1 files changed, 76 insertions, 0 deletions
diff --git a/15/2.rkt b/15/2.rkt
new file mode 100644
index 0000000..1825a65
--- /dev/null
+++ b/15/2.rkt
@@ -0,0 +1,76 @@
+#lang racket
+
+(define line
+  (call-with-input-file "input"
+    (λ (fp) (read-line fp))))
+
+(define strs (string-split line ","))
+
+(define (parse-len str)
+  (define str-len (string-length str))
+  (define last-char (string-ref str (- str-len 1)))
+  (if (char=? #\- last-char)
+    (list (substring str 0 (- str-len 1)) #\-)
+    (let ()
+      (define tp (string-split str "="))
+      (list (car tp) #\= (string->number (cadr tp))))))
+
+(define (calc-hash str)
+  (define len (string-length str))
+  (let loop ((i 0) (cur 0))
+    (if (< i len)
+      (let ()
+        (define c (string-ref str i))
+        (define ascii (char->integer c))
+        (loop (+ i 1) (modulo (* 17 (+ cur ascii)) 256)))
+      cur)))
+
+(define (remove-len lst lable)
+  (let loop ((cur lst) (ret '()))
+    (if (null? cur)
+      (reverse ret)
+      (if (equal? (caar cur) lable)
+        (loop (cdr cur) ret)
+        (loop (cdr cur) (cons (car cur) ret))))))
+
+(define (insert-len lst lable value)
+  (let loop ((cur lst) (ret '()) (inserted #f))
+    (if (null? cur)
+      (if inserted
+        (reverse ret)
+        (reverse (cons (cons lable value) ret)))
+      (if (equal? (caar cur) lable)
+        (loop (cdr cur) (cons (cons lable value) ret) #t)
+        (loop (cdr cur) (cons (car cur) ret) inserted)))))
+
+(define boxes (make-vector 256 '()))
+
+(define lens (map parse-len strs))
+
+(define (process-len len box)
+  (if (char=? #\- (cadr len))
+    (remove-len box (car len))
+    (insert-len box (car len) (caddr len))))
+
+;; do lens installation
+(let loop ((cur lens))
+  (if (null? cur)
+    (void)
+    (let ()
+      (define lable (caar cur))
+      (define hash-num (calc-hash lable))
+      (vector-set! boxes hash-num (process-len (car cur) (vector-ref boxes hash-num)))
+      (loop (cdr cur)))))
+
+;; calculate focusing power
+(let loop ((i 1) (ret 0))
+  (if (> i 256)
+    ret
+    (let ()
+      (define box (vector-ref boxes (- i 1)))
+      (define sum
+        (let loop ((j 1) (cur box) (s 0))
+          (if (null? cur)
+            s
+            (loop (+ j 1) (cdr cur) (+ s (* i j (cdar cur)))))))
+      (loop (+ i 1) (+ ret sum)))))