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+// Copyright (C) 2012-2021 Internet Systems Consortium, Inc. ("ISC")
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#ifndef TRIPLET_H
+#define TRIPLET_H
+
+#include <exceptions/exceptions.h>
+#include <util/optional.h>
+
+namespace isc {
+namespace util {
+
+/// @brief This template specifies a parameter value
+///
+/// This template class is used to store configuration parameters, like lifetime
+/// or T1. It defines 3 parameters: min, default, and max value. If the
+/// particular configuration parameter is not mandatory, it is possible to
+/// mark the parameter described by a @c Triplet "unspecified". For example, the
+/// T1 and T2 values in DHCPv4 server are optional and may be not specified
+/// in the configuration. The @c Triplets describing these parameters will be
+/// marked "unspecified". If the server finds that the particular parameter
+/// is unspecified it will not include it (e.g. option 58 or 59) in the message
+/// to a client.
+///
+/// There are 3 constructors:
+/// - without parameters - marks the parameter "unspecified"
+/// - simple (just one value that sets all parameters)
+/// - extended (that sets default value and two thresholds)
+///
+/// It will be used with integer types. It provides necessary operators, so
+/// it can be assigned to a plain integer or integer assigned to a Triplet.
+/// See TripletTest.operator test for details on an easy Triplet usage.
+template <class T>
+class Triplet : public util::Optional<T> {
+public:
+
+    using util::Optional<T>::get;
+
+    /// @brief Base type to Triplet conversion.
+    ///
+    /// Typically: uint32_t to Triplet assignment. It is very convenient
+    /// to be able to simply write Triplet<uint32_t> x = 7;
+    ///
+    /// @param other A number to be assigned as min, max and default value.
+    Triplet<T>& operator=(T other) {
+        min_ = other;
+        util::Optional<T>::default_ = other;
+        max_ = other;
+        // The value is now specified because we just assigned one.
+        util::Optional<T>::unspecified_ = false;
+        return (*this);
+    }
+
+    /// @brief Constructor without parameters.
+    ///
+    /// Marks value in @c Triplet unspecified.
+    Triplet()
+        : util::Optional<T>(), min_(0), max_(0) {
+    }
+
+    /// @brief Sets a fixed value.
+    ///
+    /// This constructor assigns a fixed (i.e. no range, just a single value)
+    /// value.
+    ///
+    /// @param value A number to be assigned as min, max and default value.
+    Triplet(T value)
+        : util::Optional<T>(value), min_(value), max_(value) {
+    }
+
+    /// @brief Sets the default value and thresholds
+    ///
+    /// @throw BadValue if min <= def <= max rule is violated
+    Triplet(T min, T def, T max)
+        : util::Optional<T>(def), min_(min), max_(max) {
+        if ( (min_ > def) || (def > max_) ) {
+            isc_throw(BadValue, "Invalid triplet values.");
+        }
+    }
+
+    /// @brief Returns a minimum allowed value
+    T getMin() const { return (min_);}
+
+    /// @brief Returns value with a hint
+    ///
+    /// DHCP protocol treats any values sent by a client as hints.
+    /// This is a method that implements that. We can assign any value
+    /// from configured range that client asks.
+    ///
+    /// @param hint A value being returned when if it is within the range
+    /// between min and max value of @c Triplet. If the hint value is lower
+    /// than min value, the min value is returned. if the hint is greater
+    /// than max value, the max value is returned.
+    ///
+    /// @return A value adjusted to the hint.
+    T get(T hint) const {
+        if (hint <= min_) {
+            return (min_);
+        }
+
+        if (hint >= max_) {
+            return (max_);
+        }
+
+        return (hint);
+    }
+
+    /// @brief Returns a maximum allowed value
+    T getMax() const { return (max_); }
+
+private:
+
+    /// @brief the minimum value
+    T min_;
+
+    /// @brief the maximum value
+    T max_;
+};
+
+
+} // namespace isc::dhcp
+} // namespace isc
+
+#endif // TRIPLET_H