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Advent of Code:   solutions & explanations
6.12

Advent of Code: solutions & explanations

Matthew Butterick <[email protected]>

 (require aoc-racket) package: aoc-racket

Dedicated to curious characters everywhere, especially those learning Racket.

Advent of Code is a series of programming puzzles designed by Eric Wastl.

I find that programming puzzles are a good way of learning something new about a programming language, or learning how to do certain things better. Documenting these solutions helped me nail down some discoveries.

Thank you to Eric Wastl. If you like Advent of Code, please pay him for it.

You can install this package (if you haven’t already) with

raco pkg install aoc-racket

    1 Day 1

      1.1 Where does the elevator land?

        1.1.1 Alternate approach: numerical conversion

      1.2 At what point does the elevator enter the basement?

        1.2.1 Alternate approaches: for/first or for/or

      1.3 Testing Day 1

    2 Day 2

      2.1 How much paper is needed to wrap the boxes?

      2.2 How much ribbon is needed to wrap the boxes?

      2.3 Testing Day 2

    3 Day 3

      3.1 How many grid cells are visited?

        3.1.1 Alternate approach: complex numbers

      3.2 How many grid cells are visited if the path is split?

      3.3 Testing Day 3

    4 Day 4

      4.1 What is the lowest-numbered MD5 hash starting with five zeroes?

      4.2 How about six zeroes?

      4.3 Testing Day 4

    5 Day 5

      5.1 How many strings are “nice”?

      5.2 How many strings are “nice” under new rules?

      5.3 Testing Day 5

    6 Day 6

      6.1 How many lights are lit after following the instructions?

      6.2 What is the total brightness of the lights if the rules are reinterpreted?

      6.3 Refactored solution

      6.4 Testing Day 6

    7 Day 7

      7.1 What’s the signal on wire a?

      7.2 What’s the signal on wire a if wire b is overridden with a’s original value?

      7.3 Testing Day 7

    8 Day 8

      8.1 What’s the difference between the literal length of the strings, and their length in memory?

      8.2 What’s the difference between the re-encoded length of the literal string, and the original length?

      8.3 Testing Day 8

    9 Day 9

      9.1 What’s the shortest route that visits all the cities?

      9.2 What’s the longest route?

      9.3 Testing Day 9

    10 Day 10

      10.1 What’s the length of the sequence after 40 iterations?

      10.2 After 50 iterations?

      10.3 Testing Day 10

    11 Day 11

      11.1 What’s the next password that meets the criteria?

      11.2 What’s the next valid password after that?

      11.3 Testing Day 11

    12 Day 12

      12.1 What’s the sum of all the numbers in the document?

      12.2 What’s the sum of all the numbers, if hash tables with value "red" are ignored?

      12.3 Testing Day 12

    13 Day 13

      13.1 What’s the optimal happiness score for a seating arrangement of eight?

        13.1.1 Optimizing in-permutations

      13.2 What’s the optimal happiness score, including ourself in the seating?

      13.3 Testing Day 13

    14 Day 14

      14.1 After 2503 seconds, what is the maximum distance any reindeer has flown?

      14.2 Under the new rule, how many points does the winning reindeer have?

      14.3 Testing Day 14

    15 Day 15

      15.1 What’s the best cookie we can make with 100 tsps of ingredients?

      15.2 What’s the best cookie we can make with 100 tsps that’s exactly 500 calories?

      15.3 Testing Day 15

    16 Day 16

      16.1 Which Sue matches the attribute input?

      16.2 Which Sue matches the attribute input, with the “retroencabulator” rules?

      16.3 Testing Day 16

    17 Day 17

      17.1 How many combinations of containers fit exactly 150 liters?

      17.2 How many combinations have the minimum number of containers?

      17.3 Testing Day 17

    18 Day 18

      18.1 How many lights are on after 100 iterations of the light-switching rules?

      18.2 How many lights are on after 100 iterations, if the corner bulbs are always lit?

      18.3 Testing Day 18

    19 Day 19

      19.1 How many distinct molecules can be created after one transformation?

      19.2 What’s the fewest number of transformations that will generate the test module?

      19.3 Testing Day 19

    20 Day 20

      20.1 What’s the first house that gets the target number of presents?

      20.2 What’s the first house that gets the target number of presents, if each elf delivers 11 gifts to 50 houses?

      20.3 Testing Day 20

    21 Day 21

      21.1 What’s the least we can spend and win?

      21.2 What’s the most we can spend and lose?

      21.3 Testing Day 21

    22 Day 22

      22.1 You’re a Wizard, Henry

      22.2 What’s the least (mana) we can spend and win?

      22.3 Hard Mode

      22.4 Testing Day 22

    23 Day 23

      23.1 What’s the value in register b after the program runs?

      23.2 What’s the value in register b if register a starts as 1?

      23.3 Testing Day 23

    24 Day 24

      24.1 What’s the score of the optimal group of packages, when divided into three groups?

      24.2 What’s the optimal score when divided into four groups?

      24.3 Testing Day 24

    25 Day 25

      25.1 What code do you give the machine?

      25.2 Testing Day 25

    Index