Altering Deal Chance Example

There are four callbacks that allow us to manipulate devil and angel chances.

MC_PRE_DEVIL_APPLY_ITEMS is run when most items that effect deal chance modify the calculation. It's called before the stage penalty is applied. If a deal has spawned anywhere on the previous two floors, the game decays the resulting deal chance by a stage penalty of either 25% or 50%, depending on how many deals have been taken.

It's important to note that even though they game displays a value of ~33% or ~66% for the 25% and 50% values respectively, devil chance is not clamped to a value between 0 and 1, and "100%" chance without items generally means a value of around ~133%.

Return a float to modify the chance in this step of the calculation.

MC_PRE_DEVIL_APPLY_STAGE_PENALTY allows you to return false to bypass the stage penalty.

MC_PRE_DEVIL_APPLY_SPECIAL_ITEMS is where the game applies modifications that bypass the stage penalty, for items such as Goat Head and Eucharist. Return a float to modify the chance in this step of the calculation.

MC_POST_DEVIL_CALCULATE is called after the deal chance has been calculated. You can return a float here to override the calculated chance.

The following example mod adds an item that will randomize the deal chance on MC_PRE_DEVIL_APPLY_SPECIAL_ITEMS while in an uncleared boss room, and keeps that chance applied after the room is cleared.

Code

You can download the full mod and its assets by clicking here.

Video Demonstration

The devil deal chance is randomized every frame while fighting Baby Plum.

Example

local mod = RegisterMod("Repentogon Devil Chance", 1)
local game = Game()

-- The item "Dark Spellcasting Tome" will set the devil deal chance to
-- a random percentage every frame while in an uncleared boss room.
local ITEM_ID = Isaac.GetItemIdByName("Dark Spellcasting Tome")

-- Store the random chance that will be set when the boss room is cleared.
local modifiedChance = 0

-- There's four callbacks for devil chance that are applied at different times.
-- The one we're using is MC_PRE_DEVIL_APPLY_SPECIAL_ITEMS.
-- This is where the game applies chances that bypass the stage penalty, such as Goat Head and Eucharist.
-- There's also MC_PRE_DEVIL_APPLY_ITEMS, which is where most items apply their modified chance.

-- If in a boss room and it isn't cleared, set the chance to something random.
function mod:RandomizeDevilChance()
    -- Check if we should actually do the item's effects.
    -- We need the RNG for later so we'll check if the following variable is nil.
    local firstPlayer = PlayerManager.FirstCollectibleOwner(ITEM_ID)
    if firstPlayer then
        local room = game:GetRoom()

        -- Only do this in a boss room.
        if room:GetType() == RoomType.ROOM_BOSS then
            -- Check if in a boss room and if isn't clear, and randomize if so.
            if not room:IsClear() then
                local rng = firstPlayer:GetCollectibleRNG(ITEM_ID)

                -- Random chance between 0% and 100%.
                modifiedChance = rng:RandomFloat()
            end

            return modifiedChance
        end
    end
end

mod:AddCallback(ModCallbacks.MC_PRE_DEVIL_APPLY_SPECIAL_ITEMS, mod.RandomizeDevilChance)