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, there are 339 items in the Treasure Room pool. Each has a weight of 1, which means that there is a 1 in 339 (0.29%) chance to get any particular item.
 * 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.

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.

The following item pools are fully decremented on sight:
 * Angel Room and Greed Mode Angel Room
 * Boss Room and Greed Mode Boss Room
 * Devil Room and Greed Mode Devil Room
 * Secret Room
 * Treasure Room and Greed Mode Treasure Room

The following item pools are decremented by 50% (0.5) on sight:
 * Beggar
 * Bomb Bum
 * Curse Room and Greed Mode Curse Room
 * Devil Beggar
 * Greed Mode Secret Room
 * 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

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 see another Notched Axe in a future Treasure Room. (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.
 * 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  file, we can see that there are some unused item pools:
 * - Likely intended to be used for the Boss Rush. However, in the actual game, the Boss Rush draws from the Treasure Room pool.
 * - Likely intended to be used for Challenge Rooms. However, in the actual game, Challenge Rooms draw from the Treasure Room pool.
 * - 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 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.
 * Similarly, just seeing set drops does not decrement them from other pools. For example, if you fight Lust in the Mini-Boss Room of Basement 1, it drops and the item remains uncollected, it will remain in the Treasure Room item pool and it can still be found in a Treasure Room later on in the run. Touching the drop will, as expected, remove it from all pools.

The set drops are as follows:
 * - Drops from Gluttony
 * - Drops from Pride
 * - Drops from Blood Donation Machines
 * - Drops from Super Sloth
 * - Drops from Sloth
 * - Drops from Super Gluttony
 * - Drops from Fortune Telling Machines
 * - Drops from Super Pride
 * - Drops from Slot Machines
 * - Drops from Super Envy
 * - Drops from Blood Donation Machines
 * - Drops from Wrath
 * - Drops from Super Wrath
 * - Drops from Super Greed
 * - Drops from Envy
 * - Drops from the Shell Game
 * - Drops from Greed
 * - Drops from Lust
 * - Drops from Super Lust

Note that Blood Donation Machines will always spawn a instead of an  if you have already touched an IV Bag during the run.

Pseudo-Set Drops

 * Destroying a pot, mushroom, or skull has a 1% chance to spawn a specific collectible; see below.
 * Like set drops, these items are hard coded and exist outside of the item pools.
 * However, unlike set drops, these collectibles will not spawn if they have already been fully decremented out of their respective pools.
 * For example, if a player sees in the Basement 1 Treasure Room and does not touch it, then they will not be able to get A Quarter from an urn for the remainder of the run.
 * Furthermore, if a psuedo-set drop is successfully spawned from a urn/mushroom/skull, it will be also be decremented from the respective pool.
 * For example, if a player gets A Quarter from destroying an urn, then they will not be able to get A Quarter from another urn for the remainder of the run.

The pseudo-set drops as as follows:
 * - 0.5% chance from destroying a polyp
 * - 0.5% chance from destroying a skull
 * - 0.5% chance from destroying a skull
 * - 0.5% chance from destroying a mushroom
 * - 0.5% chance from destroying a mushroom
 * - 0.5% chance from destroying a polyp
 * - 1% chance from destroying a urn