20

7 files changed, 2524 insertions, 0 deletions

diff --git a/20/Cargo.toml b/20/Cargo.toml
new file mode 100644
index 0000000..21adb16
--- /dev/null
+++ b/20/Cargo.toml
@@ -0,0 +1,11 @@
+[package]
+name = "advent_20"
+version = "0.1.0"
+authors = ["Irene Knapp <ireneista@gmail.com>"]
+edition = "2018"
+
+[dependencies]
+advent_lib = { path = "../lib" }
+
+[dev-dependencies]
+assert_cmd = "0.10"
diff --git a/20/input b/20/input
new file mode 100644
index 0000000..2cb7a6f
--- /dev/null
+++ b/20/input
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+
+Tile 1301:
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+
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+
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+
+Tile 3767:
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+
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+
+Tile 1931:
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+
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+
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+
+Tile 3323:
+.....#.#.#
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+
+Tile 1061:
+#....###..
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+
+Tile 1163:
+####.###.#
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+
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+
+Tile 1993:
+#..##.#.##
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+
+Tile 1213:
+#..##..#..
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+
+Tile 3373:
+#.##..#.##
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+
+Tile 2017:
+#...#.##.#
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+
+Tile 1367:
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+
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+.#.#.#.#..
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+
+Tile 1871:
+##..#.##.#
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+
+Tile 1601:
+#####.....
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+....#.#...
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+
+Tile 1471:
+.#######..
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+
+Tile 2521:
+###.....##
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+......##.#
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+........##
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+
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+
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+
+Tile 2143:
+#.#...#.#.
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+
+Tile 3517:
+####.#...#
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+#..#......
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+#......#..
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+
+Tile 2543:
+.#######.#
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+........#.
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+..#.#.##.#
+........##
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+
+Tile 1567:
+...#.##...
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+........##
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+
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+##....#.##
+......#...
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+
+Tile 1153:
+.##...###.
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+
+Tile 1811:
+###.#..#.#
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+..#......#
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+
+Tile 1667:
+.#.#.##.##
+.....#.#.#
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+
+Tile 2677:
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+
+Tile 2707:
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+
+Tile 1699:
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+
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+
+Tile 1019:
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+#.#....#..
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+....#.....
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+
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+..#...#.#.
+.##.......
+...#.#...#
+........#.
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+...##....#
+..##...##.
+.....#..##
+..###.....
+#...####..
+
+Tile 3041:
+##..#####.
+......#..#
+..#..##..#
+..........
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+..##.....#
+.#.......#
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+
+Tile 3313:
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+#.#..###..
+#....#.#..
+##........
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+#.#.#...#.
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+
+Tile 3541:
+#.#.......
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+.#......##
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+....#.#.##
+.#.......#
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+
+Tile 1319:
+#.#...#..#
+#......##.
+..#.#.####
+#.........
+#..#.....#
+..#...##.#
+#...#.##.#
+#..#....##
+....##....
+.#.#.#.###
+
+Tile 1499:
+####...#..
+#...#....#
+.....#...#
+.......#..
+#.###....#
+.......###
+.......##.
+.......#.#
+..#.#...#.
+......#.#.
+
+Tile 2239:
+..#....##.
+.####....#
+...#.....#
+......#...
+.....#..#.
+#..#.....#
+#.#.####..
+###.##....
+.........#
+...#...#..
+
+Tile 3769:
+####.####.
+#....#.#.#
+.##.......
+###.....##
+##.#...#.#
+....#.#..#
+..........
+#.#.#.....
+##.......#
+.#####.###
+
+Tile 1187:
+#..#.#....
+#.......#.
+...#....##
+#..##....#
+#.....#.##
+...##.####
+#....#....
+.....#.#..
+#.##.#...#
+#..#.##.##
+
+Tile 1913:
+#.##.#.##.
+##.#...#.#
+#.#...#...
+##.#.....#
+#.##......
+#.###....#
+..#..#.##.
+#.........
+#.#......#
+...#.###.#
+
+Tile 1753:
+..#..#....
