1 files changed, 128 insertions, 0 deletions
diff --git a/20/src/main.rs b/20/src/main.rs
new file mode 100644
index 0000000..1b4fc51
--- /dev/null
+++ b/20/src/main.rs
@@ -0,0 +1,128 @@
+use advent_lib::prelude::*;
+
+
+fn main() -> Result<()> {
+  let mut args = std::env::args();
+  if args.len() != 2 {
+    eprintln!("Usage: advent input");
+  }
+  let _ = args.next();
+  let filename = args.next().unwrap();
+
+  let input = advent_lib::read_lines_file(&filename)?;
+
+  let mut iterator = input.iter();
+
+  let mut dictionary = Vec::new();
+  for c in iterator.next().unwrap().chars() {
+    dictionary.push(match c {
+      '#' => 1,
+      '.' => 0,
+      _ => panic!("unexpected character"),
+    });
+  }
+
+  let _ = iterator.next();
+
+  let mut image = Vec::new();
+  for line in iterator {
+    let mut row = Vec::new();
+    for c in line.chars() {
+      row.push(match c {
+        '#' => 1,
+        '.' => 0,
+        _ => panic!("unexpected character"),
+      });
+    }
+    image.push(row);
+  }
+
+  let mut background = 0;
+  for _ in 0 .. 2 {
+    let (new_image, new_background) =
+      iterate(&image, background, &dictionary);
+    image = new_image;
+    background = new_background;
+  }
+  println!("{}", count_pixels(&image));
+
+  for _ in 2 .. 50 {
+    let (new_image, new_background) =
+      iterate(&image, background, &dictionary);
+    image = new_image;
+    background = new_background;
+  }
+  println!("{}", count_pixels(&image));
+
+  Ok(())
+}
+
+
+fn iterate(input: &Vec<Vec<u8>>, background: u8, dictionary: &Vec<u8>)
+  -> (Vec<Vec<u8>>, u8)
+{
+  let height = input.len() as i64;
+  let width = input[0].len() as i64;
+
+  let mut output = Vec::new();
+  for y in -2 .. height+2 {
+    let mut row = Vec::new();
+    for x in -2 .. width+2 {
+      let mut index: usize = 0;
+
+      for offset_y in -1 .. 2 {
+        for offset_x in -1 .. 2 {
+          let computed_y: i64 = y + offset_y;
+          let computed_x: i64 = x + offset_x;
+
+
+          index *= 2;
+          if (computed_y >= 0) && (computed_y < height)
+            && (computed_x >= 0) && (computed_x < width)
+          {
+            index += input[computed_y as usize][computed_x as usize] as usize;
+          } else {
+            index += background as usize;
+          }
+        }
+      }
+
+      row.push(dictionary[index]);
+    }
+    output.push(row);
+  }
+
+  let output_background = dictionary[background as usize];
+
+  (output, output_background)
+}
+
+
+fn count_pixels(image: &Vec<Vec<u8>>) -> i64 {
+  let mut count = 0;
+
+  for row in image {
+    for cell in row {
+      count += *cell as i64;
+    }
+  }
+
+  count
+}
+
+
+#[allow(dead_code)]
+fn debug_image(image: &Vec<Vec<u8>>) {
+  for row in image {
+    for cell in row {
+      if *cell == 1 {
+        print!("#");
+      } else {
+        print!(".");
+      }
+    }
+    println!("");
+  }
+  println!("");
+}
+