WARNING: a bit of a long tutorial.

When making a ROM hack, it's natural to want to add new battle terrain for battles. Emerald unfortunately handles loading these through big switch statements in functions.
FRLG on the other hand, has a few extra functions that handle the loading of these terrain by giving them the required battle terrain ID for example `BATTLE_TERRAIN_PLAIN`
at the cost of needing every single battle terrain in `sBattleTerrainTable`. These changes are done in [battle_bg.c](../blob/master/src/battle_bg.c).

This will walk you through porting this behaviour over to Emerald. Firstly, we're going to look at some things from pokefirered.

```c
// Maps map scene values to battle terrain IDs.
// Maps with any of the MAP_BATTLE_SCENE_* values seen here will use these
// regardless of whether the player is on water or in tall grass etc
static const struct {
    u8 mapScene;
    u8 battleTerrain;
} sMapBattleSceneMapping[] = {
    {MAP_BATTLE_SCENE_GYM,      BATTLE_TERRAIN_GYM},
    {MAP_BATTLE_SCENE_INDOOR_1, BATTLE_TERRAIN_INDOOR_1},
    {MAP_BATTLE_SCENE_INDOOR_2, BATTLE_TERRAIN_INDOOR_2},
    {MAP_BATTLE_SCENE_LORELEI,  BATTLE_TERRAIN_LORELEI},
    {MAP_BATTLE_SCENE_BRUNO,    BATTLE_TERRAIN_BRUNO},
    {MAP_BATTLE_SCENE_AGATHA,   BATTLE_TERRAIN_AGATHA},
    {MAP_BATTLE_SCENE_LANCE,    BATTLE_TERRAIN_LANCE},
    {MAP_BATTLE_SCENE_LINK,     BATTLE_TERRAIN_LINK}
};
```

```c
// If current map scene equals any of the values in sMapBattleSceneMapping,
// use its battle terrain value. Otherwise, use the default.
static u8 GetBattleTerrainByMapScene(u8 mapBattleScene)
{
    int i;
    for (i = 0; i < NELEMS(sMapBattleSceneMapping); i++)
    {
        if (mapBattleScene == sMapBattleSceneMapping[i].mapScene)
            return sMapBattleSceneMapping[i].battleTerrain;
    }
    return BATTLE_TERRAIN_PLAIN;
}
```

```c
// Loads the initial battle terrain.
static void LoadBattleTerrainGfx(u16 terrain)
{
    if (terrain >= NELEMS(sBattleTerrainTable))
        terrain = BATTLE_TERRAIN_PLAIN;  // If higher than the number of entries in sBattleTerrainTable, use the default.
    // Copy to bg3
    LZDecompressVram(sBattleTerrainTable[terrain].tileset, (void *)BG_CHAR_ADDR(2));
    LZDecompressVram(sBattleTerrainTable[terrain].tilemap, (void *)BG_SCREEN_ADDR(26));
    LoadCompressedPalette(sBattleTerrainTable[terrain].palette, 0x20, 0x60);
}
```

```c
// Loads the entry associated with the battle terrain.
// This can be the grass moving on the screen at the start of a wild encounter in tall grass.
static void LoadBattleTerrainEntryGfx(u16 terrain)
{
    if (terrain >= NELEMS(sBattleTerrainTable))
        terrain = BATTLE_TERRAIN_PLAIN;
    // Copy to bg1
    LZDecompressVram(sBattleTerrainTable[terrain].entryTileset, (void *)BG_CHAR_ADDR(1));
    LZDecompressVram(sBattleTerrainTable[terrain].entryTilemap, (void *)BG_SCREEN_ADDR(28));
}
```

```c
// Gets the battle terrain value if conditions are met.
// This could be a specific trainer class, battle type or wild Pokémon.
// If no conditions are met or if the map scene isn't MAP_BATTLE_SCENE_NORMAL,
// use the battle terrain mapped to map scenes.
static u8 GetBattleTerrainOverride(void)
{
    u8 battleScene;
    if (gBattleTypeFlags & (BATTLE_TYPE_TRAINER_TOWER | BATTLE_TYPE_LINK | BATTLE_TYPE_BATTLE_TOWER | BATTLE_TYPE_EREADER_TRAINER))
    {
        return BATTLE_TERRAIN_LINK;
    }
    else if (gBattleTypeFlags & BATTLE_TYPE_POKEDUDE)
    {
        gBattleTerrain = BATTLE_TERRAIN_GRASS;
        return BATTLE_TERRAIN_GRASS;
    }
    else if (gBattleTypeFlags & BATTLE_TYPE_TRAINER)
    {
        if (gTrainers[gTrainerBattleOpponent_A].trainerClass == CLASS_LEADER_2)
        {
            return BATTLE_TERRAIN_LEADER;
        }
        else if (gTrainers[gTrainerBattleOpponent_A].trainerClass == CLASS_CHAMPION_2)
        {
            return BATTLE_TERRAIN_CHAMPION;
        }
    }
    battleScene = GetCurrentMapBattleScene();
    if (battleScene == MAP_BATTLE_SCENE_NORMAL)
    {
        return gBattleTerrain;
    }
    return GetBattleTerrainByMapScene(battleScene);
}
```

