diff --git a/2017/d21/main.rkt b/2017/d21/main.rkt
new file mode 100644
index 0000000..f51c80e
--- /dev/null
+++ b/2017/d21/main.rkt
@@ -0,0 +1,51 @@
+#lang br/quicklang
+(require "../helper.rkt" racket/sequence)
+(provide read-syntax (rename-out [#%mb #%module-begin]) ★ ★★)
+
+(define (read-syntax path port)
+  (define lines (port->lines port))
+  (strip-context #`(module mod "main.rkt"
+                     #,(for/list ([datum (in-port read (open-input-string (car lines)))])
+                         datum)
+                     #,@(for/list ([line (in-list (cdr lines))])
+                          (for/list ([datum (string-split line)])
+                            datum)))))
+
+(define-macro (#%mb (STARS ITERATIONS) (LH "=>" RH) ...)
+  #`(#%module-begin
+     (time (STARS ITERATIONS '(LH . RH) ...))))
+
+(define (charify str) (map string->list (string-split str "/")))
+
+(define (grid-size grid) (length (car grid)))
+
+(define (grid-split grid size)
+  (append*
+   (for/list ([rowset (in-slice size grid)])
+     (apply map list (map (λ (row) (slice-at row size)) rowset)))))
+
+(define (enhance-grid grid rules)
+  (let/ec exit
+    (for*/fold ([rotations (list grid)])
+               ([i (in-range 4)])
+      (define rot (apply map list (reverse (car rotations))))
+      (cond
+        [(hash-ref rules rot (λ () (hash-ref rules (map reverse rot) #f))) => exit]
+        [else (cons rot rotations)]))))
+
+(define (join-grids grids)
+  (define side (sqrt (length grids)))
+  (append*
+   (for/list ([gridset (in-slice side grids)])
+     (apply map append gridset))))
+
+(define (★ iterations . rulepairs)
+  (define rules (for/hash ([rp (in-list rulepairs)])
+                  (values (charify (car rp)) (charify (cdr rp)))))
+  (for/fold ([grid (charify ".#./..#/###")]
+             #:result (count (λ (c) (char=? c #\#)) (flatten grid)))
+            ([i (in-range iterations)])
+    (define grid-pieces (grid-split grid (if (even? (grid-size grid)) 2 3)))
+    (join-grids (map (λ (gp) (enhance-grid gp rules)) grid-pieces))))
+
+(define ★★ ★)
\ No newline at end of file
diff --git a/2017/d21/star1.rkt b/2017/d21/star1.rkt
new file mode 100644
index 0000000..396f657
--- /dev/null
+++ b/2017/d21/star1.rkt
@@ -0,0 +1,109 @@
+#lang reader "main.rkt" ★ 5 ; 190
+../.. => #.#/###/#.#
+#./.. => ..#/.../###
+##/.. => .../.##/###
+.#/#. => #.#/##./.#.
+##/#. => #../#.#/..#
+##/## => #.#/#../###
+.../.../... => .###/..##/.#../###.
+#../.../... => ##.#/.###/#.../##.#
+.#./.../... => ..../#.##/..../.#.#
+##./.../... => ..#./#.../#.../.###
+#.#/.../... => ..##/####/#.#./..##
+###/.../... => .##./#.#./###./.#..
+.#./#../... => #..#/..#./...#/#.#.
+##./#../... => ##../.##./##.#/#..#
+..#/#../... => #.##/.##./##.#/.###
+#.#/#../... => ...#/#.##/..#./##..
+.##/#../... => #.#./..#./##.#/.#.#
+###/#../... => #..#/...#/..#./##.#
+.../.#./... => .#.#/#.../.##./.#.#
+#../.#./... => #.#./.##./..../.#.#
+.#./.#./... => .#../#.##/..##/..##
+##./.#./... => ..##/##.#/...#/..#.
+#.#/.#./... => ...#/.##./####/.#..