+.......#.#
+...#.#.#.#
+#.#....#..
+#......#.#
+##...#....
+#.#.....##
+#..#...##.
+.##.#.#..#
+##.###.##.
+
diff --git a/20/input.small b/20/input.small
new file mode 100644
index 0000000..b07aa4b
--- /dev/null
+++ b/20/input.small
@@ -0,0 +1,107 @@
+Tile 2311:
+..##.#..#.
+##..#.....
+#...##..#.
+####.#...#
+##.##.###.
+##...#.###
+.#.#.#..##
+..#....#..
+###...#.#.
+..###..###
+
+Tile 1951:
+#.##...##.
+#.####...#
+.....#..##
+#...######
+.##.#....#
+.###.#####
+###.##.##.
+.###....#.
+..#.#..#.#
+#...##.#..
+
+Tile 1171:
+####...##.
+#..##.#..#
+##.#..#.#.
+.###.####.
+..###.####
+.##....##.
+.#...####.
+#.##.####.
+####..#...
+.....##...
+
+Tile 1427:
+###.##.#..
+.#..#.##..
+.#.##.#..#
+#.#.#.##.#
+....#...##
+...##..##.
+...#.#####
+.#.####.#.
+..#..###.#
+..##.#..#.
+
+Tile 1489:
+##.#.#....
+..##...#..
+.##..##...
+..#...#...
+#####...#.
+#..#.#.#.#
+...#.#.#..
+##.#...##.
+..##.##.##
+###.##.#..
+
+Tile 2473:
+#....####.
+#..#.##...
+#.##..#...
+######.#.#
+.#...#.#.#
+.#########
+.###.#..#.
+########.#
+##...##.#.
+..###.#.#.
+
+Tile 2971:
+..#.#....#
+#...###...
+#.#.###...
+##.##..#..
+.#####..##
+.#..####.#
+#..#.#..#.
+..####.###
+..#.#.###.
+...#.#.#.#
+
+Tile 2729:
+...#.#.#.#
+####.#....
+..#.#.....
+....#..#.#
+.##..##.#.
+.#.####...
+####.#.#..
+##.####...
+##..#.##..
+#.##...##.
+
+Tile 3079:
+#.#.#####.
+.#..######
+..#.......
+######....
+####.#..#.
+.#...#.##.
+#.#####.##
+..#.###...
+..#.......
+..#.###...
diff --git a/20/src/main.rs b/20/src/main.rs
new file mode 100644
index 0000000..12c3dea
--- /dev/null
+++ b/20/src/main.rs
@@ -0,0 +1,655 @@
+use advent_lib::prelude::*;
+
+use std::collections::BTreeMap;
+use std::collections::BTreeSet;
+
+#[derive(Clone,Copy,Debug,Eq,Ord,PartialOrd,PartialEq)]
+pub enum Edge {
+  Top,
+  Bottom,
+  Left,
+  Right,
+}
+
+#[derive(Clone,Copy,Debug,Eq,Ord,PartialOrd,PartialEq)]
+pub struct Orientation {
+  edge_at_top: Edge, // rotation occurs *after* flipping
+  is_flipped: bool, // flips occur around the y axis
+}
+
+
+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 input = advent_lib::group_lines_by_blanks(input);
+
+  let mut tiles: BTreeMap<i64,Vec<Vec<bool>>> = BTreeMap::new();
+  for tile_input in &input {
+    let mut lines = tile_input.iter();
+
+    let rest = lines.next().unwrap();
+    let (_, rest) = rest.split_at(5);
+    let colon_point = rest.find(':').unwrap();
+    let (tile_id, _) = rest.split_at(colon_point);
+    let tile_id = tile_id.parse::<i64>().unwrap();
+
+    let mut tile = Vec::new();
+    for line in lines {
+      let mut row = Vec::new();
+      for c in line.chars() {
+        row.push(c == '#');
+      }
+      tile.push(row);
+    }
+
+    tiles.insert(tile_id, tile);
+  }
+
+  let mut tile_edges: BTreeMap<(i64,Edge),u64> = BTreeMap::new();
+  for (tile_id, tile) in tiles.iter() {
+    let top = extract_top(tile);
+    let bottom = extract_bottom(tile);
+    let left = extract_left(tile);
+    let right = extract_right(tile);
+    tile_edges.insert((*tile_id, Edge::Top), top);
+    tile_edges.insert((*tile_id, Edge::Bottom), bottom);
+    tile_edges.insert((*tile_id, Edge::Left), left);
+    tile_edges.insert((*tile_id, Edge::Right), right);
+  }
+
+  let tile_count = tiles.keys().len() as f64;
+  let grid_size = tile_count.sqrt().floor() as i64;
+
