explanation of Master Lock combination lock decoding by Samy Kamkar: https://youtu.be/qkolWO6pAL8

my method is based on Samy's research. first number is same as Samy's method: lift the shackle halfway, turn the dial clockwise, and you'll feel resistance once every revolution; from the resistance point, rotate counterclockwise 5.5 digits and that's the first number. now set the bottom code wheel to that first number my making one a full revolution clockwise and stopping at the first number. next, engage the middle code wheel by making a full revolution counterclockwise and stopping at the first number again; continue rotating counterclockwise 2 more digits and that's the first guess for the second number (the first and second numbers are offset by 2 modulo 4, due to Samy). to test a guess of the second number, turn the dial a few digits clockwise, pull on the shackle until you fall in a gate, and wiggle the dial to check if there's resistance. if there's no resistance, the second number is incorrect because the middle code wheel is not lined up, so rotate the middle code wheel counterclockwise by an additional 4 digits and that's the next guess for the second number. on the other hand, if there is resistance, then that means both the bottom and middle code wheels are lined up and only the top code wheel remains. so, simply turn the dial clockwise, pulling on the shackle gate by gate, until the lock opens

Master Lock combination locks have an alternate combination. given the original combination C, the alternate combination is C + [-7, 2, 0]. to enter the alternate combination, first rotate the dial counterclockwise, then clocwise, then counterclockwise

this second combinating thing was developped by discovering the following facts:

- bumps on code wheels are approximately 1.5 digits wide
- there is play in the system (roughly 1 digit wide). this means that the last code wheel will turn with the first code wheel for 1 digit before the friction disk keeps it from turning further
- one padlock's combination was found to be [15, 1, 7]. found alternate combination by setting the first digit to 14 and brute-forcing the two last digits. looking into the lock with a flashlight also helped align the code wheels more easily. alternate combination turned out to [8, 3, 7]