We will basically be taking advantage of these functions to get the battle terrain we are wanting to use. We will need to add some more BATTLE_TERRAIN_* values. In `include/constants/battle.h`,
We should see these:
```c
// Battle terrain defines for gBattleTerrain.
#define BATTLE_TERRAIN_GRASS        0
#define BATTLE_TERRAIN_LONG_GRASS   1
#define BATTLE_TERRAIN_SAND         2
#define BATTLE_TERRAIN_UNDERWATER   3
#define BATTLE_TERRAIN_WATER        4
#define BATTLE_TERRAIN_POND         5
#define BATTLE_TERRAIN_MOUNTAIN     6
#define BATTLE_TERRAIN_CAVE         7
#define BATTLE_TERRAIN_BUILDING     8
#define BATTLE_TERRAIN_PLAIN        9
```
It's going to need to look like this:
```c
// Battle terrain defines for gBattleTerrain.
#define BATTLE_TERRAIN_GRASS        0
#define BATTLE_TERRAIN_LONG_GRASS   1
#define BATTLE_TERRAIN_SAND         2
#define BATTLE_TERRAIN_UNDERWATER   3
#define BATTLE_TERRAIN_WATER        4
#define BATTLE_TERRAIN_POND         5
#define BATTLE_TERRAIN_MOUNTAIN     6
#define BATTLE_TERRAIN_CAVE         7
#define BATTLE_TERRAIN_BUILDING     8
#define BATTLE_TERRAIN_PLAIN        9
#define BATTLE_TERRAIN_FRONTIER     10
#define BATTLE_TERRAIN_GYM          11
#define BATTLE_TERRAIN_LEADER       12
#define BATTLE_TERRAIN_MAGMA        13
#define BATTLE_TERRAIN_AQUA         14
#define BATTLE_TERRAIN_SIDNEY       15
#define BATTLE_TERRAIN_PHOEBE       16
#define BATTLE_TERRAIN_GLACIA       17
#define BATTLE_TERRAIN_DRAKE        18
#define BATTLE_TERRAIN_CHAMPION     19
#define BATTLE_TERRAIN_GROUDON      20
#define BATTLE_TERRAIN_KYOGRE       21
#define BATTLE_TERRAIN_RAYQUAZA     22
```

