The Task for Day 04 was quite easy – compared to the previous ones. For the given password-rules, you have to calculate, how many valid passwords exists in a given range.

The code repository for my solutions can be found as usual on GitHub and GitLab.

— Day 4: Secure Container —

You arrive at the Venus fuel depot only to discover it’s protected by a password. The Elves had written the password on a sticky note, but someone threw it out.

However, they do remember a few key facts about the password:

– It is a six-digit number.
– The value is within the range given in your puzzle input.
– Two adjacent digits are the same (like 22 in 122345).
– Going from left to right, the digits never decrease; they only ever increase or stay the same (like 111123 or 135679).

Other than the range rule, the following are true:

– 111111 meets these criteria (double 11, never decreases).
– 223450 does not meet these criteria (decreasing pair of digits 50).
– 123789 does not meet these criteria (no double).

How many different passwords within the range given in your puzzle input meet these criteria?

Advent of Code 2019 – Day 04 / Part 1
Spoiler: My solution for day 04 – part 1

First, I wrote a function, which tells us, if a given password is valid for the given rules. It simply checks if the string contains 6 characters if they are not descending and have a double-digit.

Go

To calculate how many passwords are between a given range, we just count the valid passwords in this range:

Go

— Part Two —

An Elf just remembered one more important detail: the two adjacent matching digits are not part of a larger group of matching digits.

Given this additional criterion, but still ignoring the range rule, the following are now true:

– 112233 meets these criteria because the digits never decrease and all repeated digits are exactly two digits long.
– 123444 no longer meets the criteria (the repeated 44 is part of a larger group of 444).
– 111122 meets the criteria (even though 1 is repeated more than twice, it still contains a double 22).

How many different passwords within the range given in your puzzle input meet all of the criteria?

Advent of Code 2019 – Day 04 / Part 2
Spoiler: My solution for day 04 – part 2

We just need to adapt our function, which checks our password:

Go

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