My solutions (R) for Advent of Code 2017
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These are my solutions to Advent of Code 2017, a series of 25 programming puzzles.

Package overview

The /R folder contains the R functions I wrote for each day. I used some light test-driven development for the puzzles. That is, each puzzle description provides some example inputs and outputs. Before tackling the main test input, I write a unit-test in /tests that confirms that my solution can correctly reproduce the examples. The /inst directory contains the code to handle the main test input for each day.

I limited the amount of package dependencies used for these puzzles to maximize future compatibility and to make sure that it is mostly my code that solves the problems. For example, if a puzzle requires answering questions about the data in a tree-like structure, it would be kind of cheating for me to find a library for building and traversing trees to tackle the problem. It's advent of code, not advent of load.

I have allowed myself to use:

  • base, stats, utils, tools, and so on: the base R packages
  • magrittr: for the pipe %>% syntax
  • rlang: for language and code evaluation
  • stringr: for regular-expression-related functions

I've put my R source code under a GPL-3 license. It should not apply to the puzzle descriptions in the code comments at the top of each file. I did not write those descriptions.

Coding approaches

Here are the programming tasks and techniques I used for the various puzzles.

  • 01a/b Compute different kinds of sums for elements in a circular vector: Book-keeping (reordering indices).

  • 02a/b Compute different kinds of checksums on integer sequences: Functional programming.

  • 03a If positive integers are in a spiral, find taxicab distance of n from 1: Math.

  • 03b In a different kind of spiral, find first number after n: Enumerate values.

  • 04a/b Check if strings meet certain rules: Functional programming.

  • 05a/b Given a list of jump instructions, compute number of jumps required to exit: Objects.

  • 06a/b Detect when an iterative process begins to loop forever: Book-keeping (history of values).

  • 07a/b Construct a tree from a list of edges and check if it's balanced: Recursion, tree-traversal, functional programming.

  • 08a/b Simulate a register machine: Custom evaluation (parse and evaluate instructions as R code in a special environment).

  • 09a/b Count nesting depth of bracketed text that contains escape characters and ignore sequences: Consume character stream.

  • 10a/b "Twist" a circular vector and hash strings with repeated twisting: Book-keeping (reordering elements, converting strings to bits).

  • 11a/b Find shortest path between two cells in a hexagonal grid: Math.

  • 12a/b Find number of nodes in a connected graph and number of connected subgroups in a graph: Breadth first search.

  • 13a/b Collision detection and avoidance through a Frogger-style obstacle course: Math.

  • 14a/b Write a flood-fill algorithm: Breadth first search.

  • 15a/b Compare values from two generators: Enumerate values, functional programming.

  • 16a Apply a series of permutation rules to a vector: Custom evaluation.

  • 16b Repeatedly apply the rules one billion times: Book-keeping (repeat until a duplicate permutation found).

  • 17a Repeatedly insert values and jump position in a circular vector: Book-keeping (arithmetic on indices).

  • 17b Find value in a given position after 50 million cycles: Math.

  • 18a/b Simulate register machines that pass messages between each other: Custom evaluation, objects.

  • 19a/b Make a cursor follow a path: Generate and consume character stream.

  • 20a/b Find slowest moving particle in a swarm and number of particles that will eventually collide: Math.

  • 21a/b Find and replace patterns in a grid (like a fractal): Book-keeping (keeping track of indices of subgrids).

  • 22a/b Move a cursor around an infinite grid and change symbols: Objects.

  • 23a Simulate register machine: Custom evaluation, objects.

  • 23b Compute value in an inefficient register machine: Analyzing code.

  • 24a/b Find largest/longest combinations of units: Recursion, functional programming.

  • 25a Simulate a Turing Machine: Objects.

By "book-keeping", I mean basic programming programming where I keep track of some changing state like a position in a vector.

By "math", I mean studying the problem and using math to find a shortcut that lets me skip some computations.

By "custom evaluation", I mean writing a parser to convert the problem input into R code and run that R code in a special environment. This technique changes the coding task into the task of running R code with some extra book-keeping. This approach was a new one for me, so I learned a great deal along the way and I could streamline my solutions for some puzzles if I had to redo them.

By "functional programming", I mean both map/filter/reduce operations as well as using first class functions. For example, on day 04, I have to count how many passphrases have a duplicated word in Part A and count how many passphrases contain anagrams in Part B. My solution is a function count_valid_passphrases(passphrases, rule) where rule is a function tailored for Part A or Part B.

By "objects", I mean object-oriented programming using closures. Something like:

counter <- function(start = 0) {
  initial <- force(start)
  num <- initial
  inc <- function() num <<- num + 1
  dec <- function() num <<- num - 1
  reset <- function() num <<- initial
  look <- function() num
  list(inc = inc, dec = dec, reset = reset, look = look)

robot <- counter(10)
#> [1] 10
#> [1] 12
#> [1] 8
#> [1] 10