+###/.#./... => ###./####/...#/####
+.#./##./... => ...#/.#.#/#.#./#.#.
+##./##./... => ..../...#/#.#./...#
+..#/##./... => .##./#.../##.#/.#..
+#.#/##./... => .#../.#../...#/....
+.##/##./... => ..#./.##./####/.#..
+###/##./... => ##../.#.#/##../.#..
+.../#.#/... => ..#./.#../.#../.###
+#../#.#/... => ####/..../#..#/#...
+.#./#.#/... => #.##/##../##.#/##.#
+##./#.#/... => ###./..../#.##/###.
+#.#/#.#/... => ###./.#../#.#./#.#.
+###/#.#/... => ...#/#..#/#.#./..##
+.../###/... => .#../...#/...#/....
+#../###/... => ####/#.../##../.#.#
+.#./###/... => ...#/####/.##./#..#
+##./###/... => .###/##.#/..#./.#..
+#.#/###/... => ####/###./.###/.###
+###/###/... => .#.#/..##/..#./##..
+..#/.../#.. => #.../.###/###./...#
+#.#/.../#.. => ..../#.../##../..#.
+.##/.../#.. => ####/####/...#/####
+###/.../#.. => #.../.#../#.#./#.#.
+.##/#../#.. => ##../..#./.#../##.#
+###/#../#.. => ..../#..#/.###/.###
+..#/.#./#.. => ...#/##../.##./##..
+#.#/.#./#.. => #.##/.###/#.#./##.#
+.##/.#./#.. => ..../..../.#.#/#..#
+###/.#./#.. => ##../.#.#/.#.#/####
+.##/##./#.. => #.##/##.#/####/....
+###/##./#.. => ..../..##/#.#./.###
+#../..#/#.. => #.#./...#/#.##/.###
+.#./..#/#.. => ####/#.##/.#../.###
+##./..#/#.. => .##./..#./.#.#/##.#
+#.#/..#/#.. => .#.#/#.##/##../#...
+.##/..#/#.. => ..../.###/####/.#..
+###/..#/#.. => ##.#/##.#/..##/.#..
+#../#.#/#.. => #.##/###./##../....
+.#./#.#/#.. => ..../###./####/###.
+##./#.#/#.. => ##.#/#.##/##../#.##
+..#/#.#/#.. => .###/#.../.#../##..
+#.#/#.#/#.. => ##../##.#/#.../.##.
+.##/#.#/#.. => ...#/..#./.###/##.#
+###/#.#/#.. => #.../#.##/..##/..##
+#../.##/#.. => #.##/#.../.##./##..
+.#./.##/#.. => #.#./#.../..##/.#..
+##./.##/#.. => .###/.#.#/####/.#.#
+#.#/.##/#.. => ####/.#../##.#/.###
+.##/.##/#.. => .#../##.#/####/#.#.
+###/.##/#.. => #.##/#.../...#/....
+#../###/#.. => ###./.#.#/##../#..#
+.#./###/#.. => #..#/..##/..../....
+##./###/#.. => ..#./#.../...#/###.
+..#/###/#.. => ##../..##/##../#.##
+#.#/###/#.. => ..../..#./.###/##..
+.##/###/#.. => #..#/####/.#.#/.##.
+###/###/#.. => ###./#.##/##.#/.#..
+.#./#.#/.#. => #.../####/#.#./.##.
+##./#.#/.#. => ..##/..../.#.#/##..
+#.#/#.#/.#. => ####/..##/####/#...
+###/#.#/.#. => ##.#/#.#./.##./####
+.#./###/.#. => .#.#/.#.#/##.#/###.
+##./###/.#. => .#../###./#.##/#...
+#.#/###/.#. => #.../.###/.#../.#..
+###/###/.#. => #.#./.##./.###/####
+#.#/..#/##. => .#../#..#/###./#.##
+###/..#/##. => #.#./####/###./###.
