Space Image Format

Advent of Code 2019 Day 8

Try the simulator with your puzzle input!

Task: Solve for X where...

Part 1

X = the number of 1 digits multiplied by the number of 2 digits in the layer that contains the fewest 0 digits

Part 2

X = the BIOS password

Example input

123456789012

It represents

• A picture of a password
• Signified as a series of single digits
• Each digit represents the color of a single pixel

Part 1

1. Understanding the composition of the image
2. Doing just enough to succeed
3. A written description of my working algorithm
4. A visual depiction of my working algorithm

Understanding the composition of the image

The image dimensions are known:

• In the example, they are 3x2
• For my puzzle input, they are 25x6

The tricky part:

Each image actually consists of a series of identically-sized layers that are filled in

The example input contains 12 digits

123456789012

Since the area of the image is 6...
...we can divide the number of digits by the area...
...to determine the number of identically-sized layers

 12 [digits]
/ 6 [area: 3 x 2]
---
  2 [identically-sized layers]

My puzzle input contains 15,000 digits

 15000 [digits]
/  150 [area: 25 x 6]
---
   100 [identically-sized layers]

Doing just enough to succeed

• The task is to find the layer with the fewest 0s
• There is no need to identify the rows within each layer, as the sample diagram depicts
• Instead, it's enough to extract the characters representing each layer

A written description of my working algorithm

Accept three parameters:
  1. Digits
  2. Width
  3. Height

Calculate the area by multiplying width and height
Create a 2-element array:
  1. A string containing as many 0s as equivalent to the area
  2. Infinity

For i from 0 to the number of digits
  Store a string containing only the digits at locations from i up to but not including i + area
  Count the number of 0s in that string by turning the string into an array of digits and filtering to include only the 0s
  If the number of zeros is less than the value stored in the 2-element array
    Update the 2-element array such that:
      1. The newly stored string created in this iteration of the for-loop
      2. The count of 0s in the string

Return the product of the following operation performed twice on the string now stored in the 2-element array:
  1. Turn the string into an array of digits and filter to include only the 1s - return the length
  2. Turn the string into an array of digits and filter to include only the 2s - return the length

A visual depiction of my working algorithm

Part 2

1. I knew this was coming, and thought it would be harder
2. A written description of my working algorithm
3. A visual depiction of my working algorithm
4. An image-encoding simulator

I knew this was coming, and thought it would be harder

The instruction from Part 1 stated:

Images are sent as a series of digits that each represent the color of a single pixel.

• Noting that the digits were 0,1,2 - I anticipated they would represent RGB and require a complex algorithm to determine what the image depicts
• To my surprise and delight, they represent transparent, white and black - whichever color is visible when viewed top-down is the winning pixel

A written description of my working algorithm

Accept three parameters:
  1. Digits
  2. Width
  3. Height

Calculate the area by multiplying width and height

Create an empty array, layers

For i from 0 to the number of digits in the input
  Add to layers a string containing only the digits at locations from i up to but not including i + area

Using the first string in layers
  Split the string into an array of stringified digits
  Update each digit according to the following operations:
    For each character and its location in the array
      Set new variable, pixels, to an ordered list containing each digit from each layer's string at the current location
      Set new variable, position, to 0
      While the digit in pixels at position is '2'
        Increment position
      Return the digit at position in pixels
  Merge all characters back into a string

Create an empty array, image

For i from 0 to the number of digits in the updated first layer
  Add to image a string containing only the digits at locations from i up to but not including i + width

Merge each string in image using a newline character to simulate a 25x6 grid of digits
Replace all '0's with space characters to make the '1's pop out and reveal letters
Print the grid of digits to see the password

A visual depiction of my working algorithm

An image-encoding simulator

• Easy to build
• Added bonus of slowly rendering image using code from my Jurassic Jigsaw and simulator

I did it!!

• Both parts solved!
• Two GIFs designed!
• A simulator built!

I appreciated this fun, breezy puzzle as a break between Intcode computer puzzles.

Bring it on, Day 9!