AoC 2020.1 p1

1 files changed, 65 insertions, 0 deletions

diff --git a/report-repair/rr.scm b/report-repair/rr.scm
new file mode 100644
index 0000000..b327d3c
--- /dev/null
+++ b/report-repair/rr.scm
@@ -0,0 +1,65 @@
+#!/run/current-system/profile/bin/guile \
+-e main -s
+!#
+(define-module (rr)
+  #:use-module (srfi srfi-1)
+  #:use-module (ice-9 exceptions)
+  #:export (rr
+	    multiply-pair
+	    return-equivalent-pair
+	    equivalent-pair?
+	    return-friendship-pair
+	    report->pairs))
+
+
+(define (rr str)
+  (multiply-pair
+   (return-equivalent-pair
+    (report->pairs str))))
+
+(define (multiply-pair pair)
+  (* (car pair)
+     (cdr pair)))
+
+(define (return-equivalent-pair pairs)
+  "Given PAIRS friendship pairs, returns the
+ones that are found to be friends. Throws
+friendship-exception if no friends are found.
+
+We do NOT care if there are multiple pairs.
+Return the first one."
+  (let loop ((pair (car pairs)) (pairs (cdr pairs)))
+    (if (null? pairs)
+	(raise-exception
+	 (make-exception-with-message
+	  "Concerning lack of friendship!"))
+	(if (equivalent-pair? pair pairs)
+	    pair
+	    (loop (car pairs) (cdr pairs))))))
+
+(define (equivalent-pair? pair pairs)
+  "Given friendship pair PAIR and a list
+of other PAIRS, determines if PAIR is
+contained in PAIRS, irrespective of order"
+  (let ((r-pair (cons (cdr pair)
+		      (car pair))))
+    (any (lambda (pair)
+	   (equal? r-pair pair))
+	 pairs)))
+
+(define (report->pairs str)
+  "Given a report, convert it to a
+list of friendship pairs."
+  (let ((lst (string-split
+	      (string-trim-both str char-set:whitespace)
+	      char-set:whitespace)))
+    (map (lambda (str)
+	   (return-friendship-pair
+	    (string->number str)))
+	 lst)))
+
+(define (return-friendship-pair num)
+  "Given NUM, returns a cons pair with
+CAR as the original number and CDR as
+the companion number."
+  (cons num (- 2020 num)))