Effects of Pokémon Abilities on Wild Encounter Rate

Effects of Pokémon Abilities on Wild Encounter Rate

In the Pokémon Emerald disassembly:
https://github.com/pret/pokeemerald/blob/83df84e40623b79281f2397faa611cbf044170bd/src/wild_encounter.c#L509

    if (!ignoreAbility && !GetMonData(&gPlayerParty[0], MON_DATA_SANITY_IS_EGG))
    {
        u32 ability = GetMonAbility(&gPlayerParty[0]);

        if (ability == ABILITY_STENCH && gMapHeader.mapLayoutId == LAYOUT_BATTLE_FRONTIER_BATTLE_PYRAMID_FLOOR)
            encounterRate = encounterRate * 3 / 4;
        else if (ability == ABILITY_STENCH)
            encounterRate /= 2;
        else if (ability == ABILITY_ILLUMINATE)
            encounterRate *= 2;
        else if (ability == ABILITY_WHITE_SMOKE)
            encounterRate /= 2;
        else if (ability == ABILITY_ARENA_TRAP)
            encounterRate *= 2;
        else if (ability == ABILITY_SAND_VEIL && gSaveBlock1Ptr->weather == WEATHER_SANDSTORM)
            encounterRate /= 2;
    }

GetMonData() is defined here:
https://github.com/pret/pokeemerald/blob/83df84e40623b79281f2397faa611cbf044170bd/src/pokemon.c#L3642

I won't go through much detail but basically the call GetMonData(&mon, field) is dispatched to function GetMonData2(*mon, field) which is an alias for GetMonData3(*mon, field, NULL). GetMonData3() doesn't have a case for MON_DATA_SANITY_IS_EGG so it calls GetBoxMonData(&mon->box, field, data). This function dispatches to GetBoxMonData3() which then returns boxMon->isEgg.

In other words the if statement from the function from wild_encounter.c can be simplified to:
if (don't ignore ability && mon isn't egg) {
	if mon's ability is Arena Trap:
		double the encounter rate
}

This also applies to all other abilities that raise or lower the encounter rate.

Interestingly Stench multiplies the encounter rate by 3/4 (round down) only in the Battle Pyramid.