Day 2 Notebook

input<-read_lines("Day2Sample.txt")

idranges<-as_vector(str_split(input,","))

Part 1

Takes in a range, gives out the number of repeated numbers

finddoubles<-function(r){
  curr<-as.numeric(str_split_i(r,"-",i = 1))
  counter<-0
  upper<-as.numeric(str_split_i(r,"-",2))
  while(curr<=upper){
    ### find out how many digits - if odd, go to next 1010 or 100100, etc.
    evendigits<-floor(log10(curr))
    if(evendigits%%2==0){
      curr<-10^(evendigits+1)+10^(evendigits/2)
      next}
    ### otherwise, split into two halves (which can be done, because there's an even number of digits)
    ### find the smallest valid value >= current.
    ### then add 1 to the first half and keep going up until it hits the upper limit.
    ### there is a lot of redundancy in this loop.
    ed2<-(evendigits+1)/2
    ed2<-10^ed2
    fronthalf<-curr%/%ed2
    backhalf<-curr%%ed2
    if(fronthalf>backhalf){
      curr<-fronthalf*ed2+fronthalf
    }else if(backhalf>fronthalf){
      curr<-((fronthalf+1)*ed2)+fronthalf+1
    }else{
      counter<-counter+curr
      curr<-((fronthalf+1)*ed2)+fronthalf+1}}
    counter}

Takes in all of the ranges and returns the sum of all of the ids

add_all_ids<-function(rns){
  allids<-sum(sapply(rns,finddoubles))
  allids}

add_all_ids(idranges)

[1] 1227775554

Part 2

I saw it coming. I just had a feeling that this was going to be part 2. And yet, I did nothing to prepare for it in advance. I can’t think of any way to not check every number.

This tells me if a number is a repeater.

isrepeater<-function(n){
  #comparing strings. this is non-optimal, but it does work.
  n<-as.character(n)
  l<-nchar(n)
  for(i in 1:(l/2)){
    ### if this won't go evenly in, don't bother.
    if(l%%i!=0){next}
    a<-str_sub(n,1,i)
    a<-str_flatten(rep(a,l/i))
    if(a==n){return(TRUE)}}
  FALSE}

reprange<-function(r){
  repeaters<-c()
  ### ignore 1 digit numbers
  lower<-max(as.numeric(str_split_i(r,"-",1)),11)
  upper<-as.numeric(str_split_i(r,"-",2))
  if(upper>=lower){
    for(i in lower:upper){
      if(isrepeater(i)){repeaters<-c(repeaters,i)}}}
  repeaters}

findallrepeaters<-function(rs){
  allreps<-sum(unname(unlist(sapply(rs,reprange))))
  allreps}

findallrepeaters(idranges)

[1] 4174379265