[border-router] simplify local OMR prefix update logic (#12750)

This commit simplifies the logic for updating the local OMR prefix
within `RoutingManager::OmrPrefixManager`.

The process of updating the prefix in `UpdateLocalPrefix()` is
consolidated into a single flow. Instead of multiple paths clearing
the old prefix from `NetworkData` and logging changes, the method now
determines the appropriate `prefix`, `preference`, and `origin`
(`kSelfGenerated`, `kCustom`, `kDhcp6Pd`), and delegates the change
to a shared sequence at the end of the method.

It also adds a `Matches()` method to `OmrPrefix` to efficiently check
if a given `Ip6::Prefix` and `RoutePreference` match the current OMR
prefix, avoiding unnecessary copies during updates.

Additionally, this change standardizes the log output format for local
OMR prefix updates by utilizing `LocalToString()` and ensures
the prefix's route preference is consistently included.

src/core/border_router/br_types.cpp

@@ -255,6 +255,11 @@ void OmrPrefix::SetPrefix(const Ip6::Prefix &aPrefix, RoutePreference aPreferenc
     mPreference = aPreference;
 }
 
+bool OmrPrefix::Matches(const Ip6::Prefix &aPrefix, RoutePreference aPreference) const
+{
+    return (mPreference == aPreference) && (mPrefix == aPrefix);
+}
+
 //---------------------------------------------------------------------------------------------------------------------
 // FavoredOmrPrefix
 

src/core/border_router/br_types.hpp

@@ -582,6 +582,17 @@ public:
      */
     void SetPrefix(const Ip6::Prefix &aPrefix, RoutePreference aPreference);
 
+    /**
+     * Indicates whether the OMR prefix matches a given prefix and preference.
+     *
+     * @param[in] aPrefix      The prefix to compare with.
+     * @param[in] aPreference  The preference to compare with.
+     *
+     * @retval TRUE   The OMR prefix matches the given prefix and preference.
+     * @retval FALSE  The OMR prefix does not match the given prefix and preference.
+     */
+    bool Matches(const Ip6::Prefix &aPrefix, RoutePreference aPreference) const;
+
 protected:
     Ip6::Prefix     mPrefix;
     RoutePreference mPreference;

src/core/border_router/routing_manager.cpp

@@ -857,8 +857,13 @@ void RoutingManager::OmrPrefixManager::DetermineFavoredPrefixInNetData(FavoredOm
 
 void RoutingManager::OmrPrefixManager::UpdateLocalPrefix(void)
 {
-    // Determine the local prefix and remove any outdated previous
-    // local prefix which may have been added in the Network Data.
+    // Determine the local prefix, its origin and remove any outdated
+    // previous local prefix which may have been added in the Network
+    // Data.
+
+    const Ip6::Prefix *prefix = nullptr;
+    RoutePreference    preference;
+    PrefixOrigin       origin;
 
     switch (mConfig)
     {
@@ -866,45 +871,46 @@ void RoutingManager::OmrPrefixManager::UpdateLocalPrefix(void)
 #if OPENTHREAD_CONFIG_BORDER_ROUTING_DHCP6_PD_ENABLE
         if (Get<RoutingManager>().mPdPrefixManager.HasPrefix() && !Get<RoutingManager>().mPdPrefixManager.HasConflict())
         {
-            if (mLocalPrefix.GetPrefix() != Get<RoutingManager>().mPdPrefixManager.GetPrefix())
-            {
-                RemoveLocalFromNetData();
-                mLocalPrefix.SetPrefix(Get<RoutingManager>().mPdPrefixManager.GetPrefix(),
-                                       PdPrefixManager::kPdRoutePreference);
-                LogInfo("Setting local OMR prefix to PD prefix: %s", mLocalPrefix.GetPrefix().ToString().AsCString());
-            }
+            prefix     = &Get<RoutingManager>().mPdPrefixManager.GetPrefix();
+            preference = PdPrefixManager::kPdRoutePreference;
+            origin     = kDhcp6Pd;
+            break;
         }
-        else
 #endif
-
-            if (mLocalPrefix.GetPrefix() != mGeneratedPrefix)
-        {
-            RemoveLocalFromNetData();
-            mLocalPrefix.SetPrefix(mGeneratedPrefix, RoutePreference::kRoutePreferenceLow);
-            LogInfo("Setting local OMR prefix to generated prefix: %s",
-                    mLocalPrefix.GetPrefix().ToString().AsCString());
-        }
-
+        prefix     = &mGeneratedPrefix;
+        preference = RoutePreference::kRoutePreferenceLow;
+        origin     = kSelfGenerated;
         break;
 