We then need to update `sBattleTerrainTable` with our new `BATTLE_TERRAIN_*` values. They need to be in order of these values.
So if you change `BATTLE_TERRAIN_PLAIN` to be for example 20, you will need to move it further down the terrain table.
Changes needed are large so I'll show individually what's needed.

```c
    [BATTLE_TERRAIN_FRONTIER] =
    {
        .tileset = gBattleTerrainTiles_Building,
        .tilemap = gBattleTerrainTilemap_Building,
        .entryTileset = gBattleTerrainAnimTiles_Building,
        .entryTilemap = gBattleTerrainAnimTilemap_Building,
        .palette = gBattleTerrainPalette_Frontier,
    },
```

```c
    [BATTLE_TERRAIN_GYM] =
    {
        .tileset = gBattleTerrainTiles_Building,
        .tilemap = gBattleTerrainTilemap_Building,
        .entryTileset = gBattleTerrainAnimTiles_Building,
        .entryTilemap = gBattleTerrainAnimTilemap_Building,
        .palette = gBattleTerrainPalette_BuildingGym,
    },
```

```c
    [BATTLE_TERRAIN_LEADER] =
    {
        .tileset = gBattleTerrainTiles_Building,
        .tilemap = gBattleTerrainTilemap_Building,
        .entryTileset = gBattleTerrainAnimTiles_Building,
        .entryTilemap = gBattleTerrainAnimTilemap_Building,
        .palette = gBattleTerrainPalette_BuildingLeader,
    },
```

```c
    [BATTLE_TERRAIN_MAGMA] =
    {
        .tileset = gBattleTerrainTiles_Stadium,
        .tilemap = gBattleTerrainTilemap_Stadium,
        .entryTileset = gBattleTerrainAnimTiles_Building,
        .entryTilemap = gBattleTerrainAnimTilemap_Building,
        .palette = gBattleTerrainPalette_StadiumMagma,
    },
```

```c
    [BATTLE_TERRAIN_AQUA] =
    {
        .tileset = gBattleTerrainTiles_Stadium,
        .tilemap = gBattleTerrainTilemap_Stadium,
        .entryTileset = gBattleTerrainAnimTiles_Building,
        .entryTilemap = gBattleTerrainAnimTilemap_Building,
        .palette = gBattleTerrainPalette_StadiumAqua,
    },
```

```c
    [BATTLE_TERRAIN_SIDNEY] =
    {
        .tileset = gBattleTerrainTiles_Stadium,
        .tilemap = gBattleTerrainTilemap_Stadium,
        .entryTileset = gBattleTerrainAnimTiles_Building,
        .entryTilemap = gBattleTerrainAnimTilemap_Building,
        .palette = gBattleTerrainPalette_StadiumSidney,
    },
```

```c
    [BATTLE_TERRAIN_PHOEBE] =
    {
        .tileset = gBattleTerrainTiles_Stadium,
        .tilemap = gBattleTerrainTilemap_Stadium,
        .entryTileset = gBattleTerrainAnimTiles_Building,
        .entryTilemap = gBattleTerrainAnimTilemap_Building,
        .palette = gBattleTerrainPalette_StadiumPhoebe,
    },
```

```c
    [BATTLE_TERRAIN_GLACIA] =
    {
        .tileset = gBattleTerrainTiles_Stadium,
        .tilemap = gBattleTerrainTilemap_Stadium,
        .entryTileset = gBattleTerrainAnimTiles_Building,
        .entryTilemap = gBattleTerrainAnimTilemap_Building,
        .palette = gBattleTerrainPalette_StadiumGlacia,
    },
```

```c
    [BATTLE_TERRAIN_DRAKE] =
    {
        .tileset = gBattleTerrainTiles_Stadium,
        .tilemap = gBattleTerrainTilemap_Stadium,
        .entryTileset = gBattleTerrainAnimTiles_Building,
        .entryTilemap = gBattleTerrainAnimTilemap_Building,
        .palette = gBattleTerrainPalette_StadiumDrake,
    },
```

```c
    [BATTLE_TERRAIN_CHAMPION] =
    {
        .tileset = gBattleTerrainTiles_Stadium,
        .tilemap = gBattleTerrainTilemap_Stadium,
        .entryTileset = gBattleTerrainAnimTiles_Building,
        .entryTilemap = gBattleTerrainAnimTilemap_Building,
        .palette = gBattleTerrainPalette_StadiumWallace,
    },
```

```c
    [BATTLE_TERRAIN_GROUDON] =
    {
        .tileset = gBattleTerrainTiles_Cave,
        .tilemap = gBattleTerrainTilemap_Cave,
        .entryTileset = gBattleTerrainAnimTiles_Cave,
        .entryTilemap = gBattleTerrainAnimTilemap_Cave,
        .palette = gBattleTerrainPalette_Groudon,
    },
```

```c
    [BATTLE_TERRAIN_KYOGRE] =
    {
        .tileset = gBattleTerrainTiles_Water,
        .tilemap = gBattleTerrainTilemap_Water,
        .entryTileset = gBattleTerrainAnimTiles_Underwater,
        .entryTilemap = gBattleTerrainAnimTilemap_Underwater,
        .palette = gBattleTerrainPalette_Kyogre,
    },
```

```c
    [BATTLE_TERRAIN_RAYQUAZA] =
    {
        .tileset = gBattleTerrainTiles_Rayquaza,
        .tilemap = gBattleTerrainTilemap_Rayquaza,
        .entryTileset = gBattleTerrainAnimTiles_Rayquaza,
        .entryTilemap = gBattleTerrainAnimTilemap_Rayquaza,
        .palette = gBattleTerrainPalette_Rayquaza,
    },
```

The tedious bit is done. Now we need to start adding in the functions mentioned above but making changes to them so they work for Emerald's scenarios.