+  let mut grid: Vec<Vec<Option<(i64,Orientation)>>> = Vec::new();
+  for _ in 0 .. grid_size {
+    let mut grid_row = Vec::new();
+    for _ in 0 .. grid_size {
+      grid_row.push(None);
+    }
+    grid.push(grid_row);
+  }
+
+  let mut tile_ids = BTreeSet::new();
+  for tile_id in tiles.keys() {
+    tile_ids.insert(*tile_id);
+  }
+
+  let grid = fill_grid(&grid, &tiles, &tile_edges, &tile_ids).unwrap();
+  let summary = summarize_grid(&grid);
+  println!("{}", summary);
+
+  let image = assemble_grid(&grid, &tiles);
+  let sea_serpent_count = count_sea_serpents(&image);
+  println!("{}", sea_serpent_count);
+
+  Ok(())
+}
+
+
+fn fill_grid(grid: &Vec<Vec<Option<(i64,Orientation)>>>,
+             tiles: &BTreeMap<i64,Vec<Vec<bool>>>,
+             tile_edges: &BTreeMap<(i64,Edge),u64>,
+             available_tiles: &BTreeSet<i64>)
+  -> Option<Vec<Vec<Option<(i64,Orientation)>>>>
+{
+  match find_empty_cell(grid) {
+    Some((empty_x, empty_y)) => {
+      for tile_id in available_tiles {
+        let mut value_to_match_top = None;
+        if empty_y > 0 {
+          let (tile_id_above, orientation_above) =
+              grid[empty_y - 1][empty_x].unwrap();
+          let edge_above = edge_at(orientation_above, Edge::Bottom);
+          let mut edge_value_above =
+              *tile_edges.get(&(tile_id_above, edge_above)).unwrap();
+          if !orientation_above.is_flipped {
+            edge_value_above = reverse_edge(edge_value_above);
+          }
+          value_to_match_top = Some(edge_value_above);
+        }
+
+        let mut value_to_match_left = None;
+        if empty_x > 0 {
+          let (tile_id_left, orientation_left) =
+              grid[empty_y][empty_x - 1].unwrap();
+          let edge_left = edge_at(orientation_left, Edge::Right);
+          let mut edge_value_left =
+              *tile_edges.get(&(tile_id_left, edge_left)).unwrap();
+          if !orientation_left.is_flipped {
+            edge_value_left = reverse_edge(edge_value_left);
+          }
+          value_to_match_left = Some(edge_value_left);
+        }
+
+        for edge_at_top in &[Edge::Top, Edge::Right, Edge::Bottom, Edge::Left] {
+          for is_flipped in &[false, true] {
+            let orientation = Orientation {
+              edge_at_top: *edge_at_top,
+              is_flipped: *is_flipped,
+            };
+
+            match value_to_match_top {
+              Some(to_match) => {
+                let edge_top = edge_at(orientation, Edge::Top);
+                let mut value_top =
+                    *tile_edges.get(&(*tile_id, edge_top)).unwrap();
+                if *is_flipped {
+                  value_top = reverse_edge(value_top);
+                }
+
+                if value_top != to_match {
+                  continue;
+                }
+              },
+              None => { },
+            }
+
+            match value_to_match_left {
+              Some(to_match) => {
+                let edge_left = edge_at(orientation, Edge::Left);
+                let mut value_left =
+                    *tile_edges.get(&(*tile_id, edge_left)).unwrap();
+                if *is_flipped {
+                  value_left = reverse_edge(value_left);
+                }
+
+                if value_left != to_match {
+                  continue;
+                }
