## On the Subject of Black Holes

Ground control to Major Tom, do you read?

- To disarm all Black Hole modules on a bomb, obtain a code consisting of digits 0–4. The length of the code is 7 times the total number of Black Hole modules on the same bomb. At any point, you may enter the next digit of the code on any unsolved Black Hole module.
- If you solve a different module on the bomb after entering a digit of the code, the code gets shortened by two digits, except when your digit caused a Black Hole module to become solved. This way the code for a single Black Hole can be reduced from 7 to 3 digits by solving other modules between entering digits. The code for two Black Holes can be shortened from 14 to 6, etc.
- Observe the 10×10 grid on the next page. Calculate your starting location in this grid from the bomb’s serial number. The third character is the x-coordinate, the sixth character the y-coordinate. Columns and rows are counted left-to-right/top-to-bottom from 0.
- To determine your initial direction of movement, start by facing north, then turn 45° clockwise for each port on the bomb.
- Obtain the first digit of the code from that grid location, then move one space in your initial direction, then rotate your direction 45° clockwise.
- The next digit is the sum,
*modulo 5**, of two consecutive grid locations in your current direction. Obtain the two numbers, move one space beyond them, then rotate 45° clockwise. - Proceed likewise for every subsequent digit, adding one more grid location each time. The
`n`th digit of the code is the sum (modulo 5*) of`n`consecutive grid locations. - Every time you move beyond the edge of the grid, wrap around to the opposite side.
- The following page explains the gesture for entering each digit.
- The “C” gesture can be used to determine how many correct digits have already been entered in case you lost track.

^{*} “modulo 5”: Add/subtract 5 until the number is in the range 0–4.