Our `sMapBattleSceneMapping` should look like this. I recommend putting this just below the terrain table but it's a matter of preference.
```c
static const struct {
    u8 mapScene;
    u8 battleTerrain;
} sMapBattleSceneMapping[] = {
    {MAP_BATTLE_SCENE_GYM,      BATTLE_TERRAIN_GYM},
    {MAP_BATTLE_SCENE_MAGMA,    BATTLE_TERRAIN_MAGMA},
    {MAP_BATTLE_SCENE_AQUA,     BATTLE_TERRAIN_AQUA},
    {MAP_BATTLE_SCENE_SIDNEY,   BATTLE_TERRAIN_SIDNEY},
    {MAP_BATTLE_SCENE_PHOEBE,   BATTLE_TERRAIN_PHOEBE},
    {MAP_BATTLE_SCENE_GLACIA,   BATTLE_TERRAIN_GLACIA},
    {MAP_BATTLE_SCENE_DRAKE,    BATTLE_TERRAIN_DRAKE},
    {MAP_BATTLE_SCENE_FRONTIER, BATTLE_TERRAIN_FRONTIER}
};
```

`GetBattleTerrainByMapScene`,`LoadBattleTerrainGfx` and `LoadBattleTerrainEntryGfx` mentioned above can be used as is. I recommend putting these below `sMapBattleSceneMapping`, but again, up to preference.

Now we need to add `GetBattleTerrainOverride` with our changes, this should look like this:
```c
static u8 GetBattleTerrainOverride(void)
{
    u8 battleScene;
    if (gBattleTypeFlags & (BATTLE_TYPE_FRONTIER | BATTLE_TYPE_LINK | BATTLE_TYPE_RECORDED_LINK | BATTLE_TYPE_EREADER_TRAINER))
    {
        return BATTLE_TERRAIN_FRONTIER;
    }
    else if (gBattleTypeFlags & BATTLE_TYPE_GROUDON)
    {
        return BATTLE_TERRAIN_GROUDON;
    }
    else if (gBattleTypeFlags & BATTLE_TYPE_KYOGRE)
    {
        return BATTLE_TERRAIN_KYOGRE;
    }
    else if (gBattleTypeFlags & BATTLE_TYPE_RAYQUAZA)
    {
        return BATTLE_TERRAIN_RAYQUAZA;
    }
    else if (gBattleTypeFlags & BATTLE_TYPE_TRAINER)
    {
        if (gTrainers[gTrainerBattleOpponent_A].trainerClass == TRAINER_CLASS_LEADER)
        {
            return BATTLE_TERRAIN_LEADER;
        }
        else if (gTrainers[gTrainerBattleOpponent_A].trainerClass == TRAINER_CLASS_CHAMPION)
        {
            return BATTLE_TERRAIN_CHAMPION;
        }
    }
    battleScene = GetCurrentMapBattleScene();
    if (battleScene == MAP_BATTLE_SCENE_NORMAL)
    {
        return gBattleTerrain;
    }
    return GetBattleTerrainByMapScene(battleScene);
}
```
I recommend putting this just before `LoadChosenBattleElement` but up to preference.

Don't forget to extern it under `static void CB2_UnusedBattleInit(void);` like so:
```c
static void CB2_UnusedBattleInit(void);
static u8 GetBattleTerrainOverride(void);
```

We then need to replace the entirety of `DrawMainBattleBackground` with:
```c
void DrawMainBattleBackground(void)
{
    LoadBattleTerrainGfx(GetBattleTerrainOverride());
}
```

