diff --git a/average.cpp b/average.cpp
new file mode 100644
index 0000000..c5864bd
--- /dev/null
+++ b/average.cpp
@@ -0,0 +1,51 @@
+#include "average.hpp"
+
+#include <cassert>
+
+std::optional<Average::averageValue>
+    Average::getAverageValue(const Average::averageKey& sensorKey) const
+{
+    const auto it = _previousAverageMap.find(sensorKey);
+    if (it == _previousAverageMap.end())
+    {
+        return {};
+    }
+
+    return std::optional(it->second);
+}
+
+void Average::setAverageValue(const Average::averageKey& sensorKey,
+                              const Average::averageValue& sensorValue)
+{
+    _previousAverageMap[sensorKey] = sensorValue;
+}
+
+std::optional<int64_t> Average::calcAverage(int64_t preAverage,
+                                            int64_t preInterval,
+                                            int64_t curAverage,
+                                            int64_t curInterval)
+{
+    int64_t value = 0;
+    // Estimate that the interval will overflow about 292471
+    // years after it starts counting, so consider it won't
+    // overflow
+    int64_t delta = curInterval - preInterval;
+
+    assert(delta >= 0);
+    // 0 means the delta interval is too short, the value of
+    // power*_average_interval is not changed yet
+    if (delta == 0)
+    {
+        return {};
+    }
+    // Change formula (a2*i2-a1*i1)/(i2-i1) to be the
+    // following formula, to avoid multiplication overflow.
+    // (a2*i2-a1*i1)/(i2-i1) =
+    // (a2*(i1+delta)-a1*i1)/delta =
+    // (a2-a1)(i1/delta)+a2
+    value =
+        (curAverage - preAverage) * (static_cast<double>(preInterval) / delta) +
+        curAverage;
+
+    return value;
+}