[random] add helper function to add random jitter to a value (#2703)

src/core/common/random.hpp

@@ -146,6 +146,22 @@ inline void FillBuffer(uint8_t *aBuffer, uint16_t aSize)
     }
 }
 
+/**
+ * This function adds a random jitter within a given range to a given value.
+ *
+ * @param[in]  aValue     A value to which the random jitter is added.
+ * @param[in]  aJitter    Maximum jitter. Random jitter is selected from the range `[-aJitter, aJitter]`.
+ *
+ * @returns The given value with an added random jitter.
+ *
+ */
+inline uint32_t AddJitter(uint32_t aValue, uint16_t aJitter)
+{
+    aJitter = (aJitter <= aValue) ? aJitter : static_cast<uint16_t>(aValue);
+
+    return aValue + GetUint32InRange(0, 2 * aJitter + 1) - aJitter;
+}
+
 /**
  * @}
  *

src/core/utils/channel_monitor.cpp

@@ -104,29 +104,6 @@ exit:
     return occupancy;
 }
 
-void ChannelMonitor::RestartTimer(void)
-{
-    uint16_t interval = kTimerInterval;
-    int16_t  jitter;
-
-    jitter = static_cast<int16_t>(Random::GetUint16InRange(0, 2 * kMaxJitterInterval)) - kMaxJitterInterval;
-
-    if (jitter >= kTimerInterval)
-    {
-        jitter = kTimerInterval - 1;
-    }
-
-    if (jitter <= -kTimerInterval)
-    {
-        jitter = -kTimerInterval + 1;
-    }
-
-    interval += jitter;
-    mTimer.StartAt(mTimer.GetFireTime(), interval);
-
-    otLogDebgUtil(GetInstance(), "ChannelMonitor: Timer interval %u, jitter %d", interval, jitter);
-}
-
 void ChannelMonitor::HandleTimer(Timer &aTimer)
 {
     aTimer.GetOwner<ChannelMonitor>().HandleTimer();
@@ -136,7 +113,8 @@ void ChannelMonitor::HandleTimer(void)
 {
     GetInstance().Get<Mac::Mac>().EnergyScan(mScanChannelMasks[mChannelMaskIndex], 0,
                                              &ChannelMonitor::HandleEnergyScanResult, this);
-    RestartTimer();
+
+    mTimer.StartAt(mTimer.GetFireTime(), Random::AddJitter(kTimerInterval, kMaxJitterInterval));
 }
 
 void ChannelMonitor::HandleEnergyScanResult(void *aContext, otEnergyScanResult *aResult)

src/core/utils/channel_monitor.hpp

@@ -194,7 +194,6 @@ private:
         kMaxOccupancy      = 0xffff,
     };
 
-    void        RestartTimer(void);
     static void HandleTimer(Timer &aTimer);
     void        HandleTimer(void);
     static void HandleEnergyScanResult(void *aContext, otEnergyScanResult *aResult);