Item Pool

An **item pool** is the set of items that can be generated under a given circumstance. There are many item pools that exist within *The Binding of Isaac: Rebirth.*

## Item Weight

* Each item that is in an item pool has a weight associated with it. The weight represents how likely it is for the game to spawn the item. Nearly every item in the game has a weight of 1.
	+ For example, if there are 200 items in a given item pool, and each item has a weight of 1, that means there is a 1 in 200 (0.5%) chance to get any particular item from that pool.
	+ However, if an item has a weight of 0.5, that means that item is 50% as likely to spawn compared to an item that has a weight of 1. In a pool of 200 items that has only 2 items with a weight of 0.5, that would result in the total weight being 199. ((1 \* 198) + (2 \* 0.5) = 199) This means items with a weight of 1 have a 1 in 199 (0.5%) chance to spawn, while an item with a weight of 0.5 has a 1 in 398 (0.25%) chance to spawn.
* Only a few items in the game do not have a weight of 1:
	+ has a weight of 0.2 in the Bomb Bum pool, making it 80% less likely to encounter.
	+ has a weight of 0.4 in Angel Room pool, making it 60% less likely to encounter.
	+ has a weight of 0.2 in the Devil Room and Greed Mode Devil Room pools, making it 80% less likely to encounter.
	+ has a weight of 0.2 in the Curse Room, Devil Beggar, Devil Room, Greed Mode Shop, Red Chest and Shop pools, making it 80% less likely to encounter.
* Item weights were adjusted more extensively in Repentance.
	+ Treasure Room:
		- : 0.2
		- : 0.2
		- : 0.2
		- : 0.1
		- : 0.5
		- : 0.5
		- : 0.5
		- : 0.5
		- : 0.2
		- : 0.2
		- : 0.5
		- : 0.1
		- : 0.2
		- : 0.2
		- : 0.5
		- : 0.1
		- : 0.2
		- : 0.2
		- : 0.1
		- : 0.1
	+ Shop:
		- : 0.5
		- : 0.5
		- : 0.5
		- : 0.5
		- : 0.5
		- : 0.1
		- : 0.5
		- : 0.2
		- : 0.5
	+ Library:
		- : 0.2
		- : 0.2
	+ Curse Room:
		- : 0.5
		- : 0.5
		- : 0.2
		- : 0.5
	+ Boss Room:
		- : 0.5
		- : 0.5
	+ Secret Room:
		- : 0.5
		- : 0.5
	+ Devil Room:
		- : 0.5
		- : 0.5
		- : 0.5
		- : 0.2
		- : 0.5
		- : 0.5
		- : 0.2
		- : 0.2
		- : 0.5
	+ Angel Room:
		- : 0.5
		- : 0.4
		- : 0.5
		- : 0.5
	+ Golden Chest:
		- : 0.5
		- : 0.1
		- : 0.1
		- : 0.5
		- : 0.5
	+ Red Chest:
		- : 0.5
		- : 0.2
		- : 0.1
		- : 0.1
	+ Devil Beggar:
		- : 0.2
		- : 0.5
		- : 0.5
		- : 0.2
		- : 0.2
		- : 0.2
	+ Bomb Bum:
		- : 0.2
		- : 0.2
	+ Rotten Beggar:
		- : 0.5
		- : 0.5
	+ Battery Bum:
		- : 0.1
		- : 0.1
	+ Old Chest:
		- : 0.2
		- : 0.2
	+ Mom's Chest:
		- : 0.1
		- : 0.5
	+ Greed Mode Treasure Room:
		- : 0.1
		- : 0.2
		- : 0.2
		- : 0.1
		- : 0.5
		- : 0.5
		- : 0.5
		- : 0.5
		- : 0.2
		- : 0.2
		- : 0.2
		- : 0.5
		- : 0.5
		- : 0.2
		- : 0.1
		- : 0.2
		- : 0.5
		- : 0.1
		- : 0.2
		- : 0.1
		- : 0.2
		- : 0.1
	+ Greed Mode Shop:
		- : 0.1
		- : 0.5
		- : 0.5
		- : 0.1
		- : 0.1
		- : 0.1
		- : 0.1
		- : 0.1
		- : 0.1
		- : 0.1
		- : 0.5
		- : 0.2
		- : 0.1
		- : 0.1
		- : 0.1
		- : 0.1
		- : 0.1
		- : 0.1
	+ Greed Mode Curse Room:
		- : 0.5
		- : 0.5
		- : 0.2
	+ Greed Mode Boss Room:
		- : 0.2
		- : 0.5
		- : 0.5
	+ Greed Mode Secret Room:
		- : 0.5
	+ Greed Mode Devil Room:
		- : 0.5
		- : 0.5
		- : 0.5
		- : 0.5
		- : 0.5
		- : 0.2
		- : 0.2
	+ Greed Mode Boss Room:
		- : 0.5
		- : 0.5
	+ Greed Mode Golden Chest:
		- : 0.5
		- : 0.1
		- : 0.1
		- : 0.5
		- : 0.5

