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ps. look out for all my other solutions for Advent of Code challenges here.
See details of the challenge here.
Let’s start by adding a hash function that’ll take an input string and return the hexdecimal representation of its MD5 hash.
From there, we can create an infinite sequence of hash values generated by concatinating the input and an increasing index (as per the instruction). But, remember, we’re only interested in hash values that starts with 5 zeroes.
Once we have this sequence of interesting hash values, part 1 requires us to take the first 8 and make a password out of it.
As the door slides open, you are presented with a second door that uses a
slightly more inspired security mechanism. Clearly unimpressed by the last
version (in what movie is the password decrypted in order?!), the Easter Bunny
engineers have worked out a better solution.
Instead of simply filling in the password from left to right, the hash now also
indicates the position within the password to fill. You still look for hashes
that begin with five zeroes; however, now, the sixth character represents the
position (0-7), and the seventh character is the character to put in that position.
A hash result of 000001f means that f is the second character in the password.
Use only the first result for each position, and ignore invalid positions.
For example, if the Door ID is abc:
The first interesting hash is from abc3231929, which produces 0000015…; so, 5
goes in position 1: _5______.
In the previous method, 5017308 produced an interesting hash; however, it is
ignored, because it specifies an invalid position (8).
The second interesting hash is at index 5357525, which produces 000004e…; so,
e goes in position 4: _5__e___.
You almost choke on your popcorn as the final character falls into place,
producing the password 05ace8e3.
Given the actual Door ID and this new method, what is the password? Be extra
proud of your solution if it uses a cinematic “decrypting” animation.
We can reuse the inifinite sequence of interesting hash values we constructed earlier, but this time around we need to also filter out hash values whose 6th character is not 0-7.
Additionally, we need to track the first hash value for each position.
The first idea I had was to use a mutable Option<char> (length 8) to represent the password, and then iterate over the valid hash values until that array is filled with Some x. But this approach felt too imperative for my liking..
I couldn’t use a fold as it will fold over all the inputs.. however, I can use a scan which yields the intermediate results as a sequence – which I can then skip until I have 8 hash values.
I can also use an immutable Set to represent all the positions that are still unoccupied as I iterate through the hash values.
Whilst part 1 was not computationally light, it did complete in a matter of seconds on my laptop. Part 2 took more than a minute to finish so if anyone knows a good way to speed things up please leave a comment to this post.
- Day 5 challenge description
- Advent of Code 2015
- Solution for Day 4
- All my F# solutions for Advent of Code
- Github repo
Hi, I’m Yan. I’m an AWS Serverless Hero and I help companies go faster for less by adopting serverless technologies successfully.
Are you struggling with serverless or need guidance on best practices? Do you want someone to review your architecture and help you avoid costly mistakes down the line? Whatever the case, I’m here to help.
Skill up your serverless game with this hands-on workshop.
My 4-week Production-Ready Serverless online workshop is back!
This course takes you through building a production-ready serverless web application from testing, deployment, security, all the way through to observability. The motivation for this course is to give you hands-on experience building something with serverless technologies while giving you a broader view of the challenges you will face as the architecture matures and expands.
We will start at the basics and give you a firm introduction to Lambda and all the relevant concepts and service features (including the latest announcements in 2020). And then gradually ramping up and cover a wide array of topics such as API security, testing strategies, CI/CD, secret management, and operational best practices for monitoring and troubleshooting.
If you enrol now you can also get 15% OFF with the promo code “yanprs15”.
Check out my new podcast Real-World Serverless where I talk with engineers who are building amazing things with serverless technologies and discuss the real-world use cases and challenges they face. If you’re interested in what people are actually doing with serverless and what it’s really like to be working with serverless day-to-day, then this is the podcast for you.
Check out my new course, Learn you some Lambda best practice for great good! In this course, you will learn best practices for working with AWS Lambda in terms of performance, cost, security, scalability, resilience and observability. We will also cover latest features from re:Invent 2019 such as Provisioned Concurrency and Lambda Destinations. Enrol now and start learning!
Check out my video course, Complete Guide to AWS Step Functions. In this course, we’ll cover everything you need to know to use AWS Step Functions service effectively. There is something for everyone from beginners to more advanced users looking for design patterns and best practices. Enrol now and start learning!
Here is a complete list of all my posts on serverless and AWS Lambda. In the meantime, here are a few of my most popular blog posts.
- All you need to know about caching for serverless applications
- Lambda optimization tip – enable HTTP keep-alive
- You are wrong about serverless and vendor lock-in
- You are thinking about serverless costs all wrong
- Just how expensive is the full AWS SDK?
- Check-list for going live with API Gateway and Lambda
- How to choose the right API Gateway auth method
- CloudFormation protip: use !Sub instead of !Join
- AWS Lambda – should you have few monolithic functions or many single-purposed functions?
- Guys, we’re doing pagination wrong
- Top 10 Serverless framework best practices
- How to break the “senior engineer” career ceiling
- My advice to junior developers