1 files changed, 70 insertions, 0 deletions
diff --git a/09/lib.scm b/09/lib.scm
new file mode 100644
index 0000000..5eac027
--- /dev/null
+++ b/09/lib.scm
@@ -0,0 +1,70 @@
+(library (lib)
+  (export solve-part1 solve-part2)
+  (import (chezscheme))
+
+  ; Global state... a sin, but very convenient in this case!
+  (define chars '())     ; Characters still to be parsed
+  (define part1-total 0) ; Total score of groups in input
+  (define part2-total 0) ; Total non-escaped garbage chars
+
+  ; Is the next character in the stream equal to `c'?
+  (define (peek? c)
+    (char=? c (car chars)))
+
+  ; Blindly accept character and pop it. We do all detection
+  ; via `peek?', so we don't care what gets discarded here.
+  (define (next)
+    (set! chars (cdr chars)))
+
+  ; Parse a "group". We can assume all input is well-formed,
+  ; which really helps keep this code as simple as possible.
+  (define (parse-group depth)
+    (next) ; Accept initial #\{
+    (let loop ()
+      ; Expect start of "group" or "garbage"
+      (cond
+        ((peek? #\{)
+         (parse-group (+ depth 1)))
+        ((peek? #\<)
+         (parse-garbage)))
+      ; Expect comma or end of current "group"
+      (cond
+        ((peek? #\,)
+         (next)  ; Accept #\,
+         (loop)) ; Parse next "group"/"garbage"
+        ((peek? #\})
+         ; Add current group's nesting depth to total score
+         (set! part1-total (+ part1-total depth))
+         (next))))) ; Accept #\} and return
+
+  ; Parse "garbage": everything until next non-escaped #\>.
+  (define (parse-garbage)
+    (let loop ()
+      (next) ; Accept char (#\< at first)
+      (cond
+        ((peek? #\>)
+          (next)) ; Accept #\> and return
+        ((peek? #\!)
+          (next)  ; Accept #\!
+          (loop)) ; Continue to escaped char
+        (else
+          ; Add this character to the total count
+          (set! part2-total (+ part2-total 1))
+          (loop)))))
+
+  ; Reset global parser state and parse the input string
+  (define (solve-puzzle str)
+    (set! chars (string->list str))
+    (set! part1-total 0)
+    (set! part2-total 0)
+    (parse-group 1))
+
+  (define (solve-part1 str)
+    (solve-puzzle str)
+    part1-total)
+
+  (define (solve-part2 str)
+    (solve-puzzle str)
+    part2-total)
+
+)