用lisp来让计算机学会写作

        大部分的代码、思路参考了《Ansi Common Lisp》P138~P141。

问题：给一篇英文文本，如何让计算机依据此文本而生成随机但可读的文本。如：

|Venture|

  The National Venture Capital Association estimates that wealth associated with a deal a big spending by regulations that will spend one another's main reason these projects . 

这是计算机学习了Paul Graham的一些文章后生成的随机文本。它根据Venture这个单词向两边延伸成一个句子。令人惊喜的是，文本常常是可读的。

  算法：记录每个单词后面出现的单词以及出现的次数，如I leave在原文中出现了5次，I want出现了3次，除此之外，其它地方没有出现过I，所以在生成随机文章的时候，当遇到I，有5/8的概率选择leave为下一个单词。假如选择了leave的话，则看看leave后面出现过哪些单词，重复以上过程。

现用lisp来解决问题。

lisp里的符号类型，即symbol，可以很好记录各种字符串还有标点符号，所以采用它来记录。采用内附的hashtable来建立列表：

        (defparameter *words* (make-hash-table :size 10000))

那如何建立列表呢？

(let ((prev '|.|))
  (defun see (sym)
    (let ((pair (assoc sym (gethash prev *words*))))
      (if pair
	  (incf (cdr pair))
	  (push (cons sym 1) (gethash prev *words*))))
    (setf prev sym)))

以当前单词为keyword，以assoc-list关系列表为该keyword下的值。

        如I下有( (|leave| . 5) (|want| . 3) )。没有单词word的话，则push入(word . 1)。

        如何随机选一个词呢？

(defun random-word (word ht)
  (let* ((choices (gethash word ht))
	 (x (random (reduce #'+ choices :key #'cdr))))
    (dolist (pair choices)
      (decf x (cdr pair))
      (if (minusp x)
	  (return (car pair))))))

        这里巧妙用了reduce函数。

        现在再来思考，如何将给定一个词向两侧延伸成一句话呢？

1）先将文本反向，得到一个反向的列表，也即I leave,I want变成leave I,want I。

2）将hashtable反向，得到另外一个hashtable，以后一个单词为关键字，前面可能出现的单词及次数构成assoc-list。

3）碰运气，从一个标点开始延续文章，直到出现给定单词为止。

        我用了第二个方法：

(defparameter *r-words* (make-hash-table :size 10000))

(defun push-words (w1 w2 n)
  (push (cons w2 n) (gethash w1 *r-words*)))

(defun get-reversed-words ();a cat -> cat a
  (maphash #'(lambda (k lst)
	       (dolist (pair lst)
		 (push-words (car pair) k (cdr pair))))
	   *words*))

         遍历原来的hashtable，再把每一对单词先后换个位置插入另外一个hashtable。

给出双向延伸句子的自动生成文本代码：

(defparameter *words* (make-hash-table :size 10000))
(defconstant maxword 100)
(defparameter nwords 0)
(defconstant debug nil)
(let ((prev '|.|))
  (defun see (sym)
    (incf nwords)
    (let ((pair (assoc sym (gethash prev *words*))))
      (if pair
	  (incf (cdr pair))
	  (push (cons sym 1) (gethash prev *words*))))
    (setf prev sym)))

(defun check-punc (c);char to symbol
  (case c
    (#\. '|.|) (#\, '|,|)
    (#\; '|;|) (#\? '|?|)
    (#\: '|:|) (#\! '|!|)))

(defun read-text (pathname)
  (with-open-file (str pathname :direction :input)
    (let ((buf (make-string maxword))
	  (pos 0))
      (do ((c (read-char str nil 'eof)
	      (read-char str nil 'eof)))
	  ((eql c 'eof))
	(if (or (alpha-char-p c)
		(eql c #\'))
	    (progn
	      (setf (char buf pos) c)
	      (incf pos))
	    (progn
	      (unless (zerop pos)
		(see (intern (subseq buf 0 pos)))
		(setf pos 0))
	      (let ((punc (check-punc c)))
		(if punc
		    (see punc)))))))))

(defun print-ht (ht)
  (maphash #'(lambda (k v)
		(format t "~A ~A~%" k v))
	     ht))

(defparameter *r-words* (make-hash-table :size 10000))

(defun push-words (w1 w2 n)
  (push (cons w2 n) (gethash w1 *r-words*)))

(defun get-reversed-words ();a cat -> cat a
  (maphash #'(lambda (k lst)
	       (dolist (pair lst)
		 (push-words (car pair) k (cdr pair))))
	   *words*))

(defun print-a-word (word ht)
  (maphash #'(lambda (k lst)
	       (if (eql k word)
		   (format t "~A ~A~%" k lst)))
	   ht))

(if debug
    (print-a-word '|leave| *r-words*))

(defun punc-p (sym);symbol to char,nil when fails.
  (check-punc (char (symbol-name sym) 0)))

(defun random-word (word ht)
  (let* ((choices (gethash word ht))
	 (x (random (reduce #'+ choices :key #'cdr))))
    (dolist (pair choices)
      (decf x (cdr pair))
      (if (minusp x)
	  (return (car pair))))))

(defun gen-former (word str)
  (let ((last (random-word word *r-words*)))
    (if (not (punc-p last))
	(progn 
	  (gen-former last str)
	  (format str "~A " last)))))

(defun gen-latter (word str)
  (let ((next (random-word word *words*)))
    (format str "~A " next)
    (if (not (punc-p next))
        (gen-latter next str))))

;(gen-latter '|leave| t)

(defun get-a-word (ht);get a random word
  (let ((x (random nwords)))
    (maphash #'(lambda (k v)
		 (dolist (pair v)
		   (decf x (cdr pair))
		   (if (minusp x)
		       (return-from get-a-word (car pair)))))
	     ht)))
;(get-a-word *words*)
(defun gen-sentence (word str)
  (gen-former word str)
  (format str "~A " word)
  (gen-latter word str))

(defun test ()
  (setf nwords 0)
  (read-text "essay.txt")
  (get-reversed-words)
  (let ((word (get-a-word *words*)))
    (print word)
    (gen-sentence word t)))
(test)

     文本语料库、lisp源代码见： Here