Day 18 Notebook

input<-read_lines("Day18Sample.txt")
### Just read these into a dictionary - save the position in line
corrupt<-dict()
for(i in 1:length(input)){
  corrupt$set(input[i],i)}

Part 1

Search. Use time as a way to figure out if something is in your way

computerstorm<-function(cs,sx,sy,ex,ey,maxx,maxy,t){
  beenthere<-dict()
  pq<-priority_queue()
  ## push x,y,steps
  pq$push(c(sx,sy,0),0)
  while(pq$size()>0){
    curr<-pq$pop()
    ### check to see if have been here before - if not - note
    if(beenthere$has(str_flatten(c(curr[1],curr[2]),","))){next}
    beenthere$set(str_flatten(c(curr[1],curr[2]),","),0)
    ### check to see if we're at the end
    if(all(curr[1:2]==c(ex,ey))){
      return(curr[3])}
    ### otherwise move
    ##up
    if(curr[2]>0){
      ### check to see if that space is corrupt - if not, move there
      checkup<-cs$get(str_flatten(c(curr[1],curr[2]-1),","),default=Inf)
      if(t<checkup){pq$push(c(curr[1],curr[2]-1,curr[3]+1),-sum(1,curr[3],abs(c(ex,ey)-c(curr[1],curr[2]-1))))}}
    ##down
    if(curr[2]<maxy){
      ### check to see if that space is corrupt - if not, move there
      checkdown<-cs$get(str_flatten(c(curr[1],curr[2]+1),","),default=Inf)
      if(t<checkdown){pq$push(c(curr[1],curr[2]+1,curr[3]+1),-sum(1,curr[3],abs(c(ex,ey)-c(curr[1],curr[2]+1))))}}
    ##left
    if(curr[1]>0){
      ### check to see if that space is corrupt - if not, move there
      checkleft<-cs$get(str_flatten(c(curr[1]-1,curr[2]),","),default=Inf)
      if(t<checkleft){pq$push(c(curr[1]-1,curr[2],curr[3]+1),-sum(1,curr[3],abs(c(ex,ey)-c(curr[1]-1,curr[2]))))}}
    ##right
    if(curr[1]<maxx){
      ### check to see if that space is corrupt - if not, move there
      checkleft<-cs$get(str_flatten(c(curr[1]+1,curr[2]),","),default=Inf)
      if(t<checkleft){pq$push(c(curr[1]+1,curr[2],curr[3]+1),-sum(1,curr[3],abs(c(ex,ey)-c(curr[1]+1,curr[2]))))}}
  }
  ### for part 2, return 0 if there's no way through
return(0)}

computerstorm(corrupt,0,0,6,6,6,6,12)

[1] 22

Part 2

Find the limits to t

findt<-function(cs,sx,sy,ex,ey,maxx,maxy){
  ### what are the limits to t
  lowlimit<-1
  highlimit<-cs$size()
  #### while there's a difference between low & high limit
  while(lowlimit+1<highlimit){
    ### try a random time...because I can.
    trytime<-sample((lowlimit+1):(highlimit-1),1)
    x<-computerstorm(cs,sx,sy,ex,ey,maxx,maxy,trytime)
    if(x==0){highlimit<-trytime}else{lowlimit<-trytime}}
  highlimit}

p2<-findt(corrupt,0,0,6,6,6,6)
part2<-input[p2]
part2

[1] "6,1"

6,1