Added SafeNum type SERVER-6399

1 files changed, 160 insertions, 0 deletions

diff --git a/src/mongo/util/safe_num.h b/src/mongo/util/safe_num.h
new file mode 100644
index 00000000000..e8e099939ec
--- /dev/null
+++ b/src/mongo/util/safe_num.h
@@ -0,0 +1,160 @@
+/*    Copyright 2012 10gen Inc.
+ *
+ *    Licensed under the Apache License, Version 2.0 (the "License");
+ *    you may not use this file except in compliance with the License.
+ *    You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *    Unless required by applicable law or agreed to in writing, software
+ *    distributed under the License is distributed on an "AS IS" BASIS,
+ *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *    See the License for the specific language governing permissions and
+ *    limitations under the License.
+ */
+
+#pragma once
+
+#include <iosfwd>
+#include <string>
+
+#include "mongo/bson/bsonelement.h"
+#include "mongo/bson/bsonobj.h"
+
+namespace mongo {
+
+    /**
+     * SafeNum holds and does arithmetic on a number in a safe way, handling overflow
+     * and casting for the user. 32-bit integers will overflow into 64-bit integers. But
+     * 64-bit integers will NOT overflow to doubles. Also, this class does NOT
+     * downcast. This class should be as conservative as possible about upcasting, but
+     * should never lose precision.
+     *
+     * This class does not throw any exceptions, so the user should call type() before
+     * using a SafeNum to ensure that it is valid.  A SafeNum could be invalid
+     * from creation (if, for example, a non-numeric BSONElement was passed to the
+     * constructor) or due to overflow.  NAN is a valid value.
+     *
+     * Usage example:
+     *
+     *      SafeNum counter(doc["count"]);
+     *
+     *      SafeNum newValue = counter + 10;
+     *      // check if valid
+     *      if (newValue.type() == EOO) {
+     *          return;
+     *      }
+     *      // append SafeNum to a BSONObj
+     *      bsonObjBuilder.append(newValue);
+     *
+     */
+    class SafeNum {
+    public:
+        SafeNum();
+        ~SafeNum() { }
+
+        //
+        // construction support
+        //
+
+        // Copy ctor and assignment are allowed.
+        SafeNum(const SafeNum& rhs);
+        SafeNum& operator=(const SafeNum& rhs);
+
+        // Implicit conversions are allowed.
+        SafeNum(const BSONElement& element);
+        SafeNum(int num);
+        SafeNum(long long int num);
+        SafeNum(double num);
+        // TODO: add Paul's mutablebson::Element ctor
+
+        //
+        // comparison support
+        //
+
+        /**
+         * Returns true if the numeric quantity of 'rhs' and 'this' are the same. That is,
+         * an int32(10), an int64(10), and a double(10) are equivalent. An EOO-typed safe
+         * num is equivalent only to another EOO-typed instance. Otherwise, returns false.
+         */
+        bool isEquivalent(const SafeNum& rhs) const;
+        bool operator==(const SafeNum& rhs) const;
+        bool operator!=(const SafeNum& rhs) const;
+
+        /**
+         * Returns true if 'rsh' is equivalent to 'this' (see isEquivalent) _and_ both
+         * types are exactly the same. An EOO-typed safe num is never identical to
+         * anything else, even another EOO-typed instance. Otherwise, returns false.
+         */
+        bool isIdentical(const SafeNum& rhs) const;
+
+        //
+        // arithmetic support
+        //
+
+        /**
+         * Sums the 'rhs' -- right-hand side -- safe num with this, taking care of
+         * upconvertions and overflow (see class header).
+         */
+        SafeNum operator+(const SafeNum& rhs) const;
+        SafeNum& operator+=(const SafeNum& rhs);
+        // TODO other operations than sum
+
+        //
+        // output support
+        //
+
+        // TODO: output to builder
+        // TODO: output to Paul's class Element
+
+        //
+        // accessors
+        //
+
+        BSONType type() const { return _type; }
+        std::string debugString() const;
+
+        //
+        // Below exposed for testing purposes. Treat as private.
+        //
+
+        // Maximum integer that can be converted accuratelly into a double, assuming a
+        // double precission IEEE 754 representation.
+        // TODO use numeric_limits to make this portable
+        static const long long maxIntInDouble = 9007199254740992LL; // 2^53
+
+    private:
+        // One of the following: NumberInt, NumberLong, NumberDouble, or EOO.
+        BSONType _type;
+
+        // Value of the safe num. Indeterminate if _type is EOO.
+        union {
+            int int32Val;
+            long long int int64Val;
+            double doubleVal;
+        } _value;
+
+        /**
+         * Returns the sum of 'lhs' and 'rhs', taking into consideration their types. The
+         * type of the result would upcast, if necessary and permitted. Otherwise, returns
+         * an EOO-type instance.
+         */
+        static SafeNum addInternal(const SafeNum& lhs, const SafeNum& rhs);
+
+        /**
+         * Extracts the value of 'snum' in a long format. It assumes 'snum' is an NumberInt
+         * or a NumberDouble.
+         */
+        static long long getLongLong(const SafeNum& snum);
+
+        /**
+         * Extracts the value of 'snum' in a double format. It assumes 'snum' is a valid
+         * SafeNum, i.e., that _type is not EOO.
+         */
+        static double getDouble(const SafeNum& snum);
+    };
+
+    // Convenience method for unittest code. Please use accessors otherwise.
+    ostream& operator<<(ostream& os, const SafeNum& snum);
+
+} // namespace mongo