## Weight Decrementation

* When you see an item spawn, the weight of that item is automatically decremented inside of the respective item pool. However, the amount of the decrementation varies depending on the specific item pool.
* If an item happens to be in more than one item pool, the weight decrementation will also apply to other pools.
* Touching an item will remove it from all item pools.
* If a pedestal cycles between multiple items, such as from , each item option will count as being seen.

The following item pools are decremented by 50% (0.5) on sight:

* Beggar
* Bomb Bum
* Curse Room and Greed Mode Curse Room
* Devil Beggar
* Key Master
* Shop and Greed Mode Shop

The following item pools are decremented by 40% (0.4) on sight:

* Golden Chest and Greed Mode Golden Chest
* Red Chest

The following item pools are decremented by 10% (0.1) on sight:

* Library and Greed Mode Library

All other item pools not listed above are fully decremented on sight.

### Examples

* All items in the Treasure Room pool are completely decremented on sight. Thus, seeing a  in the Basement 1 Treasure Room will make it impossible to spawn another Notched Axe in the run. (In this case, players do **not** have to touch the Notched Axe in order to prevent the game from ever spawning another one.)
* All items in the shop pool are decremented by 0.5 on sight. Thus, if you use a Restock Machine to reroll a , you have a chance for another Treasure Map to appear. However, after the Treasure Map has already appeared twice on a run, it is completely decremented from the shop pool and it will never appear again.
	+ All items in the shop pool are fully decremented on sight. Thus, if you use a Restock Machine to reroll a , it will never appear again for the rest of the run.
* If the player sees  in the Basement 1 Treasure Room, it is completely removed from all other pools for the rest of the run even without touching it. Seeing it in a Library and not touching it, however, allows it still to be encountered as a Treasure item, with slightly lowered weight.

## Unused Item Pools

In the `itempools.xml` file, we can see that there are some unused item pools:

* `bossrush` - Likely intended to be used for the Boss Rush. However, in the actual game, the Boss Rush draws from the Treasure Room pool.
* `challenge` - Likely intended to be used for Challenge Rooms. However, in the actual game, Challenge Rooms draw from the Treasure Room pool.
* `dungeon` - Likely intended to be used for the Crawl Spaces. However, in the actual game, Crawl Spaces draw from the Treasure Room pool.

## Set Drops

* Some specific collectible items are hard-coded to have a chance to drop from certain special events. These are referred to as "set drops".
* Set drops from the Seven Deadly Sins are not affected by item pools. For example, if you touch  in the Treasure Room on Basement 1, and then you fight Lust in the Mini-Boss Room of Basement 2, Lust will still have the normal chance to drop . If it does, you can take it and have two Viruses.
* Set drops from most Machines can be obtained multiple times.
	+ With the exception of , , and .
		- If an  has already been touched ( or seen), then Blood Donation Machines will only spawn s after.
* Set drops from bosses will always replace the Boss Room Item if the set drop wasn't already obtained or depleted from pools, with some exceptions.
	+ Famine, Pestilence, War, and Death will always drop  or , even if either was already obtained.
		- Respectively, if  or  have been touched, then The Headless Horseman and Conquest will drop a Boss Room Item instead.
		- Respectively, if  or  have been touched, then The Headless Horseman and Conquest will always drop either  or .
	+ Krampus will always spawn either  or , even if either was already obtained.
	+ Krampus will only spawn  once, then will only spawn  after, even if it was already obtained.
	+ Mom will always drop  and  if they are unlocked, even if either was already obtained.
	+ Bumbino will drop his items in addition to the boss room item, and will always drop  once all his drops are obtained.
	+ Hornfel will drop his items in addition to the boss room item, and will only drop 3 bombs once all his drops are seen.
* 's drops are deceptively not from the Devil Room Pool, and are considered set drops.
	+ Once all of its drops are seen, it will always drop .
* All other set drops not mentioned above can only spawn if the item hasn't already been obtained or depleted from pools.

