Room Clear Awards

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When clearing a room, the game has a chance of rewarding a pickup or trinket. This is based on many things, including your current luck and whether you have some particular items.

Process

First, the game generates a random number using this formula:

(RandomFloat * Luck * 0.1) + pickupPercent

  • RandomFloat and pickupPercent are random numbers between 0 and 1
  • Luck is clamped to 0 if Isaac’s luck is less than 0 and 10 if Isaac’s luck is greater than 10
  • This chance is modified by the following items/trinkets:
    • Collectible Lucky Foot icon.pngLucky Foot multiplies pickupPercent by 0.9, then adds 0.1 to it.
    • Lucky ToeLucky Toe multiplies the final value by 0.9, then adds 0.1 to it. (But if Isaac also has Lucky Foot and your luck is greater than 0, the value is instead multiplied by 0.98, plus 0.02, with the Lucky Foot bonus still applying.)

That number is then used to determine the reward given after clearing the room, using the following index:

  • Nothing (< 0.22, base 22% chance)
  • A tarot card, pill, or trinket (0.22 - 0.3, base 8% chance)
    • All three possibilities are equally likely to be chosen, giving them individual base 2.66% chances.
  • A coin (0.3 - 0.45, base 15% chance)
    • The range increases to 0.3 - 0.5 (base 20%) if Isaac has (except in Rebirth)Rib of GreedRib of Greed
  • A heart (0.45 - 0.6, base 15% chance)
    • The range decreases to 0.5 - 0.6 (base 10%) if Isaac has Rib of Greed
    • If Isaac has Daemon's TailDaemon's Tail, the heart has an 80% chance of being replaced with a key.
  • A key (0.6 - 0.8, base 20% chance)
  • A bomb (0.8 - 0.95, base 15% chance)
  • A chest (> 0.95, base 5% chance)
    • If Isaac has positive luck, the chance for a chest increases and all other chances decrease.
      • Lucky Foot/Toe decrease the chance of getting nothing, but do not themselves increase the chance for chests to spawn.


The reward then has a chance to be replaced/modified by the following, in order:

  • A lil' battery (base 5% chance, plus an additional 6.67% chance if Isaac has (except in Rebirth)Watch BatteryWatch Battery)
  • A sack (2% chance, always in effect)
  • A tarot card (10% chance if Isaac has Ace of SpadesAce of Spades)
  • A pill (10% chance if Isaac has Safety CapSafety Cap)
  • A bomb (10% chance if Isaac has Match StickMatch Stick)
  • A heart (10% chance if Isaac has Child's HeartChild's Heart or 2% if Isaac also has Daemon's TailDaemon's Tail)
  • A key (10% chance if Isaac has Rusted KeyRusted Key)
    • If Isaac has multiple of these, the first one to activate is the one that replaces the room reward
  • A trinket (2% chance if Isaac has (except in Rebirth and Afterbirth)Collectible Smelter icon.pngSmelter)
  • Collectible Guppy's Tail icon.pngGuppy's Tail has a 33% chance to replace the reward with a chest (either gray or locked with equal chances) and a 22% chance to replace it with nothing.
  • Collectible Contract From Below icon.pngContract From Below causes an extra copy of the reward to spawn, but has a 33% chance to replace it with nothing.
    • Each extra contract Isaac has causes another copy of the reward to spawn and reduces the chance of the reward becoming nothing by 2/3.
  • If playing on Hard Mode, heart rewards have a (except in Repentance and Repentance+) 66%/(in Repentance and Repentance+) 33% chance to be replaced with nothing.
  • (except in Rebirth and Afterbirth)Collectible Broken Modem icon.pngBroken Modem has a 25% chance to create an additional copy of the reward if it’s a key, coin, heart, bomb, or sack.

Pseudo-code

The full logic is contained within the following pseudo-code: 

local room = Game():GetLevel():GetCurrentRoom()
local awardSeed = room.AwardSeed
local player = Game():GetPlayer(0)
local difficulty = Game().Difficulty

local rng = RNG()
rng:SetSeed(awardSeed, 35) --5, 9, 7

local pickupPercent = rng:RandomFloat()
if (player:HasCollectible(COLLECTIBLE_LUCKYFOOT)) then
	pickupPercent = (pickupPercent * 0.9) + 0.1
end

local luck = math.max(math.min(player.Luck, 10), 0) --Clamp to 0-10

--Max luck increases the pickupPercent range from 0-1 to 0-2
--That means the more luck you have, the more likely you are to get chests.
pickupPercent = rng:RandomFloat() * luck * 0.1 + pickupPercent

if (player:HasTrinket(TRINKET_LUCKY_TOE)) then
	if (player:HasCollectible(COLLECTIBLE_LUCKYFOOT) and luck > 0) then
		pickupPercent = (pickupPercent * 0.98) + 0.02
	else
		pickupPercent = (pickupPercent * 0.9) + 0.1
	end
end