+              },
+              None => { },
+            }
+
+            let mut modified_grid = Vec::new();
+            for y in 0 .. grid.len() {
+              let mut modified_row = Vec::new();
+              for x in 0 .. grid[y].len() {
+                if y == empty_y && x == empty_x {
+                  modified_row.push(Some((*tile_id, orientation)));
+                } else {
+                  modified_row.push(grid[y][x]);
+                }
+              }
+              modified_grid.push(modified_row);
+            }
+
+            let mut modified_available_tiles = available_tiles.clone();
+            modified_available_tiles.remove(tile_id);
+
+            if modified_available_tiles.iter().count() == 0 {
+              return Some(modified_grid);
+            } else {
+              let result = fill_grid(&modified_grid, tiles,
+                                     tile_edges, &modified_available_tiles);
+              if result.is_some() {
+                return result;
+              }
+            }
+          }
+        }
+      }
+
+      None
+    },
+    None => {
+      Some(grid.clone())
+    },
+  }
+}
+
+
+fn find_empty_cell(grid: &Vec<Vec<Option<(i64,Orientation)>>>)
+  -> Option<(usize, usize)>
+{
+  let mut found_empty_cell = false;
+  let mut empty_y = 0;
+  let mut empty_x = 0;
+
+  for y in 0 .. grid.len() {
+    for x in 0 .. grid[y].len() {
+      match grid[y][x] {
+        None => {
+          empty_x = x;
+          empty_y = y;
+          found_empty_cell = true;
+          break;
+        },
+        _ => { },
+      }
+    }
+    if found_empty_cell {
+      break;
+    }
+  }
+
+  if found_empty_cell {
+    Some((empty_x, empty_y))
+  } else {
+    None
+  }
+}
+
+
+fn extract_top(tile: &Vec<Vec<bool>>) -> u64 {
+  let mut result = 0;
+  let y = 0;
+  for x in 0 .. tile[y].len() {
+    result *= 2;
+    if tile[y][x] {
+      result += 1;
+    }
+  }
+  result
+}
+
+
+fn extract_bottom(tile: &Vec<Vec<bool>>) -> u64 {
+  let mut result = 0;
+  let y = tile.len() - 1;
+  let width = tile[y].len();
+  for x_reversed in 0 .. width {
+    let x = width - x_reversed - 1;
+    result *= 2;
+    if tile[y][x] {
+      result += 1;
+    }
+  }
+  result
+}
+
+
+fn extract_left(tile: &Vec<Vec<bool>>) -> u64 {
+  let mut result = 0;
+  let x = 0;
+  let height = tile.len();
+  for y_reversed in 0 .. height {
+    let y = height - y_reversed - 1;
+    result *= 2;
+    if tile[y][x] {
+      result += 1;
+    }
+  }
+  result
+}
+
+
+fn extract_right(tile: &Vec<Vec<bool>>) -> u64 {
+  let mut result = 0;
+  let x = tile[0].len() - 1;
+  for y in 0 .. tile.len() {
+    result *= 2;
+    if tile[y][x] {
+      result += 1;
+    }
+  }
+  result
+}
+
+
+fn reverse_edge(input: u64) -> u64 {
+  let mut input = input;
+  let mut result = 0;
+  for _ in 0 .. 10 {
+    result *= 2;
+    if input & 1 != 0 {
+      result += 1;
+    }
+    input /= 2;
+  }
+  result
+}
+
+
+fn edge_clockwise(edge: Edge) -> Edge {
+  match edge {
+    Edge::Top => Edge::Right,
+    Edge::Right => Edge::Bottom,
+    Edge::Bottom => Edge::Left,
+    Edge::Left => Edge::Top,
+  }
+}
+
+
+fn edge_counterclockwise(edge: Edge) -> Edge {
+  match edge {
+    Edge::Top => Edge::Left,
+    Edge::Left => Edge::Bottom,
+    Edge::Bottom => Edge::Right,