+.##/#.#/##. => .#.#/...#/..../#.##
+###/#.#/##. => ...#/..../#.##/####
+#.#/.##/##. => ##../.#../.#.#/##..
+###/.##/##. => #.../#.#./#.#./#.#.
+.##/###/##. => ..../#.##/#.##/..##
+###/###/##. => ####/##.#/#..#/.##.
+#.#/.../#.# => ##.#/.#.#/####/.##.
+###/.../#.# => #..#/.#.#/#.../#..#
+###/#../#.# => ..##/###./.###/..##
+#.#/.#./#.# => #.##/#.#./##../...#
+###/.#./#.# => ..#./.###/..##/#...
+###/##./#.# => #.../...#/..##/.###
+#.#/#.#/#.# => #..#/.#../...#/#..#
+###/#.#/#.# => ###./#.../##../.##.
+#.#/###/#.# => ...#/..#./...#/#..#
+###/###/#.# => ###./####/.###/###.
+###/#.#/### => .###/.#../..../##.#
+###/###/### => #..#/.###/##../.##.
\ No newline at end of file
diff --git a/2017/d21/star2.rkt b/2017/d21/star2.rkt
new file mode 100644
index 0000000..458b556
--- /dev/null
+++ b/2017/d21/star2.rkt
@@ -0,0 +1,109 @@
+#lang reader "main.rkt" ★★ 18 ; 2335049
+../.. => #.#/###/#.#
+#./.. => ..#/.../###
+##/.. => .../.##/###
+.#/#. => #.#/##./.#.
+##/#. => #../#.#/..#
+##/## => #.#/#../###
+.../.../... => .###/..##/.#../###.
+#../.../... => ##.#/.###/#.../##.#
+.#./.../... => ..../#.##/..../.#.#
+##./.../... => ..#./#.../#.../.###
+#.#/.../... => ..##/####/#.#./..##
+###/.../... => .##./#.#./###./.#..
+.#./#../... => #..#/..#./...#/#.#.
+##./#../... => ##../.##./##.#/#..#
+..#/#../... => #.##/.##./##.#/.###
+#.#/#../... => ...#/#.##/..#./##..
+.##/#../... => #.#./..#./##.#/.#.#
+###/#../... => #..#/...#/..#./##.#
+.../.#./... => .#.#/#.../.##./.#.#
+#../.#./... => #.#./.##./..../.#.#
+.#./.#./... => .#../#.##/..##/..##
+##./.#./... => ..##/##.#/...#/..#.
+#.#/.#./... => ...#/.##./####/.#..
+###/.#./... => ###./####/...#/####
+.#./##./... => ...#/.#.#/#.#./#.#.
+##./##./... => ..../...#/#.#./...#
+..#/##./... => .##./#.../##.#/.#..
+#.#/##./... => .#../.#../...#/....
+.##/##./... => ..#./.##./####/.#..
+###/##./... => ##../.#.#/##../.#..
+.../#.#/... => ..#./.#../.#../.###
+#../#.#/... => ####/..../#..#/#...
+.#./#.#/... => #.##/##../##.#/##.#
+##./#.#/... => ###./..../#.##/###.
+#.#/#.#/... => ###./.#../#.#./#.#.
+###/#.#/... => ...#/#..#/#.#./..##
+.../###/... => .#../...#/...#/....
+#../###/... => ####/#.../##../.#.#
+.#./###/... => ...#/####/.##./#..#
+##./###/... => .###/##.#/..#./.#..
+#.#/###/... => ####/###./.###/.###
+###/###/... => .#.#/..##/..#./##..
+..#/.../#.. => #.../.###/###./...#
+#.#/.../#.. => ..../#.../##../..#.
+.##/.../#.. => ####/####/...#/####
+###/.../#.. => #.../.#../#.#./#.#.
+.##/#../#.. => ##../..#./.#../##.#
+###/#../#.. => ..../#..#/.###/.###
+..#/.#./#.. => ...#/##../.##./##..