local pickupAward = COLLECTIBLE_NULL
local pickupCount = 1

if (pickupPercent > 0.22) then
	if (pickupPercent < 0.3) then
		if (rng:RandomInt(3) == 0) then
			pickupAward = PICKUP_TAROTCARD
		elseif (rng:RandomInt(2) == 0) then
			pickupAward = PICKUP_TRINKET
		else
			pickupAward = PICKUP_PILL
		end
	elseif (pickupPercent < 0.45) then
		pickupAward = PICKUP_COIN
	elseif (pickupPercent < 0.5 and player:HasTrinket(TRINKET_RIB_OF_GREED)) then
		pickupAward = PICKUP_COIN
	elseif (pickupPercent < 0.6 and (not player:HasTrinket(TRINKET_DAEMONS_TAIL) or rng:RandomInt(5) == 0)) then
		pickupAward = PICKUP_HEART
	elseif (pickupPercent < 0.8) then
		pickupAward = PICKUP_KEY
	elseif (pickupPercent < 0.95) then
		pickupAward = PICKUP_BOMB
	else
		pickupAward = PICKUP_CHEST
	end
	
	if (rng:RandomInt(20) == 0 or (rng:RandomInt(15) == 0 and player:HasTrinket(TRINKET_WATCH_BATTERY))) then
		pickupAward = PICKUP_LIL_BATTERY
	end
	
	if (rng:RandomInt(50) == 0) then
		pickupAward = PICKUP_GRAB_BAG
	end
	
	if (player:HasTrinket(TRINKET_ACE_SPADES) and rng:RandomInt(10) == 0) then
		pickupAward = PICKUP_TAROTCARD
	elseif (player:HasTrinket(TRINKET_SAFETY_CAP) and rng:RandomInt(10) == 0) then
		pickupAward = PICKUP_PILL
	elseif (player:HasTrinket(TRINKET_MATCH_STICK) and rng:RandomInt(10) == 0) then
		pickupAward = PICKUP_BOMB
	elseif (player:HasTrinket(TRINKET_CHILDS_HEART) and rng:RandomInt(10) == 0 and (not player:HasTrinket(TRINKET_DAEMONS_TAIL) or rng:RandomInt(5) == 0)) then
		pickupAward = PICKUP_HEART
	elseif (player:HasTrinket(TRINKET_RUSTED_KEY) and rng:RandomInt(10) == 0) then
		pickupAward = PICKUP_KEY
	end
	
		
	if (player:HasCollectible(COLLECTIBLE_SMELTER) and rng:RandomInt(50) == 0) then
		pickupAward = PICKUP_TRINKET
	end
end


if (player:HasCollectible(COLLECTIBLE_GUPPYS_TAIL)) then
	if (rng:RandomInt(3) != 0) then
		if (rng:RandomInt(3) == 0) then
			pickupAward = PICKUP_NULL
		end
	else
		if (rng:RandomInt(2) != 0) then
			pickupAward = PICKUP_LOCKEDCHEST
		else
			pickupAward = PICKUP_CHEST
		end
	end
end

if (player:HasCollectible(COLLECTIBLE_CONTRACT_FROM_BELOW) and pickupAward != PICKUP_TRINKET) then
	pickupCount = player:GetCollectibleNum(COLLECTIBLE_CONTRACT_FROM_BELOW) + 1
	--The chance of getting nothing goes down with each contract exponentially
	local nothingChance = math.pow(0.666, pickupCount - 1)
	if (nothingChance * 0.5 > rng:NextFloat()) then
		pickupCount = 0
	end
end

if (difficulty == 1 and pickupAward == PICKUP_HEART) then
	if rng:RandomInt(100) >= 35 then
		pickupAward = PICKUP_NULL
	end
end

if (player:HasCollectible(COLLECTIBLE_BROKEN_MODEM) and rng:RandomInt(4) == 0 and pickupCount >= 1 and
		(pickupAward == PICKUP_COIN or pickupAward == PICKUP_HEART or pickupAward == PICKUP_KEY or pickupAward == PICKUP_GRAB_BAG or pickupAward == PICKUP_BOMB) then
	pickupCount = pickupCount + 1
end

if (pickupCount > 0 and pickupAward != PICKUP_NULL) then
	local subType = 0
	for i=1, pickupCount do
		local ent = Game():Spawn(ENTITY_PICKUP, pickupAward, nearCenter, Vector(0, 0), 0, subtype, rng:Next())
		subType = ent.SubType
	end
end

Notes

This pseudo-code is taken from Blade's GitHub Gist. (Blade is also known as blcd / Will.) He reverse engineered the game using a disassembler in order to create this pseudo-code.


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