+    Edge::Right => Edge::Top,
+  }
+}
+
+
+fn edge_at(orientation: Orientation, edge_as_placed: Edge) -> Edge {
+  if !orientation.is_flipped {
+    let n_clockwise_from_top = match edge_as_placed {
+      Edge::Top => 0,
+      Edge::Right => 1,
+      Edge::Bottom => 2,
+      Edge::Left => 3,
+    };
+
+    let mut rotated_edge = orientation.edge_at_top;
+    for _ in 0 .. n_clockwise_from_top {
+      rotated_edge = edge_clockwise(rotated_edge);
+    }
+
+    rotated_edge
+  } else {
+    let n_counterclockwise_from_top = match edge_as_placed {
+      Edge::Top => 0,
+      Edge::Right => 1,
+      Edge::Bottom => 2,
+      Edge::Left => 3,
+    };
+
+    let mut rotated_edge = orientation.edge_at_top;
+    for _ in 0 .. n_counterclockwise_from_top {
+      rotated_edge = edge_counterclockwise(rotated_edge);
+    }
+
+    rotated_edge
+  }
+}
+
+
+fn summarize_grid(grid: &Vec<Vec<Option<(i64,Orientation)>>>) -> i64 {
+  let mut summary = 1;
+  let height = grid.len();
+  let width = grid[0].len();
+  summary *= match grid[0][0] {
+    Some((tile_id, _)) => { tile_id },
+    _ => 0,
+  };
+  summary *= match grid[height - 1][0] {
+    Some((tile_id, _)) => { tile_id },
+    _ => 0,
+  };
+  summary *= match grid[0][width - 1] {
+    Some((tile_id, _)) => { tile_id },
+    _ => 0,
+  };
+  summary *= match grid[height - 1][width - 1] {
+    Some((tile_id, _)) => { tile_id },
+    _ => 0,
+  };
+  summary
+}
+
+
+fn assemble_grid(grid: &Vec<Vec<Option<(i64,Orientation)>>>,
+                 tiles: &BTreeMap<i64,Vec<Vec<bool>>>)
+  -> Vec<Vec<bool>>
+{
+  let mut image: Vec<Vec<bool>> = Vec::new();
+
+  let grid_size = grid.len();
+  let (sample_tile_id, _) = grid[0][0].unwrap();
+  let sample_tile = tiles.get(&sample_tile_id).unwrap();
+  let tile_size = sample_tile.len();
+
+  for outer_y in 0 .. grid_size {
+    for inner_y_shifted in 0 .. tile_size - 2 {
+      let mut image_row = Vec::new();
+
+      let inner_y = inner_y_shifted + 1;
+
+      for outer_x in 0 .. grid_size {
+        let (tile_id, orientation) = grid[outer_y][outer_x].unwrap();
+        let tile = tiles.get(&tile_id).unwrap();
+
+        for inner_x_shifted in 0 .. tile_size - 2 {
+          let inner_x = inner_x_shifted + 1;
+          let (tile_x, tile_y) = transform_placed_coordinates_to_tile(
+              orientation, tile_size, inner_x, inner_y);
+          let cell = tile[tile_y][tile_x];
+          image_row.push(cell);
+        }
+      }
+
+      image.push(image_row);
+    }
+  }
+
+  image
+}
+
+
+pub fn debug_grid(grid: &Vec<Vec<Option<(i64,Orientation)>>>,
+              tiles: &BTreeMap<i64,Vec<Vec<bool>>>)
+{
+  let mut debug_output = String::new();
+
+  let grid_size = grid.len();
+  let (sample_tile_id, _) = grid[0][0].unwrap();
+  let sample_tile = tiles.get(&sample_tile_id).unwrap();
+  let tile_size = sample_tile.len();
+
+  for outer_y in 0 .. grid_size {
+    if outer_y > 0 {
+      debug_output.push_str("\n");
+    }
+
+    for inner_y in 0 .. tile_size {
+      for outer_x in 0 .. grid_size {
+        debug_output.push_str("  ");
+
+        let (tile_id, orientation) = grid[outer_y][outer_x].unwrap();
+        let tile = tiles.get(&tile_id).unwrap();
+
+        for inner_x in 0 .. tile_size {
+          let (tile_x, tile_y) = transform_placed_coordinates_to_tile(
