Add scanf feature to scheme-lib

Scanf is a common input and output function for C++/C, and I implemented scheme's scanf, requesting to be added to scheme-lib

Author: FrankYu

docs/第三章 输入输出/3.1 输入输出.md

@@ -1,9 +1,16 @@
 # 1 输入
 
+读取一个输入
 ```scheme
     (read)
 ```
 
+读取一行输入，按格式标记符分配到变量中，和C++的scanf功能类似，目前支持~s 和~d标记
+```scheme
+    (define h 0)
+    (scanf "/~s:" h)
+```
+
 # 2 输出
 
 ```scheme

packages/io/scanf.ss

@@ -0,0 +1,31 @@
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;Copyright 2016-2080 evilbinary.
+;;作者:evilbinary on 12/24/16.
+;;邮箱:rootdebug@163.com
+;;其中 scanf 由 yupengkun 友情贡献
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+(library (io scanf)
+  (export
+    scanf)
+  (import (scheme))
+    (define (scanf fmt . var)
+    (define (sf-f c str)
+      (do ([n 0 (+ 1 n)]) ((equal? c (string-ref str n)) n)))
+    (define c (list #\d string->number #\s eval))
+    (set! fmt (string-append fmt "\r"))
+    (let foo ([i 0] [tar (get-line (current-input-port))])
+      (when (<= (+ i 3) (string-length fmt))
+        (let ([f (string-ref fmt i)]
+              [m (string-ref fmt (+ i 1))]
+              [l (string-ref fmt (+ i 2))])
+          (cond
+            [(and (eq? f #\~) (or (eq? m #\d) (eq? m #\s)))
+            (set! temp (+ (sf-f l tar) 1))
+            (set-box!
+              (car var)
+              ((cadr (memv m c)) (substring tar 0 (- temp 1))))
+            (set! var (cdr var))
+            (foo (+ i 3) (substring tar temp (string-length tar)))]
+            [else
+            (foo (+ 1 i) (substring tar 1 (string-length tar)))])))))
+)