More information about Advent of Code can be found at https://adventofcode.com/2020.

• Use .Net 5 / and latest versions of C#
• Use Visual Studio for Mac and Rider
• TDD with decent code coverage
• No external libraries to solve problem... allowed NUnit and Moq for testing

• Taking a break on a tropical island
• Island is cash only
• Local currency is 'Starfish', often abbreviated to 'stars'
• No currency exchanges deal in stars, so only way to obtain enough stars for deposit is to solve puzzles
• Need 50 stars to pay for room deposit in 25 days time
• Two puzzles per day, with each puzzle paying 1 star

• Elves need me to fix my 'expense report' (puzzle input) before leaving on vacation

• Requirement: need to find the two entries that sum to 2020, and then multiply them together. This number is what we are looking for.

• Example: in list:

  • 1721
  • 979
  • 366
  • 299
  • 675
  • 1456

  Answer would be 514579 as (1721 + 299) = 2020 and (1721 x 299) = 514579

• Password database corrupted

• List of password provided in following format:

  • 1-3 a: abcde
  • 1-3 b: cdefg
  • 2-9 c: ccccccccc

  where the row is made up of a password policy and password delimited my a : character. A policy of 1-3 a indicates a must be used 1-3 times in the password. The first and last passwords are valid, but the second one isn't valid as b doesn't occur 1-3 times in cdefg.

• Requirement: How many passwords are valid according to their policies?

• Requirement: password policy of part 1 incorrect, numbers describe positions not occurrence count, such that:

  • 1-3 a: abcde is valid: position 1 contains a and position 3 does not.
  • 1-3 b: cdefg is invalid: neither position 1 nor position 3 contains b.
  • 2-9 c: ccccccccc is invalid: both position 2 and position 9 contain c.

  Exactly one of these positions must contain the specified letter.

• Puzzle input is a map of trees where . represents open space, and # represents a tree.
• Trees grow on a grid pattern.
• Pattern repeats horizontally.
• Navigate from top-left to off bottom of map in a move right 3 places and down 1. In this way you navigate diagonally downward.
• Aim of puzzle is to count the number of trees you stop on i.e. after the 3-right 1-down move.

• Passprts have following fields:

  • byr (Birth Year)
  • iyr (Issue Year)
  • eyr (Expiration Year)
  • hgt (Height)
  • hcl (Hair Color)
  • ecl (Eye Color)
  • pid (Passport ID)
  • cid (Country ID)

• North Pole Credentials are the same as a passport, but without the cid field.

• Scanner processes passports in batch files (puzzle input).

• Passport made up of key:value pairs separated by spaces or new lines. Passports are separated by new lines.

• Example batch file:

  ecl:gry pid:860033327 eyr:2020 hcl:#fffffd
  byr:1937 iyr:2017 cid:147 hgt:183cm
  
  iyr:2013 ecl:amb cid:350 eyr:2023 pid:028048884
  hcl:#cfa07d byr:1929
  
  hcl:#ae17e1 iyr:2013
  eyr:2024
  ecl:brn pid:760753108 byr:1931
  hgt:179cm
  
  hcl:#cfa07d eyr:2025 pid:166559648
  iyr:2011 ecl:brn hgt:59in 
  

  where:

  • passport 1 is valid as all fields are present
  • passport 2 is invalid as hgt field is missing
  • passport 3 is in valid as cid filed is missing
  • passport 4 is invalid as cid and byr fields missing.

• As a one-off we should also treat the cid as optional, and hence allow North Pole Credentials to be used like a passport i.e passport 3 should be treated as apassport even though it is a North Pole Credential

• Requirement: how many passports are valid there?

• Your airline you are using to fly to the tropical island identifies your seat using a binary space partition map where:

  • F indicates front
  • B indicates back
  • L indicates left
  • R indicates

• First 7 characters of your sead number will be either F or B, and the final 3 L or R where:

  • L indicates left
  • R indicates right

• First 7 characters describe which row of 0- 128 you are in.

• Last 3 characters describe which column of 0-7 you are in.

• Seat ID is calculated by (row x 8 + col)

• Example boarding passes are:

Here are some other boarding passes:

• FBFBBFFRLR: row 44, column 5, seat ID (44 * 8 + 5) = 357

• BFFFBBFRRR: row 70, column 7, seat ID 567.

• FFFBBBFRRR: row 14, column 7, seat ID 119.

• BBFFBBFRLL: row 102, column 4, seat ID 820.

• Requirement: What is the highest seat ID on a boarding pass (puzzle input)?

• Boarding pass list missing items from front and back as these seats don't exist on this plane.
• Occupied seats is continuous apart from where I'm sitting (ID+1 and -1 will be in my list).
• Requirement: what is my seat ID?

My thinking:

• Seat ID is effectively seat number as SeatID = row x 8 + col where 8 is number of seats in row. This means it should be continuous, apart from a block at the start and end we are told is missing. We are also told our seat ±1 is occupied, so we are looking for unoccupied seats... turns out there is only 1 unoccupied seat, so this must be ours.

• Customs declaration forms have 26 yes/no questions marked a-z.

• Identify any question a member of your group answers yes.