+#.#/.#./#.. => #.##/.###/#.#./##.#
+.##/.#./#.. => ..../..../.#.#/#..#
+###/.#./#.. => ##../.#.#/.#.#/####
+.##/##./#.. => #.##/##.#/####/....
+###/##./#.. => ..../..##/#.#./.###
+#../..#/#.. => #.#./...#/#.##/.###
+.#./..#/#.. => ####/#.##/.#../.###
+##./..#/#.. => .##./..#./.#.#/##.#
+#.#/..#/#.. => .#.#/#.##/##../#...
+.##/..#/#.. => ..../.###/####/.#..
+###/..#/#.. => ##.#/##.#/..##/.#..
+#../#.#/#.. => #.##/###./##../....
+.#./#.#/#.. => ..../###./####/###.
+##./#.#/#.. => ##.#/#.##/##../#.##
+..#/#.#/#.. => .###/#.../.#../##..
+#.#/#.#/#.. => ##../##.#/#.../.##.
+.##/#.#/#.. => ...#/..#./.###/##.#
+###/#.#/#.. => #.../#.##/..##/..##
+#../.##/#.. => #.##/#.../.##./##..
+.#./.##/#.. => #.#./#.../..##/.#..
+##./.##/#.. => .###/.#.#/####/.#.#
+#.#/.##/#.. => ####/.#../##.#/.###
+.##/.##/#.. => .#../##.#/####/#.#.
+###/.##/#.. => #.##/#.../...#/....
+#../###/#.. => ###./.#.#/##../#..#
+.#./###/#.. => #..#/..##/..../....
+##./###/#.. => ..#./#.../...#/###.
+..#/###/#.. => ##../..##/##../#.##
+#.#/###/#.. => ..../..#./.###/##..
+.##/###/#.. => #..#/####/.#.#/.##.
+###/###/#.. => ###./#.##/##.#/.#..
+.#./#.#/.#. => #.../####/#.#./.##.
+##./#.#/.#. => ..##/..../.#.#/##..
+#.#/#.#/.#. => ####/..##/####/#...
+###/#.#/.#. => ##.#/#.#./.##./####
+.#./###/.#. => .#.#/.#.#/##.#/###.
+##./###/.#. => .#../###./#.##/#...
+#.#/###/.#. => #.../.###/.#../.#..
+###/###/.#. => #.#./.##./.###/####
+#.#/..#/##. => .#../#..#/###./#.##
+###/..#/##. => #.#./####/###./###.
+.##/#.#/##. => .#.#/...#/..../#.##
+###/#.#/##. => ...#/..../#.##/####
+#.#/.##/##. => ##../.#../.#.#/##..
+###/.##/##. => #.../#.#./#.#./#.#.
+.##/###/##. => ..../#.##/#.##/..##
+###/###/##. => ####/##.#/#..#/.##.
+#.#/.../#.# => ##.#/.#.#/####/.##.
+###/.../#.# => #..#/.#.#/#.../#..#
+###/#../#.# => ..##/###./.###/..##
+#.#/.#./#.# => #.##/#.#./##../...#
+###/.#./#.# => ..#./.###/..##/#...
+###/##./#.# => #.../...#/..##/.###
+#.#/#.#/#.# => #..#/.#../...#/#..#
+###/#.#/#.# => ###./#.../##../.##.
+#.#/###/#.# => ...#/..#./...#/#..#
+###/###/#.# => ###./####/.###/###.
+###/#.#/### => .###/.#../..../##.#
+###/###/### => #..#/.###/##../.##.
\ No newline at end of file
diff --git a/2017/d21/test1.rkt b/2017/d21/test1.rkt
new file mode 100644
index 0000000..1ecd487
--- /dev/null
+++ b/2017/d21/test1.rkt
@@ -0,0 +1,3 @@
+#lang reader "main.rkt" ★ 2 ; 12
+../.# => ##./#../...
+.#./..#/### => #..#/..../..../#..#
\ No newline at end of file