Programming

The earliest memory I have of ‘programming’ is in the early/mid 90s when my father brought home a computer from work. We could play games on it … so of course I took the spreadsheet program he used (LOTUS 123, did I date myself with that?) and tried to modify it to print out a helpful message for him. It … halfway worked? At least I could undo it so he could get back to work…

After that, I picked up programming for real in QBASIC (I still have a few of those programs lying around), got my own (junky) Linux desktop from my cousin, tried to learn VBasic (without a Windows machine), and eventually made it to high school… In college, I studied computer science and mathematics, mostly programming in Java/.NET, although with a bit of everything in the mix. A few of my oldest programming posts on this blog are from that time.

After that, on to grad school! Originally, I was going to study computational linguistics, but that fell through. Then programming languages (the school’s specialty). And finally I ended up studying censorship and computer security… before taking a hard turn into the private sector to follow my PhD advisor.

Since then, I’ve worked in the computer security space at a couple of different companies. Some don’t exist any more, some you’ve probably heard of. I still program for fun too, and not just in security.

But really, I still have a habit of doing a little bit of everything. Whatever seems interesting at the time!

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AoC 2016 Day 24: Venti

2016-12-24

Source: Air Duct Spelunking

Part 1: Given a map of the form:

########### #0.1…..2# #.#######.# #4…….3# ###########


> Find the shortest route to visit each of the points, starting at `0`.





First, we want to take the map that we were given and simplify it. We know that we want to visit all of the points, so lets take the original map and turn it just into a map of distances between any two named points.

```python
walls = set()
name_to_point = {}
point_to_name = {}

# Load the input file into a set of walls and the location of points of interest
for y, line in enumerate(fileinput.input(args.files)):
    for x, c in enumerate(line.strip()):
        if c.isdigit():
            name_to_point[int(c)] = (x, y)
            point_to_name[(x, y)] = int(c)

        elif c == '#':
            walls.add((x, y))

# Dynamically fill a distance map to a given point
def distances_to(name):
    to_scan = queue.Queue()
    to_scan.put((name_to_point[name], 0))

    scanned = set()

    result = {}

    while not to_scan.empty():
        point, distance = to_scan.get()

        if point in point_to_name:
            name = point_to_name[point]
            if name not in result:
                result[name] = distance

        if point in scanned:
            continue
        else:
            scanned.add(point)

        x, y = point
        for xd, yd in [(-1, 0), (1, 0), (0, -1), (0, 1)]:
            neighbor = (x + xd, y + yd)
            if neighbor not in walls:
                to_scan.put((neighbor, distance + 1))

    return result

distances = {
    name: distances_to(name)
    for name in name_to_point
}
names = list(sorted(name_to_point.keys()))

The first part loads the map and the second part will flood fill out from each point. This will find the minimum distance from each named point to the given one, somewhat simplifying the problem.

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AoC 2016 Day 23: Assembunny2

2016-12-23

Source: Safe Cracking

Part 1: Take the assembunny interpreter from day 12 and add an instruction (tgl X) that modifies the code at an offset of X instructions.

  • inc becomes dec; any other one argument instruction (including tgl) becomes inc
  • jnz becomes cpy; any other two argument instructions become jnz
  • Toggling an instruction outside of the program does nothing (it does not halt execution)
  • If toggling produces an invalid instruction, ignore it

Run the given program with the initial register of a = 7. What is the final value in register a?

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AoC 2016 Day 21: Scrambler

2016-12-21

Source: Scrambled Letters and Hash

Part 1: Another virtual machine, of sorts. Start with the string abcdefgh and apply a sequence of the following commands to it:

  • swap position X with position Y = swap two positions
  • swap letter X with letter Y = swap to letters, no matter where they are
  • rotate (left|right) X steps = rotate forward or backward
  • rotate based on position of letter X = find X, rotate right based on its position; if the original position was >= 4, rotate one more1
  • reverse positions X through Y = reverse a subset of the string
  • move position X to position Y = take a character at a position out of the string and put it somewhere else specific

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AoC 2016 Day 18: Its A Trap

2016-12-18

Source: Like a Rogue

Part 1: Starting with a sequence of . and ^, generate additional rows using the rules based on the three characters above the new position.

  • ^^. -> ^
  • .^^ -> ^
  • ^.. -> ^
  • ..^ -> ^
  • Otherwise -> .

How many safe tiles (.) are there after 40 generations?

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AoC 2016 Day 17: Md5 Maze

2016-12-17

Source: Two Steps Forward

Part 1: Create a 4x4 grid of rooms with doors Up, Down, Left, and Right from each location. To determine if a door is currently open:

  • Calculate MD5(salt + sequence) where sequence is a string containing any combination of UDLR depending on how you got to this room
  • The first four hex values represent the doors Up, Down, Left, and Right respectively: bcdef means open; anything else is closed

Find the shortest path from (0, 0) to (3, 3).

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AoC 2016 Day 16: Dragon Data

2016-12-16

Source: Dragon Checksum

Part 1: Generate noise using a modified dragon curve:

  • Start with data a
  • Create a copy of the data b, reverse and invert it (0 <-> 1)
  • Create the string a0b

Repeat until you have enough data, truncate at the end if needed.

From this string calculate a checksum as follows:

  • xor each pair of bits, concatenate the results
  • If the resulting string has an even length, repeat; if it’s odd, stop

Calculate the checksum of a given initial state expanded to 272 bits.

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