BoI:Afterbirth Angel Room Mechanics

Angel room generation is almost unchanged from Rebirth.

Whenever the game successfully spawns a Devil door (or you try to enter the devil room via teleport) for the first time on a floor, it checks whether it can spawn an Angel room instead if:

* You have seen a Devil DOOR spawn before on this run (teleporting in doesn't count)
* You have not bought an item in exchange for health (Devil Deals, Black Market)

If the prerequisites are fulfilled, the game does a kind of series of coin flips to determine whether you get an Angel room. Chance for an Angel room does not add up linearly like it does for a Devil room - it will never be either guaranteed or impossible. Also, the chance calculation always runs all possible checks, even if your base chance of 50% already got you an Angel room - this is important for the last step.

Applied example: The game flips the coin for your base chance at the Angel room. 50% chance to succeed, and 50% chance to fail, that being not getting an Angel room here.
But you have a Key Piece. In that case it rolls another 1 out of 4 for a 25% chance. Your chance of failure is now 0.5 * 0.75 = 0.375, which is a success rate of 62.5%.
Good thing you bombed that devil beggar earlier for another +25%. The game rolls another 1 out of 4 and now you have a 0.5 * 0.75 * 0.75 = 0.28125 chance for failure, meaning a ca. 72% chance for success.

As for all the different chance checks, there are:
* 1 out of 2 base chance for an Angel room (50%)
* 1 out of 2 chance for an Angel room from the Rosary Bead trinket (50%)
* 1 out of 2 chance for an Angel room for donating 10 or more coins on this floor (50%)
* 1 out of 4 chance for an Angel room for holding Key Piece #1 (25%)
* 1 out of 4 chance for an Angel room for holding Key Piece #2 (25%)
* 1 out of 4 chance for an Angel room for killing a Devil Beggar on this floor (25%)
* 1 out of 10 chance for an Angel room for satisfying a Beggar on this floor (10%)
* 1 out of 10 chance for NO Angel room for satisfying a Devil Beggar on this floor (-10%)

These last two will cancel each other out if you did both.

Before, i said that the game runs all checks regardless of your outcome, and the last one is why: If you satisfied a Devil Beggar (giving it health until it warps away), you have a 1 in 10 chance to NOT get an Angel room, even if one of the previous checks came up positive. This is the only thing in the game that negatively impacts your Angel room chances.

All of this was in the original Rebirth, too. The only thing that changed is that ONLY killing Devil Beggars will give you the bonus now; shellgames or key beggars will no longer count.

Your highest chance for an Angel room with all bonuses will be ~95.25% - your lowest (just satisfying a Devil Beggar) will be 45%.