Day 1 - Part 1

How many measurements are larger than the previous measurement?

199  (N/A - no previous measurement)
200  (increased)
208  (increased)
210  (increased)
200  (decreased)
207  (increased)
240  (increased)
269  (increased)
260  (decreased)
263  (increased)

Day 1 - Part 2

How many sliding three-measurement sums are larger than the previous sum?

199  A           607  (N/A - no previous sum)
200  A B         618  (increased)
208  A B C       618  (no change)
210    B C D     617  (decreased)
200  E   C D     647  (increased)
207  E F   D     716  (increased)
240  E F G       769  (increased)
269    F G H     792  (increased)
260      G H
263        H

Day 1: How to make life harder for yourself

By Claire Little 10,000 μs

library(tidyverse)
# Part 1:
input %>% mutate(change = V1 - lag(V1),
                 sign = change / abs(change)) %>%
  group_by(sign) %>%
  summarise(n = n())
# Part 2:
input %>% mutate(window = V1 + lag(V1) + lag(V1, 2),
                 change = window - lag(window),
                 sign = change / abs(change)) %>%
  group_by(sign) %>%
  summarise(n = n())

Day 1: base R using diff()

By David Selby ~100 μs

increases <- function(depths) {
  sum(diff(depths) > 0)
}
rolling_sum <- function(depths) {
  head(depths, -2) + tail(depths, -2) +
    head(tail(depths, -1), -1)
}
input1 <- scan('input01.txt')
increases(input1)
increases(rolling_sum(input1))

Or you could use zoo::rollsum(depths, 3)

Day 1: one-liner using diff(lag = n)

By David Schoch ~100 μs

sapply(c(1, 3), function(n) sum(diff(input1, lag = n) > 0))

Or with R's new anonymous function \(x) syntax:

sapply(c(1, 3), \(n) sum(diff(input1, n) > 0))

Day 2 - Part 1

• forward X increases the horizontal position by X units.
• down X increases the depth by X units.
• up X decreases the depth by X units.

                  x  depth
forward 5   -->   5      -
down 5      -->          5
forward 8   -->  13
up 3        -->          2
down 8      -->         10
forward 2   -->  15
==> x = 15, depth = 10

Day 2 - Part 2

• down X increases your aim by X units.
• up X decreases your aim by X units.
• forward X does two things:
  • It increases your horizontal position by X units.
  • It increases your depth by your aim multiplied by X.

                  x  aim  depth
forward 5   -->   5    -      - 
down 5      -->        5      
forward 8   -->  13          40
up 3        -->        2
down 8      -->       10
forward 2   -->  15          60
==> x = 15, depth = 60

Day 2: I don't care if loops are inefficient

By Claire Little 60,000 μs

inst = read.table("path/to/Day2.csv")
ninst = dim(inst)[1]
# Day 2 - Part 1
position = c(0, 0)
for (i in 1:ninst) {
  if (inst[i, 1] == 'forward') {
    position = position + c(inst[i, 2], 0)
  } else if (inst[i, 1] == 'down') {
    position = position + c(0, inst[i, 2])
  } else {
    position = position - c(0, inst[i, 2])
  }
}
answer1 = position[1] * position[2]

Day 2: I don't care if loops are inefficient

By Claire Little 60,000 μs

# Day 2 - Part 2
position = c(0, 0, 0)
for (i in 1:ninst) {
  if (inst[i, 1] == 'forward') {
    position = position + c(inst[i, 2], 0, 0)
    position = position + c(0, (inst[i, 2] * position[3]), 0)
  } else if (inst[i, 1] == 'down') {
    position = position + c(0, 0, inst[i, 2])
  } else {
    position = position - c(0, 0, inst[i, 2])
  }
}
answer2 = position[1] * position[2]

Day 2: no ifs, no loops

By David Selby 100 μs

course <- read.table('input.txt', col.names = c('dir', 'value'))
with(course, {
  x <- (dir == 'forward') * value
  y <- ((dir == 'down') - (dir == 'up')) * value
  sum(x) * sum(y)
})

with(course, {
  x <- (dir == 'forward') * value
  y <- ((dir == 'down') - (dir == 'up')) * value
  depth <- cumsum(y) * x
  sum(x) * sum(depth)
})

Day 2: tidyverse-style

By David Selby 3000 μs

course %>%
  mutate(x = if_else(dir == 'forward', value, 0L),
         y = if_else(dir == 'down', value, -value) * !x,
         depth = cumsum(y) * x) %>%
  summarise(part1 = sum(x) * sum(y),
            part2 = sum(x) * sum(depth))