* - Drops from Gluttony
* - Drops from Pride
* - Drops from Super Sloth
* - Drops from Sloth
* - Drops from Super Gluttony
* - Drops from Super Pride
* - Drops from Ultra Pride when Florian is killed last
* - Drops from Super Envy
* - Drops from Wrath
* - Drops from Super Wrath
* - Drops from Super Greed
* - Drops from Envy
* - Drops from Greed
* - Drops from Lust
* - Drops from Super Lust

From Bosses:

* - Drops from Famine, Pestilence, War, and Death
* - Drops from Famine, Pestilence, War, and Death
* - Drops from Krampus
* - Drops from Uriel and Gabriel
* - Drops from Gabriel and Uriel
* - Drops from C.H.A.D.
* - Drops from Gish
* - Drops from Steven
* - Drops from Krampus
* - Drops from Mom
* - Drops from Baby Plum after not damaging her for 30 seconds
* - Drops from Mom
* - Drops from The Headless Horseman
* - Drops from Steven
* - Drops from Conquest

From Machines:

* - Drops from Blood Donation Machines
* - Drops from Fortune Telling Machines
* - Drops from Slot Machines
* - Drops from Blood Donation Machines
* - Drops from  Mom's Dressing Table
* - Drops from  Mom's Dressing Table
* - Drops from  Mom's Dressing Table
* - Drops from  Mom's Dressing Table
* - Drops from  Mom's Dressing Table
* - Drops from  Mom's Dressing Table
* - Drops from  Mom's Dressing Table
* - Drops from  Mom's Dressing Table
* - Drops from  Mom's Dressing Table
* - Drops from  Mom's Dressing Table
* - Drops from Confessionals after taking a Devil Deal
* - Drops from the Shell Game

From Rocks:

* - Drops from destroying polyps
* - Drops from destroying skulls
* - Drops from destroying skulls
* - Drops from destroying buckets in Downpour
* - Drops from destroying mushrooms
* - Drops from destroying mushrooms
* - Drops from destroying polyps
* - Drops from destroying buckets in Dross
* - Drops from destroying urns
* - Drops from destroying polyps in the Corpse
* - Drops from destroying polyps in the Corpse
* - Drops from destroying tinted rocks and super special rocks
* - Drops from destroying mushrooms in a Secret Room

* - Drops from  on Womb II
* - Drops from
* - Drops from
* - Drops from

From Rooms:

* - Found by bombing the first starting room after defeating the first boss in under 1 minute
* - Found by defeating Boss Rush while holding
* - Found in Mausoleum and Gehenna II's Boss Room after entering The Strange Door
* - Found in a rare room in the Mines
* - Found in the hidden closet in Home
* - Found in the Mirrored World's Treasure Room
* - Found at the end of the Mine Cart Room area
* - Found in a rare room in Downpour and Dross
* - Drops from Sacrifice Rooms after taking a Devil Deal
* - Found in Grave Rooms

 From Bumbino after he collects 12 cents:

 Chance from Hornfel after defeating his 2nd phase before he escapes:

 From sacrificing familiars with

## Notes

* Item pools can be exhausted by having the game create every item in that specific pool. Items do not need to be picked up for this to happen, the game simply has to generate them (rerolling the item with  for example).
	+ When an item pool is exhausted, the game will generate items from the Treasure Room pool instead.
	+ If the Treasure Room pool is exhausted, the only item that can be generated from it is .
* Even though you technically cannot see items during Curse of the Blind, their weights are still being reduced upon encountering them.
	+ The same applies to blind items in alternative chapters.