2021-13: Transparent Origami

library(stringr)
library(data.table)
library(tidyverse)

── Attaching packages ─────────────────────────────────────── tidyverse 1.3.2 ──
✔ ggplot2 3.4.0      ✔ purrr   0.3.5 
✔ tibble  3.1.8      ✔ dplyr   1.0.10
✔ tidyr   1.2.1      ✔ forcats 0.5.2 
✔ readr   2.1.3      
── Conflicts ────────────────────────────────────────── tidyverse_conflicts() ──
✖ dplyr::between()   masks data.table::between()
✖ dplyr::filter()    masks stats::filter()
✖ dplyr::first()     masks data.table::first()
✖ dplyr::lag()       masks stats::lag()
✖ dplyr::last()      masks data.table::last()
✖ purrr::transpose() masks data.table::transpose()

dt <- readLines("input.txt")

coords <- dt[grepl("[0-9]{1,4},[0-9]{1,4}", dt)]
coords <- data.frame(str_split(coords, ",", simplify = TRUE))
coords <- data.frame(sapply(coords, as.numeric))
coords <- coords + 1

folds <- str_remove_all(dt[grepl("fold along", dt)], "fold along ")
folds <- data.frame(str_split(folds, "=", simplify = TRUE))
folds$X2 <- as.numeric(folds$X2)

paper <- matrix(data = FALSE,
                nrow = (max(folds$X2[folds$X1 == "y"]) * 2) + 1,
                ncol = (max(folds$X2[folds$X1 == "x"]) * 2) + 1)
for(r in 1:nrow(coords)) { paper[coords$X2[r], coords$X1[r]] <- TRUE }

Part 1

fold <- function(dir, n, paper) {
  if (dir == "x") {
    paper1 <- paper[,1:(n)]
    paper2 <- paper[,(n+2):ncol(paper)]
    paper2 <- t(apply(paper2, 1, rev))
  } else if (dir == "y") {
    paper1 <- paper[1:(n),]
    paper2 <- paper[(n+2):nrow(paper), ]
    paper2 <- apply(paper2, 2, rev)
  }

  return(ifelse(paper1 | paper2, TRUE, FALSE))
}

folded <- fold(folds$X1[1], folds$X2[1], paper)
length(which(folded))

[1] 684

Part 2

fold <- function(dir, n, paper) {
  if (dir == "x") {
    paper1 <- paper[,1:(n)]
    paper2 <- paper[,(n+2):ncol(paper)]
    paper2 <- t(apply(paper2, 1, rev))
  } else if (dir == "y") {
    paper1 <- paper[1:(n),]
    paper2 <- paper[(n+2):nrow(paper), ]
    paper2 <- apply(paper2, 2, rev)
  }
  
  return(ifelse(paper1 | paper2, TRUE, FALSE))
}

for(i in 1:nrow(folds)) {
  paper <- fold(folds$X1[i], folds$X2[i], paper)
  
  coordX <- c()
  coordY <- c()
  for(r in 1:nrow(paper)) {
    for(c in 1:ncol(paper)) {
      if(paper[r,c]) {
        coordX <- append(coordX, r)
        coordY <- append(coordY, c)
      }
    }
  }
  
  points <- data.frame(cbind(coordX, coordY)) %>%
    ggplot() +
    geom_point(aes(x = coordY, y = -coordX)) +
    xlim(0, max(c(coordX, coordY))) +
    ylim(-max(c(coordX, coordY)), 0)
  if(i == nrow(folds))
    print(points)
}