A tip about boss room and top secret room locations and some supplements to secret room rules by gjchangmu ([email protected]) Jul. 7, 2014.

(revision 3)

Hello everyone! Last year I looked into the source code of The Binding of Isaac v1.48 aiming for knowing how the game decides the locations of boss rooms and secret rooms. I found that there are indeed some patterns for us to follow to predict the locations. I wrote a document describing 5 secret room location rules which can be found at https://docs.google.com/file/d/0B3jxLUI5Lb1rSUFTYk1jZ3Ruajg/edit. I also found a method to better predict boss room locations when you have got The Treasure Map, but I didn’t make it published because I found it hard to explain well, besides it is not very useful anyway. Months later /u/breetmcduck made a related post in which I briefly explained the method, and then /u/aggromidget made an excellent video about the method titled The Treasure Map Perfected which can be found at https://www.youtube.com/watch?v=LQrbQk04kE0. Today I make this short document to be a source to refer to while making the method a bit more clear, and to give a new related tip about the top secret rooms. Also, there are some supplements I was always hoping to add to the secret room rules, including a simplified rule 5, so here it comes. I’m very lucky for the 5 secret room location rules seeming to be correct as I haven’t heard about a real exception. But this time please consider the boss room tip to be more experimental and controversial, and please point out my mistakes and show exceptions if you found some. Feedbacks are very much appreciated, thank you all! I will keep updating this document for a few days adding things that I forget to mention and correcting mistakes.

A tip about boss room and top secret room locations A method to predict boss room locations in non-XL levels after you have got The Treasure Map You can watch aggromidget’s vivid video, instead of reading this section. The map of a non-XL level generates like a tree, the spawn room is the root and there are at most 4 branches of rooms. In the following picture, the spawn room is marked white, and rooms that belong to different branches are marked by different colors.

There is only one room in each branch that is possible to become the boss room, thus there are at most 4 rooms in a map that is possible. There is a method for us to tell for sure which is the only possible room to be the boss room in each branch. Firstly, in a branch, the boss Room must be among the rooms that are furthest from

the spawn room. So, if there is only one room that is 6 rooms away from the spawn room and there are no rooms that are 7 or more rooms away from the spawn room, then that is the only possible location for the boss room in this branch. Secondly, what if there are 2 rooms that share an equal distance from the spawn room? Let's call them suspected rooms. You need to identify the root room for the 2 suspected rooms, i.e. the room from which the way towards the 2 suspected rooms split. Count how much rooms away it is between the suspected rooms and this root room. For example, a root room that branches 2 end rooms, the number is 1. If the number is an odd number, then the priority of the direction you should go from that root room in order to get to the boss room is: 1. to right 2. to left 3. to below 4. to above. And if the number is an even number, then the priority is: 1. to up 2. to down 3. to left 4. to right. Check in order of the priority list and choose the first direction that is valid and is not a backward way to the spawn room, and that will be the direction towards the only possible location for the boss room in this branch. Thirdly, what if there are 3 or more rooms that share an equal distance from the spawn room? Find the root room for ALL the 3 or more suspected rooms, and use the method described above to find out which direction you should go from this root room, thus some of the suspected rooms that does not lay in the correct direction are ruled out. Then, if still more than 1 suspected room lay in the correct direction, use the same method for the remaining suspected rooms, until all but one suspected rooms are ruled out. The only remaining suspected room is the only possible location to be the boss room in this branch. Using the method described above, you can find out for sure which is the only possible location in each branch. This may be useful to rule out some of the rooms that you may consider possible before knowing the tip. If there are N branches rooted at the spawn room in a map, then there are N possible locations to be the boss room (exception: branches that contain only one room do not contain a possible location for the boss room). The only possible location for

each branch is highlighted in the example pictures below.

