AoC 2020.2 p1

1 files changed, 43 insertions, 0 deletions

diff --git a/password-philosophy/pp.scm b/password-philosophy/pp.scm
new file mode 100644
index 0000000..9431ffa
--- /dev/null
+++ b/password-philosophy/pp.scm
@@ -0,0 +1,43 @@
+(define-module (pp)
+  #:use-module (srfi srfi-1)
+  #:export (pp
+	    parse-line
+	    count-occurrences
+	    valid-password?))
+
+(define (pp str)
+  (count (lambda (x) x)
+	 (map (lambda (line)
+		(let ((parsed (parse-line line)))
+		  (valid-password? (car parsed)
+				   (cadr parsed)
+				   (caddr parsed))))
+	      (string-split
+	       (string-trim-both str char-set:whitespace)
+	       #\newline))))
+
+(define (parse-line str)
+  "Parses a line of the puzzle format
+into a workable datastructure:
+(passwd (char (min max)))"
+  (let* ((fields (string-split str char-set:whitespace))
+	 (range (map string->number (string-split (car fields) #\-)))
+	 (char (string-ref (cadr fields) 0))
+	 (passwd (caddr fields)))
+    (list passwd
+	  char
+	  (cons (car range)
+		(cadr range)))))
+
+(define (valid-password? passwd char range)
+  "Given PASSWD, checks that CHAR appears
+within RANGE number of times. RANGE is a
+cons pair."
+  (let ((occurrences (count-occurrences passwd char)))
+    (and (>= occurrences (car range))
+	 (>= (cdr range) occurrences))))
+
+(define (count-occurrences str char)
+  "Returns the number of times CHR
+appears in STR."
+  (string-length (string-filter char str)))