+              orientation, tile_size, inner_x, inner_y);
+          let cell = tile[tile_y][tile_x];
+
+          if cell {
+            debug_output.push_str("#");
+          } else {
+            debug_output.push_str(".");
+          }
+        }
+      }
+
+      debug_output.push_str("\n");
+    }
+  }
+
+  println!("{}", debug_output);
+}
+
+
+pub fn debug_image(image: &Vec<Vec<bool>>) {
+  let mut output = String::new();
+
+  for row in image {
+    for cell in row {
+      if *cell {
+        output.push_str("#");
+      } else {
+        output.push_str(".");
+      }
+    }
+    output.push_str("\n");
+  }
+  println!("{}", output);
+}
+
+
+pub fn debug_map(image: &Vec<Vec<bool>>, map: &Vec<Vec<bool>>) {
+  let mut output = String::new();
+  let image_size = image.len();
+
+  for y in 0 .. image_size {
+    for x in 0 .. image_size {
+      if map[y][x] {
+        if image[y][x] {
+          output.push_str("O");
+        } else {
+          output.push_str("@");
+        }
+      } else {
+        if image[y][x] {
+          output.push_str("#");
+        } else {
+          output.push_str(".");
+        }
+      }
+    }
+    output.push_str("\n");
+  }
+  println!("{}", output);
+}
+
+
+fn transform_placed_coordinates_to_tile(
+    orientation: Orientation, tile_size: usize, placed_x: usize, placed_y: usize)
+  -> (usize, usize)
+{
+  /*
+   * at top    TOP     RIGHT      BOTTOM     LEFT
+   *
+   * normal    A.      .2         2+         +A
+   *           +2      A+         .A         2.
+   *
+   * flipped   .A      2.         +2         A+
+   *           2+      +A         A.         .2
+   */
+
+  if !orientation.is_flipped {
+    match orientation.edge_at_top {
+      Edge::Top => { (placed_x, placed_y) },
+      Edge::Left => { (placed_y, tile_size - placed_x - 1) },
+      Edge::Bottom => { (tile_size - placed_x - 1, tile_size - placed_y - 1) },
+      Edge::Right => { (tile_size - placed_y - 1, placed_x) },
+    }
+  } else {
+    match orientation.edge_at_top {
+      Edge::Top => { (tile_size - placed_x - 1, placed_y) },
+      Edge::Left => { (placed_y, placed_x) },
+      Edge::Bottom => { (placed_x, tile_size - placed_y - 1) },
+      Edge::Right => { (tile_size - placed_y - 1, tile_size - placed_x - 1) },
+    }
+  }
+}
+
+
+fn transform_image(input_image: &Vec<Vec<bool>>, orientation: Orientation)
+  -> Vec<Vec<bool>>
+{
+  let image_size = input_image.len();
+
+  let mut output_image = Vec::new();
+  for y_as_placed in 0 .. image_size {
+    let mut output_row = Vec::new();
+    for x_as_placed in 0 .. image_size {
+      let (x_in_input, y_in_input) = transform_placed_coordinates_to_tile(
+        orientation, image_size, x_as_placed, y_as_placed);
+      if input_image[y_in_input][x_in_input] {
+        output_row.push(true);
+      } else {
+        output_row.push(false);
+      }
+    }
+    output_image.push(output_row);
+  }
+
+  output_image
+}
+
+
+fn count_sea_serpents(image: &Vec<Vec<bool>>) -> usize {
+  let mut sea_serpent: Vec<Vec<bool>> = Vec::new();
+  for line in &["                  # ",
+                "#    ##    ##    ###",
+                " #  #  #  #  #  #   "]
+  {
+    let mut sea_serpent_row = Vec::new();
+    for c in line.chars() {
+      sea_serpent_row.push(c == '#');
+    }
+    sea_serpent.push(sea_serpent_row);
