Created Map Based Trainer Battle Music (markdown)

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+For this tutorial, we will be changing the battle music for generic trainers depending on the map the player is on.
+ 
+In `GetBattleBGM` in [pokemon.c](https://github.com/pret/pokeemerald/blob/master/src/pokemon.c), this is where the magic happens.
+ 
+First, we need to define the region map sections constants at the top of file:
+ 
+```diff
+...
+#include “constants/trainers.h”
++ #include ”constants/region_map_sections.h“
+```
+ 
+Next, go to `GetBattleBGM` which should look like this:
+ 
+```c
+ 
+u16 GetBattleBGM(void)
+{
+    if (gBattleTypeFlags & BATTLE_TYPE_KYOGRE_GROUDON)
+        return MUS_VS_KYOGRE_GROUDON;
+    else if (gBattleTypeFlags & BATTLE_TYPE_REGI)
+        return MUS_VS_REGI;
+    else if (gBattleTypeFlags & (BATTLE_TYPE_LINK | BATTLE_TYPE_x2000000))
+        return MUS_VS_TRAINER;
+    else if (gBattleTypeFlags & BATTLE_TYPE_TRAINER)
+    {
+        u8 trainerClass;
+ 
+        if (gBattleTypeFlags & BATTLE_TYPE_FRONTIER)
+            trainerClass = GetFrontierOpponentClass(gTrainerBattleOpponent_A);
+        else if (gBattleTypeFlags & BATTLE_TYPE_TRAINER_HILL)
+            trainerClass = TRAINER_CLASS_EXPERT;
+        else
+            trainerClass = gTrainers[gTrainerBattleOpponent_A].trainerClass;
+ 
+        switch (trainerClass)
+        {
+        case TRAINER_CLASS_AQUA_LEADER:
+        case TRAINER_CLASS_MAGMA_LEADER:
+            return MUS_VS_AQUA_MAGMA_LEADER;
+        case TRAINER_CLASS_TEAM_AQUA:
+        case TRAINER_CLASS_TEAM_MAGMA:
+        case TRAINER_CLASS_AQUA_ADMIN:
+        case TRAINER_CLASS_MAGMA_ADMIN:
+            return MUS_VS_AQUA_MAGMA;
+        case TRAINER_CLASS_LEADER:
+            return MUS_VS_GYM_LEADER;
+        case TRAINER_CLASS_CHAMPION:
+            return MUS_VS_CHAMPION;
+        case TRAINER_CLASS_PKMN_TRAINER_3:
+            if (gBattleTypeFlags & BATTLE_TYPE_FRONTIER)
+                return MUS_VS_RIVAL;
+            if (!StringCompare(gTrainers[gTrainerBattleOpponent_A].trainerName, gText_BattleWallyName))
+                return MUS_VS_TRAINER;
+            return MUS_VS_RIVAL;
+        case TRAINER_CLASS_ELITE_FOUR:
+            return MUS_VS_ELITE_FOUR;
+        case TRAINER_CLASS_SALON_MAIDEN:
+        case TRAINER_CLASS_DOME_ACE:
+        case TRAINER_CLASS_PALACE_MAVEN:
+        case TRAINER_CLASS_ARENA_TYCOON:
+        case TRAINER_CLASS_FACTORY_HEAD:
+        case TRAINER_CLASS_PIKE_QUEEN:
+        case TRAINER_CLASS_PYRAMID_KING:
+            return MUS_VS_FRONTIER_BRAIN;
+        default:
+            return MUS_VS_TRAINER;
+        }
+    }
+    else
+        return MUS_VS_WILD;
+}
+```
+On `default` under the switch statement for trainer classes is where the normal trainer battle music plays for trainer classes that don’t have special music. It’s here that we’ll be adding in a check for the map section the player is on.
+ 
+For the sake of this tutorial, we will be making trainers on Route 102 map play `MUS_RG_VS_TRAINER` which is the standard trainer battle music for Pokémon FireRed and LeafGreen.
+ 
+Here’s how that would look:
+ 
+```diff
+...
+- default:
+-   return MUS_VS_TRAINER;
+ 
+...
++ default:
++   if (gMapHeader.regionMapSectionId == MAPSEC_ROUTE_102)
++     return MUS_RG_VS_TRAINER;
++   else
++     return MUS_VS_TRAINER;
+```
+
+`region_map_sections.h` needs to be included because without it, we won’t be able to use `MAPSEC_*` defines with `*` replacing the map name.
+ 
+If I didn’t mess anything up, that should work correctly.
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