• Example group og three people might answer:

  • abcx
  • abcy
  • abcz

  In this example 6 questions were marked as yes: a, b, c, x, y, and z.

• Puzzle input is every group on the plane's answers, where each person has their own line, and groups are separated by a blank line.

• Example:

  abc
  
  a
  b
  c
  
  ab
  ac
  
  a
  a
  a
  a
  
  b
  

  where:

  • The first group contains one person who answered "yes" to 3 questions: a, b, and c.
  • The second group contains three people; combined, they answered "yes" to 3 questions: a, b, and c.
  • The third group contains two people; combined, they answered "yes" to 3 questions: a, b, and c.
  • The fourth group contains four people; combined, they answered "yes" to only 1 question, a.
  • The last group contains one person who answered "yes" to only 1 question, b.
  • Sum of yes counts is 3 + 3 + 3 + 1 + 1 = 11
  • Requirement: for each group count the number of questions to which someone answered yes to, and calculat the sum of those counts.

• Aviation regulations say bags must be colour coded and must contain specific number of other bags.

• Example rules are:

  • light red bags contain 1 bright white bag, 2 muted yellow bags.
  • dark orange bags contain 3 bright white bags, 4 muted yellow bags.
  • bright white bags contain 1 shiny gold bag.
  • muted yellow bags contain 2 shiny gold bags, 9 faded blue bags.
  • shiny gold bags contain 1 dark olive bag, 2 vibrant plum bags.
  • dark olive bags contain 3 faded blue bags, 4 dotted black bags.
  • vibrant plum bags contain 5 faded blue bags, 6 dotted black bags.
  • faded blue bags contain no other bags.
  • dotted black bags contain no other bags.

  In this example:

  • every faded blue bag is empty
  • every vibrant plum bag contains 11 bags (5 faded blue and 6 dotted black)

• You have a shiny gold bag. If you wanted to carry it in at least one other bag, how many different bag colors would be valid for the outermost bag? (In other words: how many colors can, eventually, contain at least one shiny gold bag?):

• In the above rules, the following options would be available to you:

  • A bright white bag, which can hold your shiny gold bag directly.
  • A muted yellow bag, which can hold your shiny gold bag directly, plus some other bags.
  • A dark orange bag, which can hold bright white and muted yellow bags, either of which could then hold your shiny gold bag.
  • A light red bag, which can hold bright white and muted yellow bags, either of which could then hold your shiny gold bag.

• So, in this example, the number of bag colors that can eventually contain at least one shiny gold bag is 4.

• Requirement: How many bag colors can eventually contain at least one shiny gold bag?

• Operation (acc, jmp, or nop) and an argument (a signed number like +4 or -20).
• acc = accumulator that starts at zero. Argument increases / decreases by the given amount.
• jmp jumps to the instruction given by the argument, i.e. jmp 2 would jump to the the line following the next line, whilst jmp -1 would go to the proceeding line.
• nop is no operation, and the next line executes next.
• We know we are in an infinite loop when any instruction is run for a second time.
• Requirement: find the value in the accumulator before an instruction is run for a second time.

• Data encrypted with eXchange-Masking Addition System (XMAS)
• XMAS startes with a 25 number preamble
• After that each number should be the sum of any 2 numbers in the immediate 25 numbers
• Requirement: what is the first number that doesn't follow this rule in the supplied data?

• Adapter can take an input 1, 2, or 3 jolts lower than its rating and still produce its rated output joltage.
• Device has built in joltage adapter that is rated for 3 jolts higher than the highest-rated in bag e.g. if adapter in bag are rated for 3, 9, and 6, your device's built-in adapter would be rated for 12 jolts.
• Local charging outlet has an effective rating of 0 jolts.
• Requirement 1: target joltage is joltage of highest rated adapter plus 3 jolts
• Requirement 2: chain adapters together using each adapter only once such that the increase in rating is plus 1-3 jolts each adapter.
• See worked example for detailed example.
  • Always take lowest available adapter.
• Longer example also provided.
• Requirement 3: count number of 1, 2, and 3 jolt leaps. What is the number of 1-jolt differences multiplied by the number of 3-jolt differences?

• Conway Cubes
• The pocket dimension contains an infinite 3-dimensional grid.
• State at each point in grid either active or inactive.
• Initial state almost all points start inactive (.). Small number start as active (#).
• Energy source executes 6 cycles.
• Each cube only considers its neighbours, i.e. point x=1,y=2,z=3 would consider amongst others x=2,y=2,z=2 and x=0,y=2,z=3.
• During each cycle every cube itterates according to:
  • If a cube is active and exactly 2 or 3 of its neighbors are also active, the cube remains active. Otherwise, the cube becomes inactive.
  • If a cube is inactive but exactly 3 of its neighbors are active, the cube becomes active. Otherwise, the cube remains inactive.

Observations:

• Relevant space grows by 2 in x, y, and z direction each iteration.

• Now 4 spatial dimensions: x, y, z, and w.
• Same rules of interaction, but interacts with nearest neighbours over 4 rather than 3 dimensions.
• Example in part 1 now produces 848 cubes left in active state after 6 iterations.