diff --git a/2021/README.md b/2021/README.md
index f92b694..36ecdbb 100644
--- a/2021/README.md
+++ b/2021/README.md
@@ -2,33 +2,33 @@
## Progress
-| Day | Part 1 | Part 2 | Language |
-| :-: | :----: | :----: | :------- |
-| 01 | ✓ | ✓ | [Ruby] |
-| 02 | ✓ | ✓ | [Ada] |
-| 03 | ✓ | ✓ | [Perl] |
-| 04 | ✓ | ✓ | [Go] |
-| 05 | ✓ | ✓ | [Nim] |
-| 06 | ✓ | ✓ | [Inko] |
-| 07 | ✓ | ✓ | [Swift] |
-| 08 | ✓ | ✓ | [C#] |
-| 09 | ✓ | ✓ | [C] |
-| 10 | ✓ | ✓ | [OCaml] |
-| 11 | | | |
-| 12 | | | |
-| 13 | | | |
-| 14 | | | |
-| 15 | | | |
-| 16 | | | |
-| 17 | | | |
-| 18 | | | |
-| 19 | | | |
-| 20 | | | |
-| 21 | | | |
-| 22 | | | |
-| 23 | | | |
-| 24 | | | |
-| 25 | | | |
+| Day | Part 1 | Part 2 | Language |
+| :-: | :----: | :----: | :-------- |
+| 01 | ✓ | ✓ | [Ruby] |
+| 02 | ✓ | ✓ | [Ada] |
+| 03 | ✓ | ✓ | [Perl] |
+| 04 | ✓ | ✓ | [Go] |
+| 05 | ✓ | ✓ | [Nim] |
+| 06 | ✓ | ✓ | [Inko] |
+| 07 | ✓ | ✓ | [Swift] |
+| 08 | ✓ | ✓ | [C#] |
+| 09 | ✓ | ✓ | [C] |
+| 10 | ✓ | ✓ | [OCaml] |
+| 11 | ✓ | ✓ | [Crystal] |
+| 12 | | | |
+| 13 | | | |
+| 14 | | | |
+| 15 | | | |
+| 16 | | | |
+| 17 | | | |
+| 18 | | | |
+| 19 | | | |
+| 20 | | | |
+| 21 | | | |
+| 22 | | | |
+| 23 | | | |
+| 24 | | | |
+| 25 | | | |
## Additional Challenges
@@ -46,3 +46,4 @@
[c#]: https://docs.microsoft.com/en-us/dotnet/csharp/
[c]: http://www.open-std.org/jtc1/sc22/wg14
[ocaml]: https://ocaml.org
+[crystal]: https://crystal-lang.org
diff --git a/2021/day-11/Justfile b/2021/day-11/Justfile
new file mode 100644
index 0000000..1b34677
--- /dev/null
+++ b/2021/day-11/Justfile
@@ -0,0 +1,6 @@
+@part PART INPUT_FILE="inputs/puzzle.txt":
+ crystal build --error-trace part_{{PART}}.cr
+ ./part_{{PART}} {{INPUT_FILE}}
+
+clean:
+ rm -f part_one part_two
diff --git a/2021/day-11/common.cr b/2021/day-11/common.cr
new file mode 100644
index 0000000..4a4b8b7
--- /dev/null
+++ b/2021/day-11/common.cr
@@ -0,0 +1,70 @@
+alias OctopusMap = Array(Array(Int32))
+alias FlashMap = Array(Array(Bool))
+
+SURROUNDING = [-1, 0, 1]
+ .repeated_permutations(2)
+ .reject { |v| v == [0, 0] }
+ .map { |v| Tuple(Int32, Int32).from(v) }
+ .to_set
+
+def parse_file(path : String) : OctopusMap
+ File.read_lines(path, chomp: true).map &.chars.map(&.to_i)
+end
+
+def print_map(octopi : OctopusMap)
+ puts "-" * octopi.first.size
+ octopi.each do |row|
+ puts row
+ end
+ puts "-" * octopi.first.size
+end
+
+def flash(x : Int32, y : Int32, octopi : OctopusMap, flashes : FlashMap) : Int32
+ flashes[y][x] = true
+ octopi[y][x] = 0
+
+ return 1 + mark_surrounding(x, y, octopi, flashes) do |nx, ny|
+ next 0 if flashes[ny][nx]
+ next flash(nx, ny, octopi, flashes) if octopi[ny][nx] > 9
+ 0
+ end
+end
+
+def mark_surrounding(x : Int32, y : Int32, octopi : OctopusMap, flashes : FlashMap) : Int32
