2024 advent of code day 5

day_5.R

# first star
input <- readLines("input.txt")

pages <- as.data.frame(t(sapply(strsplit(input[grep("\\|", input)], "\\|"), as.integer)))
updates <- lapply(strsplit(input[grep(",", input)], ","), as.integer)


get_order <- function(u){
  ordered <- pages[(pages$V1 %in% u) & (pages$V2 %in% u), ] # which page pairs apply to update
  ordered$V1_idx <- sapply(ordered$V1, function(x){which(u == x)}) # get indices
  ordered$V2_idx <- sapply(ordered$V2, function(x){which(u == x)})                         
  ordered$check <- (ordered$V1_idx < ordered$V2_idx) # check if indices in correct order
  ordered <- all(ordered$check)
  if (ordered){return(u[ceiling(length(u)/2)])} # return midpoint value
  else {return(0)}
  }

sum(sapply(updates, get_order))

# second star
# isolate the pages left to update
order_up <- function(u){
  if(get_order(u)){return(0)} 
  else{return(u)}
}

to_update <- Filter(function(x){is.integer(x)}, sapply(updates, order_up))


# table sort idea + application courtesy of u/cettbycett 
check_order <- function(u) {
  pages <- t(pages)
  to_order <- pages[ ,apply(pages, 2, function(z) sum(u %in% z) == 2)] 
  ordered <- as.integer(names(sort(-table(to_order[1, ]))))
  ordered <- c(ordered, setdiff(u, to_order[1,]))
  ordered[ceiling(length(u)/2)] * if (identical(ordered, u)) 1 else -1
}

ordered <- sapply(to_update, check_order)
sum(-ordered[ordered < 0])