripple
+{
+
+// compute (value)*(mul)/(div) - avoid overflow but keep precision
+std::pair<bool, std::uint64_t>
+mulDiv(std::uint64_t value, std::uint64_t mul, std::uint64_t div)
+{
+    if ((value == 0 || mul == 0) && div != 0)
+        return{ true, 0 };
+    lowestTerms(value, div);
+    lowestTerms(mul, div);
+
+    if (value < mul)
+        std::swap(value, mul);
+    constexpr std::uint64_t max =
+        std::numeric_limits<std::uint64_t>::max();
+    const auto limit = max / mul;
+    if (value > limit)
+    {
+        value /= div;
+        if (value > limit)
+            return{ false, max };
+        return{ true, value * mul };
+    }
+    return{ true, value * mul / div };
+}
+
+// compute (value)*(mul)/(div) - avoid overflow but keep precision
+std::uint64_t
+mulDivThrow(std::uint64_t value, std::uint64_t mul, std::uint64_t div)
+{
+    auto const result = mulDiv(value, mul, div);
+    if(!result.first)
+        Throw<std::overflow_error>("mulDiv");
+    return result.second;
+}
+
+
+} // ripple