Making Fish Shell Smile

When working in a shell, from time to time, I need to know if a command succeeded or failed. Sometimes, it’s easy:

$ make noise

make: *** No rule to make target `noise'.  Stop.

Sometimes, less so:

$ grep frog podcasts.json > podcasts-about-frogs.txt

Since, alas, I don’t have any podcasts about frogs, that command would fail silently. But that’s fixable!

$ grep frog podcasts.json > podcasts-about-frogs.txt

$ # Bash/Zsh
$ echo $?

$ # Fish
$ echo $status

Advent of Code 2017

As I did with last year / yesterday, I’ve written up a series of posts for the Advent of Code 2017 problems. Again, I didn’t manage to write them up as I did them, but this time around I least I finished mostly on time.

Advent of Code 2016

As I did last year, I’m going to solve the Advent of Code problems again this year.

Or that was the plan. It turns out that instead I put down my blog for almost a year and a half and never quite got around to doing these problems. So I’m actually backdating these posts from the early days of 2018 to where they would have been had I solved them on time. They’re still interesting problems, so give them a read.

AoC 2017 Day 25: Turing

Source: The Halting Problem

Part 1: Implement a Turing machine defined as such:

Begin in state A.
Perform a diagnostic checksum after 6 steps.

In state A:
  If the current value is 0:
    - Write the value 1.
    - Move one slot to the right.
    - Continue with state B.
  If the current value is 1:
    - Write the value 0.
    - Move one slot to the left.
    - Continue with state B.


What is the final number of 1s on the tape?

AoC 2017 Day 24: Maker Of Bridges

Source: Electromagnetic Moat

Part 1: Given a series of reversible components of the form 3/4 (can connect a 3 on one end to a 4 on the other), form a bridge of components. The bridge’s strength is equal to the sum of component values. So 0/3, 3/7, and 7/4 has a strength of 0+3 + 3+7 + 7+4 = 24.

What is the strongest possible bridge?

AoC 2017 Day 23: Duetvmc

Source: Coprocessor Conflagration

Part 1: Create a variation of the previous DuetVM with only the following four instructions:

  • set X Y sets register X to Y
  • sub X Y set register X to X - Y
  • mul X Y sets register X to X * Y
  • jnz X Y jumps with an offset of the value of Y, iff X is not equal to zero

If you run the given program, how many times is mul invoked?

AoC 2017 Day 22: Langton's Ant

Source: Sporifica Virus

Part 1: Implement a cellular automaton on an infinite grid of . and # pixels such that:

  1. Start at (0, 0), facing Up
  2. Repeat:
    • If the cursor is on . swap it to # and turn Left
    • If the cursor is on # swap it to . and turn Right
    • Either way, after turning, move forward once

After 10,000 iterations, how many pixels were turned from . to #?

AoC 2017 Day 21: Fractal Expander

Source: Fractal Art

Part 1: Start with an input image made of . and # pixels. For n iterations, break the image into blocks:

  • If the current size is even, break the image into 2x2 chunks and replace each with a 3x3 chunk
  • If the current size is odd, break the image into 3x3 chunks and replace each with a 4x4 chunk

The replacement rules will be specified in the following format (example is a 3x3 -> 4x4 rule):

.#./..#/### => #..#/..../..../#..#

In that example, replace this:


With this:


Any rotation or reflection of a chunk can be used to match the input of a replacement rule.

After n = 18 iterations, how many # pixels are there?

AoC 2017 Day 19: Networkout

Source: A Series of Tubes

Part 1: Take a network diagram of the following form:

    |  +--+    
    A  |  C    
    |  |  |  D
    +B-+  +--+

Starting at the single node at the top and following the lines, what order would the nodes be visited in?