     case kOmrConfigCustom:
-        if (mLocalPrefix != mCustomPrefix)
-        {
-            RemoveLocalFromNetData();
-            mLocalPrefix = mCustomPrefix;
-            LogInfo("Setting local OMR prefix to custom prefix: %s", mLocalPrefix.GetPrefix().ToString().AsCString());
-        }
-
+        prefix     = &mCustomPrefix.GetPrefix();
+        preference = mCustomPrefix.GetPreference();
+        origin     = kCustom;
         break;
 
     case kOmrConfigDisabled:
-        if (!mLocalPrefix.IsEmpty())
-        {
-            RemoveLocalFromNetData();
-            mLocalPrefix.Clear();
-        }
         break;
     }
+
+    if (prefix == nullptr)
+    {
+        VerifyOrExit(!mLocalPrefix.IsEmpty());
+        RemoveLocalFromNetData();
+        mLocalPrefix.Clear();
+        LogInfo("Cleared local OMR prefix");
+        ExitNow();
+    }
+
+    VerifyOrExit(!mLocalPrefix.Matches(*prefix, preference) || (origin != mLocalPrefixOrigin));
+
+    RemoveLocalFromNetData();
+    mLocalPrefix.SetPrefix(*prefix, preference);
+    mLocalPrefixOrigin = origin;
+
+    LogInfo("Set %s", LocalToString().AsCString());
+
+exit:
+    return;
 }
 
 void RoutingManager::OmrPrefixManager::Evaluate(void)
@@ -1040,8 +1046,11 @@ RoutingManager::OmrPrefixManager::InfoString RoutingManager::OmrPrefixManager::L
 {
     InfoString string;
 
-    string.Append("local OMR prefix %s (def-route:%s)", mLocalPrefix.GetPrefix().ToString().AsCString(),
-                  ToYesNo(mDefaultRoute));
+    string.Append("local OMR prefix %s (prf:%s, def-route:%s, origin:%s)",
+                  mLocalPrefix.GetPrefix().ToString().AsCString(),
+                  RoutePreferenceToString(mLocalPrefix.GetPreference()), ToYesNo(mDefaultRoute),
+                  PrefixOriginToString(mLocalPrefixOrigin));
+
     return string;
 }
 
@@ -1087,6 +1096,18 @@ const char *RoutingManager::OmrPrefixManager::OmrConfigToString(OmrConfig aConfi
     return kStrings[aConfig];
 }
 
+const char *RoutingManager::OmrPrefixManager::PrefixOriginToString(PrefixOrigin aOrigin)
+{
+#define PrefixOriginMapList(_)    \
+    _(kSelfGenerated, "self-gen") \
+    _(kCustom, "custom")          \
+    _(kDhcp6Pd, "dhcp6-pd")
+
+    DefineEnumStringArray(PrefixOriginMapList);
+
+    return kStrings[aOrigin];
+}
+
 //---------------------------------------------------------------------------------------------------------------------
 // OnLinkPrefixManager
 

src/core/border_router/routing_manager.hpp

@@ -640,6 +640,13 @@ private:
 
         typedef String<kInfoStringSize> InfoString;
 
+        enum PrefixOrigin : uint8_t // of `mLocalPrefix`
+        {
+            kSelfGenerated,
+            kCustom,
+            kDhcp6Pd,
+        };
+
         void       SetFavoredPrefix(const OmrPrefix &aOmrPrefix);
         void       ClearFavoredPrefix(void) { SetFavoredPrefix(OmrPrefix()); }
         void       DetermineFavoredPrefixInNetData(FavoredOmrPrefix &aFavoredPrefix);
@@ -651,12 +658,14 @@ private:
         InfoString FavoredToString(const FavoredOmrPrefix &aFavoredPrefix) const;
 
         static const char *OmrConfigToString(OmrConfig aConfig);
+        static const char *PrefixOriginToString(PrefixOrigin aOrigin);
 
         OmrConfig        mConfig;
         OmrPrefix        mLocalPrefix;
         OmrPrefix        mCustomPrefix;
         Ip6::Prefix      mGeneratedPrefix;
         FavoredOmrPrefix mFavoredPrefix;
+        PrefixOrigin     mLocalPrefixOrigin;
         bool             mIsLocalAddedInNetData;
         bool             mDefaultRoute;
     };