+ valid_surrounding = SURROUNDING.reject do |(dx, dy)|
+ val = false
+ val ||= dx == -1 if x == 0
+ val ||= dx == 1 if x == octopi.first.size - 1
+ val ||= dy == -1 if y == 0
+ val ||= dy == 1 if y == octopi.size - 1
+ val
+ end
+
+ return valid_surrounding.sum do |(dx, dy)|
+ octopi[y + dy][x + dx] += 1 unless flashes[y + dy][x + dx]
+ yield x + dx, y + dy
+ end
+end
+
+def generation(octopi : OctopusMap, flashes : FlashMap) : Int32
+ gen_flash_count = 0
+
+ octopi.each_with_index do |row, y|
+ row.each_with_index do |_, x|
+ next if flashes[y][x]
+ octopi[y][x] += 1
+ gen_flash_count += flash(x, y, octopi, flashes) if octopi[y][x] > 9
+ end
+ end
+
+ gen_flash_count
+end
+
+def generations(steps : Int32, octopi : OctopusMap, flashes : FlashMap, flash_count : Int32) : Int32
+ return flash_count if steps == 0
+
+ flash_count += generation(octopi, flashes)
+
+ flashes = octopi.map &.map { false }
+ return generations(steps - 1, octopi, flashes, flash_count)
+end
diff --git a/2021/day-11/inputs/puzzle.txt b/2021/day-11/inputs/puzzle.txt
new file mode 100644
index 0000000..1a0fab9
--- /dev/null
+++ b/2021/day-11/inputs/puzzle.txt
@@ -0,0 +1,10 @@
+4134384626
+7114585257
+1582536488
+4865715538
+5733423513
+8532144181
+1288614583
+2248711141
+6415871681
+7881531438
diff --git a/2021/day-11/inputs/sample.txt b/2021/day-11/inputs/sample.txt
new file mode 100644
index 0000000..03743f6
--- /dev/null
+++ b/2021/day-11/inputs/sample.txt
@@ -0,0 +1,10 @@
+5483143223
+2745854711
+5264556173
+6141336146
+6357385478
+4167524645
+2176841721
+6882881134
+4846848554
+5283751526
diff --git a/2021/day-11/part_one.cr b/2021/day-11/part_one.cr
new file mode 100644
index 0000000..dd7a8c1
--- /dev/null
+++ b/2021/day-11/part_one.cr
@@ -0,0 +1,10 @@
+require "./common"
+
+def main(path)
+ octopi = parse_file(path)
+ flashes = octopi.map &.map { false }
+
+ puts generations(100, octopi, flashes, 0)
+end
+
+main(ARGV.first)
diff --git a/2021/day-11/part_two.cr b/2021/day-11/part_two.cr
new file mode 100644
index 0000000..b443e04
--- /dev/null
+++ b/2021/day-11/part_two.cr
@@ -0,0 +1,18 @@
+require "./common"
+
+def main(path)
+ octopi = parse_file(path)
+
+ req_flashes = octopi.size * octopi.first.size
+
+ (1..).each do |generation|
+ flashes = octopi.map &.map { false }
+ gen_flashes = generation(octopi, flashes)
+ if gen_flashes == req_flashes
+ puts generation
+ break
+ end
+ end
+end
+
+main(ARGV.first)
diff --git a/README.md b/README.md
index e188ad2..38b14bc 100644
--- a/README.md
+++ b/README.md
@@ -8,4 +8,4 @@
- [2018](2018/README.md) (0% completed)
- [2019](2019/README.md) (0% completed)
- [2020](2020/README.md) (16% completed)
-- [2021](2021/README.md) (40% completed)
+- [2021](2021/README.md) (44% completed)