How About a Nice Game of Chess?

Advent of Code 2016 Day 5

Part 1

1. Going backwards helped
2. Playing the waiting game...again

Going backwards helped

I'm quite familiar with MD5 hashing by now:

• Day 17: Two Steps Forward
• Day 14: One-Time Pad
• And now Day 5, which I presume is the first MD5-themed puzzle for this year - though I could be wrong

Playing the waiting game...again

• Generate a hash
• Check for a match of 00000 as the first five characters in the hash
• Repeat until eight matches are found

My algorithm in JavaScript:

let index = 0, password = "", doorID = 'abc'
while (password.length < 8) {
  let hash = MD5(doorID + index).toString()
  if (hash.slice(0,5) == '00000') {
    password += hash[5]
  }
  index++
}
return password

This may take a while...especially since the match is five 0s in a row!

...

It didn't take as long as Day 14 - where I had to generate 2017 hashes each iteration!

But it still took several minutes to run.

Thankfully, it generated the correct answer for my door ID!

Part 2

A cool twist and a longer waiting game

• I updated my algorithm to account for an array, a position and a value

But I initially neglected to account for this portion of the instructions:

Use only the first result for each position

• I wasn't quite sure what that meant
• Until I ran my algorithm on the example input
• And I generated an answer different from the expected one
• Because I had modified some of the values already set

After updating my code to account for this:

password[hash[5]] = password[hash[5]] || hash[6]

My algorithm generated the correct answer for the example door ID and my door ID!

My updated algorithm in JavaScript:

let index = 0, doorID = 'abc'
let password = new Array(8).fill(null)
while (password.includes(null)) {
  let hash = MD5(doorID + index).toString()
  if (hash.slice(0,5) == '00000' && hash[5] < 8) {
    password[hash[5]] = password[hash[5]] || hash[6]
  }
  index++
}
return password.join('')

I did it!!

• I solved both parts!
• After a short time writing and a lot of time waiting!
• And accommodating one important use case in my algorithm!