Part 1

Read in our puzzle input:

day <- 3

input <- here::here(
  "2022", "input",
  paste0("day_", stringr::str_pad(day, 2, "left", "0"))
  ) |>
  readLines()

Tidyverse version

priorities <- c(letters, LETTERS)

tibble::tibble(
  input = input
) |>
  dplyr::mutate(
    halfway = nchar(input) / 2,
    part_1 = substr(input, 1, halfway),
    part_2 = substr(input, halfway + 1, 2 * halfway)) |>
  dplyr::rowwise() |>
  dplyr::mutate(common_letter = intersect(stringr::str_split_1(part_1, ""), stringr::str_split_1(part_2, "")),
    priority = stringr::str_which(priorities, common_letter)
  ) |>
  dplyr::pull(priority) |>
  sum()

## [1] 7821

Base R version

halfways <- nchar(input) / 2
part_1 <- mapply(substr, input, 1, halfways)
part_2 <- mapply(substr, input, halfways + 1, halfways * 2)

part_1_letters <- lapply(part_1, strsplit, split = "")
part_2_letters <- lapply(part_2, strsplit, split = "")

part_1_letters <- lapply(part_1_letters, unlist)
part_2_letters <- lapply(part_2_letters, unlist)

common_letters <- mapply(intersect, part_1_letters, part_2_letters,
                         SIMPLIFY = TRUE, USE.NAMES = FALSE)

sum(vapply(common_letters, grep, FUN.VALUE = 1, x = priorities,
      ignore.case = FALSE, value = FALSE))

## [1] 7821

Part 2

input |>
  split(f = rep(1:(length(input) / 3), each = 3)) |>
  lapply(strsplit, split = "") |>
  vapply(Reduce, FUN.VALUE = "a", f = intersect, USE.NAMES = FALSE) |>
  vapply(grep, FUN.VALUE = 1, x = priorities,
      ignore.case = FALSE, value = FALSE) |>
  sum()

## [1] 2752