+  }
+  let sea_serpent_height = sea_serpent.len();
+  let sea_serpent_width = sea_serpent[0].len();
+  let image_size = image.len();
+
+  let mut sea_serpent_map: Vec<Vec<bool>> = Vec::new();
+  for _ in 0 .. image_size {
+    let mut map_row = Vec::new();
+    for _ in 0 .. image_size {
+      map_row.push(false);
+    }
+    sea_serpent_map.push(map_row);
+  }
+
+  for edge_at_top in &[Edge::Top, Edge::Right, Edge::Bottom, Edge::Left] {
+    for is_flipped in &[false, true] {
+      let orientation = Orientation {
+        edge_at_top: *edge_at_top,
+        is_flipped: *is_flipped,
+      };
+
+      let transformed_image = transform_image(image, orientation);
+      let mut sea_serpent_count = 0;
+
+      for image_y in 0 .. image_size - sea_serpent_height + 1 {
+        for image_x in 0 .. image_size - sea_serpent_width + 1 {
+          let mut possible_sea_serpent = true;
+
+          for sea_serpent_y in 0 .. sea_serpent_height {
+            for sea_serpent_x in 0 .. sea_serpent_width {
+              if sea_serpent[sea_serpent_y][sea_serpent_x] {
+                let cell_x = image_x + sea_serpent_x;
+                let cell_y = image_y + sea_serpent_y;
+                let cell = transformed_image[cell_y][cell_x];
+                if !cell {
+                  possible_sea_serpent = false;
+                  break;
+                }
+              }
+            }
+            if !possible_sea_serpent {
+              break;
+            }
+          }
+
+          if possible_sea_serpent {
+            sea_serpent_count += 1;
+
+            for sea_serpent_y in 0 .. sea_serpent_height {
+              for sea_serpent_x in 0 .. sea_serpent_width {
+                if sea_serpent[sea_serpent_y][sea_serpent_x] {
+                  let map_x = image_x + sea_serpent_x;
+                  let map_y = image_y + sea_serpent_y;
+                  sea_serpent_map[map_y][map_x] = true;
+                }
+              }
+            }
+          }
+        }
+      }
+
+      if sea_serpent_count > 0 {
+        let mut roughness = 0;
+        for y in 0 .. image_size {
+          for x in 0 .. image_size {
+            if transformed_image[y][x] && !sea_serpent_map[y][x] {
+              roughness += 1;
+            }
+          }
+        }
+
+        return roughness;
+      }
+    }
+  }
+
+  0
+}
+
diff --git a/20/tests/main.rs b/20/tests/main.rs
new file mode 100644
index 0000000..feb9fe3
--- /dev/null
+++ b/20/tests/main.rs
@@ -0,0 +1,14 @@
+use assert_cmd::prelude::*;
+use std::process::Command;
+
+
+#[test]
+fn personal_input() -> Result<(), Box<dyn std::error::Error>> {
+  let mut command = Command::cargo_bin("advent_20")?;
+
+  command.arg("input");
+  command.assert().success().stdout("29125888761511\n2219\n");
+
+  Ok(())
+}
+
diff --git a/Cargo.lock b/Cargo.lock
index 6047458..1a72d14 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -157,6 +157,14 @@ dependencies = [
 ]
 
 [[package]]
+name = "advent_20"
+version = "0.1.0"
+dependencies = [
+ "advent_lib",
+ "assert_cmd",
+]
+
+[[package]]
 name = "advent_lib"
 version = "0.1.0"
 dependencies = [
diff --git a/Cargo.toml b/Cargo.toml
index 8e0da29..154b354 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -20,4 +20,5 @@ members = [
   "17",
   "18",
   "19",
+  "20",
 ]