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

1Day 1

1.1Where does the elevator land?

1.1.1Alternate approach: numerical conversion

1.2At what point does the elevator enter the basement?

1.2.1Alternate approaches: for/first or for/or

1.3Testing Day 1

2Day 2

2.1How much paper is needed to wrap the boxes?

2.2How much ribbon is needed to wrap the boxes?

2.3Testing Day 2

3Day 3

3.1How many grid cells are visited?

3.1.1Alternate approach: complex numbers

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

3.3Testing Day 3

4Day 4

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

4.2How about six zeroes?

4.3Testing Day 4

5Day 5

5.1How many strings are “nice”?

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

5.3Testing Day 5

6Day 6

6.1How many lights are lit after following the instructions?

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

6.3Refactored solution

6.4Testing Day 6

7Day 7

7.1What’s the signal on wire a?

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

7.3Testing Day 7

8Day 8

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

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

8.3Testing Day 8

9Day 9

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

9.2What’s the longest route?

9.3Testing Day 9

10Day 10

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

10.2After 50 iterations?

10.3Testing Day 10

11Day 11

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

11.2What’s the next valid password after that?

11.3Testing Day 11

12Day 12

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

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

12.3Testing Day 12

13Day 13

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

13.1.1Optimizing in-permutations

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

13.3Testing Day 13

14Day 14

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

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

14.3Testing Day 14

15Day 15

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

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

15.3Testing Day 15

16Day 16

16.1Which Sue matches the attribute input?

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

16.3Testing Day 16

17Day 17

17.1How many combinations of containers fit exactly 150 liters?

17.2How many combinations have the minimum number of containers?

17.3Testing Day 17

18Day 18

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

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

18.3Testing Day 18

19Day 19

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

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

19.3Testing Day 19

20Day 20

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

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

20.3Testing Day 20

21Day 21

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

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

21.3Testing Day 21

22Day 22

22.1You’re a Wizard, Henry

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

22.3Hard Mode

22.4Testing Day 22

23Day 23

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

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

23.3Testing Day 23

24Day 24

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

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

24.3Testing Day 24

25Day 25

25.1What code do you give the machine?

25.2Testing Day 25

Index

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