The N possible rooms definitely don’t have equal chances to become the boss room, so can we tell which one is more likely to be the boss room? Here comes an unlucky story. I do roughly calculated the probabilities for each of the N rooms, and the probabilities seem to be not far from accurate according to many test runs I did, but I also found that it seems to be far from accurate for someone others, for example, bisnap’s videos. I will write down the method on how to tell which one is more likely among the N rooms in the next paragraph, but please do not trust it. The branch that has the longest path is the most likely (more than 66%) branch to contain the boss room. If 2 or more branches share an equal longest length, use the method described above (the one that you use to rule out suspected rooms in a branch) rooted at the spawn room to pick out one direction towards the branch that is more likely to contain the boss room than the other branches. I made a program to fully simulate the method described in this section, which allows you to draw a map without any icon and it will predict the N possible locations for boss room and their probabilities. You can use the program to test and help

evaluate the method. The program along with the source code can be downloaded at https://docs.google.com/file/d/0B3jxLUI5Lb1rcWwtWXVMUnN3b0k/edit. But again, do not totally trust the probabilities showed on the program. Sorry for lacking details about the calculation of the probabilities for you to discuss and criticize, as that would take a lot of time to describe, and that is just a rough estimation anyway. The estimations for the examples are shown below.

Note that the method should be altered a bit for applying to SpiderMod 0.9, that seems to be, the priority order are always 1. to up 2. to down 3. to left 4. to right, no matter whether the number is even or odd.

How it is used to rule out some locations for top secret rooms After the game has generated a blank map layout (without setting any special rooms), the game will firstly choose the last generated end room to be the boss room, and then randomly choose one of the remaining end rooms to be the top secret room. That is saying the decision of the game on the boss room is prior to the decision on the top secret room. So, if the top secret room and the boss room are in the same branch, the only possible location for the boss room in that branch must still hold if you consider the top secret room as a regular room. In the pictures below, the blank spaces marked by crosses cannot become the top secret room, or else it will replace the true boss room location as the only possible location for boss room in the branch.

Another use of the tip to guess the direction to go towards the boss rooms after you have got The Compass /u/aggromidget came up with this idea. After you have got The Compass but not The

Treasure Map, when you found yourself at a place that is exactly diagonal from the boss room (the horizontal distance and the vertical distance from the boss room is the same) and have to guess whether to go horizontally or vertically in order to get to the boss room as quick as possible, just go vertically as that would seem to be a more likely way to the boss room. I’m not sure if this idea is right, feedbacks are highly appreciated.

Supplements to secret room rules Rule 6 If the Boss Room neighbors the Secret Room, it must not be the only End Room that neighbors the Secret Room. In the picture below, the blank space marked by a red cross is not possible to be the Secret Room, as the Boss Room is the only End Room neighboring it.

Simplified Rule 5 With the help of someone others, I found a more simple way to express rule 5, and that just removes most of the complication while retaining most of the content of the original rule 5. I hope this simplified version of rule 5 may help you a bit if you found the original rule 5 is hard to understand. If a blank space is surrounded by exactly 3 rooms with the only open being at either left or right side of the space, and one or both of the 2 rooms above and below the blank is an Icon Room, then this space is not possible to become the Secret Room.

Note that the simplified rule 5 is not a subset of the original rule 5, as it covers some cases that are not covered by the original rule 5. See an example below, the blank space under the Boss Room is not possible to become the Secret Room according to simplified rule 5. This example is not covered by the original rule 5 because it is not a complete C shape surrounding the blank.

Also note that in contrast to the original rule 5, the simplified rule 5 DOES apply to XL floors.

Priority of possible secret room locations The 6 rules rule out some spaces, but this does not mean spaces that remain possible have equal chances. There are spaces that are more likely to become Secret Room than others. If you want to try in an order from the most likely positions to the most unlikely positions, the order to bomb should be: 1st tier: spaces neighboring 3 or 4 non-end & non-boss rooms (all 3 or 4 neighboring rooms are non-end & non-boss) 2nd tier: spaces neighboring 3 or 4 rooms but are not in 1st tier 3rd tier: spaces neighboring 2 rooms 4th tier: spaces neighboring 1 room If spaces that are in a higher tier exist, then the chances for spaces in a lower tier to become Secret Room are relatively small. The more rooms there are in a higher tier, the less possible for spaces in a lower tier.