We need to look for `DrawBattleEntryBackground` and replace that with:
```c
void DrawBattleEntryBackground(void)
{
    if (gBattleTypeFlags & BATTLE_TYPE_LINK)
    {
        LZDecompressVram(gBattleVSFrame_Gfx, (void*)(BG_CHAR_ADDR(1)));
        LZDecompressVram(gVsLettersGfx, (void*)OBJ_VRAM0);
        LoadCompressedPalette(gBattleVSFrame_Pal, 0x60, 0x20);
        SetBgAttribute(1, BG_ATTR_SCREENSIZE, 1);
        SetGpuReg(REG_OFFSET_BG1CNT, 0x5C04);
        CopyToBgTilemapBuffer(1, gBattleVSFrame_Tilemap, 0, 0);
        CopyToBgTilemapBuffer(2, gBattleVSFrame_Tilemap, 0, 0);
        CopyBgTilemapBufferToVram(1);
        CopyBgTilemapBufferToVram(2);
        SetGpuReg(REG_OFFSET_WININ, WININ_WIN0_BG1 | WININ_WIN0_BG2 | WININ_WIN0_OBJ | WININ_WIN0_CLR);
        SetGpuReg(REG_OFFSET_WINOUT, WINOUT_WIN01_BG1 | WINOUT_WIN01_BG2 | WINOUT_WIN01_OBJ | WINOUT_WIN01_CLR);
        gBattle_BG1_Y = 0xFF5C;
        gBattle_BG2_Y = 0xFF5C;
        LoadCompressedSpriteSheetUsingHeap(&sVsLettersSpriteSheet);
    }
    else if (gBattleTypeFlags & (BATTLE_TYPE_FRONTIER | BATTLE_TYPE_LINK | BATTLE_TYPE_RECORDED_LINK | BATTLE_TYPE_EREADER_TRAINER))
    {
        if (!(gBattleTypeFlags & BATTLE_TYPE_INGAME_PARTNER) || gPartnerTrainerId == TRAINER_STEVEN_PARTNER)
        {
            LoadBattleTerrainEntryGfx(BATTLE_TERRAIN_BUILDING);
        }
        else
        {
            // Set up bg for the multi battle intro where both teams slide in facing the screen.
            // Note Steven's multi battle (which has a dedicated back pic) is excluded above.
            SetBgAttribute(1, BG_ATTR_CHARBASEINDEX, 2);
            SetBgAttribute(2, BG_ATTR_CHARBASEINDEX, 2);
            CopyToBgTilemapBuffer(1, gMultiBattleIntroBg_Opponent_Tilemap, 0, 0);
            CopyToBgTilemapBuffer(2, gMultiBattleIntroBg_Player_Tilemap, 0, 0);
            CopyBgTilemapBufferToVram(1);
            CopyBgTilemapBufferToVram(2);
        }
    }
    else if (gBattleTypeFlags & BATTLE_TYPE_GROUDON)
    {
        LoadBattleTerrainEntryGfx(BATTLE_TERRAIN_CAVE);
    }
    else if (gBattleTypeFlags & BATTLE_TYPE_KYOGRE)
    {
        LoadBattleTerrainEntryGfx(BATTLE_TERRAIN_UNDERWATER);
    }
    else if (gBattleTypeFlags & BATTLE_TYPE_RAYQUAZA)
    {
        LoadBattleTerrainEntryGfx(BATTLE_TERRAIN_RAYQUAZA);
    }
    else
    {
        if (gBattleTypeFlags & BATTLE_TYPE_TRAINER)
        {
            u8 trainerClass = gTrainers[gTrainerBattleOpponent_A].trainerClass;
            if (trainerClass == TRAINER_CLASS_LEADER)
            {
                LoadBattleTerrainEntryGfx(BATTLE_TERRAIN_BUILDING);
                return;
            }
            else if (trainerClass == TRAINER_CLASS_CHAMPION)
            {
                LoadBattleTerrainEntryGfx(BATTLE_TERRAIN_BUILDING);
                return;
            }
        }

        if (GetCurrentMapBattleScene() == MAP_BATTLE_SCENE_NORMAL)
        {
            LoadBattleTerrainEntryGfx(gBattleTerrain);
        }
        else
        {
            LoadBattleTerrainEntryGfx(BATTLE_TERRAIN_BUILDING);
        }
    }
}
```

And then finally, we replace `LoadChosenBattleElement` with:
```c
bool8 LoadChosenBattleElement(u8 caseId)
{
    bool8 ret = FALSE;

    switch (caseId)
    {
    case 0:
        LZDecompressVram(gBattleTextboxTiles, (void*)(BG_CHAR_ADDR(0)));
        break;
    case 1:
        CopyToBgTilemapBuffer(0, gBattleTextboxTilemap, 0, 0);
        CopyBgTilemapBufferToVram(0);
        break;
    case 2:
        LoadCompressedPalette(gBattleTextboxPalette, 0, 0x40);
        break;
    case 3:
        LZDecompressVram(sBattleTerrainTable[GetBattleTerrainOverride()].tileset, (void *)BG_CHAR_ADDR(2));
        break;
    case 4:
        LZDecompressVram(sBattleTerrainTable[GetBattleTerrainOverride()].tilemap, (void *)BG_SCREEN_ADDR(26));
        break;
    case 5:
        LoadCompressedPalette(sBattleTerrainTable[GetBattleTerrainOverride()].palette, 0x20, 0x60);
        break;
    case 6:
        LoadBattleMenuWindowGfx();
        break;
    default:
        ret = TRUE;
        break;
    }

    return ret;
}
```

And with that, if I didn't miss anything out, battle terrain should be functionally equivalent to before without the need for big switch statements which can be a pain to work with and just a waste, in my opinion.