Skip to content
Swizec Teller - a geek with a hatswizec.com

Advent of Code Day 19 – A Series of Tubes

Advent of Code Day 19 was one of those light, easy challenges. Just hard enough to make you think a little, easy enough to solve in 20 minutes.

This could make a great interview question πŸ€”

The gist of the problem is this:

  • you are given an ASCII drawing of a path
  • you have to follow it to the end
  • collect letters as you pass them
  • print them in sequence

For a map like this πŸ‘‡

|
| +--+
A | C
F---|----E|--+
| | | D
+B-+ +--+

Your result is ABCDEF.

The real map is… bigger. 201x201 characters. Or at least mine was.

Far too large to trace with your finger. I mean, you could if you had the patience of a gnat, but that's why the 2nd part of today's puzzle asks you to count the number of steps.

Ain't nobody got the patience to count the number of steps by hand.

Luckily coding up a solution in Python is pretty simple.

You take a current position, pos, that starts at the only | in top row – your entry point. And a vector (vy, vx) pointing downwards, (1, 0), and you travel through the map.

If you encounter a corner, input[y][x] == '+', you change direction – the movement vector. If so far you were traveling vertically, then you go either left or right depending on which direction has inputs. If you were traveling horizontally, then you switch to vertical.

if input[y][x] == "+":
if vy != 0:
# go left or right
if input[y][x-1] == "-" or input[y][x-1] in "ABCDEFGHIJKLMNOPQRSTUVWXYZ":
vy = 0
vx = -1
else:
vy = 0
vx = 1
else:
# go up or down
if input[y-1][x] == "|" or input[y-1][x] in "ABCDEFGHIJKLMNOPQRSTUVWXYZ":
vy = -1
vx = 0
else:
vy = 1
vx = 0

vy being different than 0 tells you that you've been moving vertically. If there's something to the left, you go there; otherwise, you go the opposite direction.

Same approach for the horizontal change.

When you encounter a letter, you add it to the list of letters.

elif input[y][x] not in ["-", "|"]:
letters.append(input[y][x])

In the end, you make sure to update your current position and steps counter.

steps += 1
pos = (y + vy, x + vx)

That applies the movement vector to our position pos and increments the steps counter. Easy πŸ‘Œ

When we put it all together and add a loop, the path following implementation looks like this πŸ‘‡

def followpath(input):
linelen = max(len(line) for line in input.split("\n"))
input = [line.ljust(linelen, " ") for line in input.split("\n") if len(line) > 0]
pos = (0, input[0].index("|"))
vx = 0
vy = 1
letters = []
steps = 0
print linelen, len(input)
while True:
y, x = pos
if y < 0 or y >= len(input) or x < 0 or x >= linelen or input[y][x] == " ":
return steps, letters
if input[y][x] == "+":
if vy != 0:
# go left or right
if input[y][x-1] == "-" or input[y][x-1] in "ABCDEFGHIJKLMNOPQRSTUVWXYZ":
vy = 0
vx = -1
else:
vy = 0
vx = 1
else:
# go up or down
if input[y-1][x] == "|" or input[y-1][x] in "ABCDEFGHIJKLMNOPQRSTUVWXYZ":
vy = -1
vx = 0
else:
vy = 1
vx = 0
elif input[y][x] not in ["-", "|"]:
letters.append(input[y][x])
steps += 1
pos = (y + vy, x + vx)

We prep the input, start an infinite loop, break out of the loop when we fall outside the input field or hit an empty spot (meaning line is over), and use our path-following logic to move around.

Eventually, we reach the end of the line and return our result: The number of steps and the sequence of letters encountered.

You can see everything on Github.

That was fun πŸ€™

Did you enjoy this article?

Published on December 19th, 2017 in Front End, python, Technical

Learned something new?
Want to become a high value JavaScript expert?

Here's how it works πŸ‘‡

Leave your email and I'll send you an Interactive Modern JavaScript Cheatsheet πŸ“–right away. After that you'll get thoughtfully written emails every week about React, JavaScript, and your career. Lessons learned over my 20 years in the industry working with companies ranging from tiny startups to Fortune5 behemoths.

Start with an interactive cheatsheet πŸ“–

Then get thoughtful letters πŸ’Œ on mindsets, tactics, and technical skills for your career.

"Man, love your simple writing! Yours is the only email I open from marketers and only blog that I give a fuck to read & scroll till the end. And wow always take away lessons with me. Inspiring! And very relatable. πŸ‘Œ"

~ Ashish Kumar

Join over 10,000 engineers just like you already improving their careers with my letters, workshops, courses, and talks. ✌️

Have a burning question that you think I can answer?Β I don't have all of the answers, but I have some! Hit me up on twitter or book a 30min ama for in-depth help.

Ready to Stop copy pasting D3 examples and create data visualizations of your own? Β Learn how to build scalable dataviz components your whole team can understand with React for Data Visualization

Curious about Serverless and the modern backend? Check out Serverless Handbook, modern backend for the frontend engineer.

Ready to learn how it all fits together and build a modern webapp from scratch? Learn how to launch a webapp and make your first πŸ’° on the side with ServerlessReact.Dev

Want to brush up on your modern JavaScript syntax?Β Check out my interactive cheatsheet: es6cheatsheet.com

By the way, just in case no one has told you it yet today: I love and appreciate you for who you are ❀️