advent-of-code/2019/day04/README.org

• — Day 4: Secure Container —
• — Part Two —

— 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?

Your puzzle answer was 2090.

— 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?

Your puzzle answer was 1419.