(define char-downcase*
  (lambda (ch)
    (if (char? ch)
        (char-downcase ch)
        ch)))

(define port->reader
  (lambda (port)
    (list (lambda () (read-char port))
          eof-object?)))

(define reader/pushback-buffer
  (lambda (read-function eof?)
    (let ((pushback-buffer '()))
      (letrec ((reader
                (lambda ()
                  (if (null? pushback-buffer)
                      (read-function)
                      (let ((ch (car pushback-buffer)))
                        (set! pushback-buffer (cdr pushback-buffer))
                        ch))))
               (pushback
                (lambda (ch)
                  (if (not (eof? ch))
                      (set! pushback-buffer (cons ch pushback-buffer))))))
        (list pushback reader eof?)))))

(define reader-words
  (lambda (part-of-a-word? pushback reader eof?)
    (letrec ((delimiter?
              (lambda (ch)
                (or (eof? ch)
                    (not (part-of-a-word? ch))
                    (eqv? ch #\space))))
             (read-until-delimiter
              (lambda (lst)
                (let ((ch (reader)))
                  (if (delimiter? ch)
                      (begin
                        (pushback ch)
                        (list->string (reverse lst)))
                      (read-until-delimiter (cons ch lst))))))
             (reader-wrapper
              (lambda ()
                (let ((ch (reader)))
                  (cond
                    ((not (char? ch)) #f)
                    ((char-whitespace? ch) (reader-wrapper))
                    (else (read-until-delimiter (list ch))))))))
      (list reader-wrapper))))

(define port->reader-words
  (lambda (port)
    (apply reader-words
           char-alphabetic?
           (apply reader/pushback-buffer
                  (port->reader port)))))

(define reader-pairs
  (lambda (reader)
    (let ((previous-word #f))
      (lambda ()
        (if (not previous-word)
            (set! previous-word (reader)))
        (if previous-word
            (let ((next-word (reader))
                  (previous-word* previous-word))
              (if next-word
                  (begin
                    (set! previous-word next-word)
                    (cons previous-word* next-word))
                  #f))
            #f)))))

(define port->reader-of-pairs
  (lambda (port)
    (apply reader-pairs (port->reader-words port))))

(define dump-pairs
  (lambda (file)
    (let ((port (open-input-file file)))
      (let ((reader (port->reader-of-pairs port)))
        (letrec ((loop
                  (lambda ()
                    (let ((pair (reader)))
                      (if pair
                          (begin
                            (display pair)
                            (loop)))))))
          (loop)
          (newline))))))

(define collect-occurences
  (lambda (file)
    (let ((port (open-input-file file))
          (table (make-hash-table)))
      (let ((reader (port->reader-of-pairs port)))
        (letrec ((handle
                  (lambda (first next)
                    (hash-table-update!
                     table
                     first
                     (lambda (next-table)
                       (hash-table-update!/default
                        next-table
                        next
                        (lambda (count) (+ count 1))
                        1)
                       next-table)
                     (lambda () (make-hash-table)))
                    (looper)))
                 (looper
                  (lambda ()
                    (let ((pair (reader)))
                      (if pair
                          (handle (car pair) (cdr pair)))))))
          (looper)
          (close-input-port port)
          table)))))

(define display-occurences
  (lambda (table)
    (hash-table-walk table
                     (lambda (first next-table)
                       (hash-table-walk next-table
                                        (lambda (next count)
                                          (display (list first next count))))))
    (newline)))

(define occurence-table->probabilities
  (lambda (next-table)
    (let ((total (hash-table-fold next-table
                                  (lambda (_ val total)
                                    (+ val total))
                                  0)))
      (let ((acc
             (hash-table-fold next-table
                              (lambda (key val acc)
                                (let ((summed-probability (car acc))
                                      (alist (cdr acc)))
                                  (let ((summed-probability
                                         (+ summed-probability
                                            (/ val total))))
                                    (cons summed-probability
                                          (cons (cons key summed-probability)
                                                alist)))))
                              (cons 0 '()))))
        (reverse (cdr acc))))))

(define occurences->probabilities
  (lambda (table)
    (let ((probability-table
           (hash-table-fold table
                            (lambda (name next-table new-table)
                              (hash-table-set! new-table
                                               name
                                               (occurence-table->probabilities
                                                next-table))
                              new-table)
                            (make-hash-table))))
      probability-table)))

(define dump-probabilities
  (lambda (table)
    (hash-table-walk table
                     (lambda (name probs)
                       (display (list name probs))))
    (newline)))

;;; Lehmer random number generator
(define random
  (let ((m 65537)
        (a 75)
        (previous 10000))
    (lambda ()
      (set! previous (remainder (* a previous)
                                m))
      (/ previous (- m 1)))))

(define probability-list->guess
  (lambda (lst normalized-random-number)
    (if (null? lst)
        (error (list "bad number or list" lst normalized-random-number))
        (let ((name (caar lst))
              (sum-prob (cdar lst)))
          (if (< sum-prob normalized-random-number)
              (probability-list->guess (cdr lst)
                                       normalized-random-number)
              name)))))

(define generate-next-word
  (lambda (probabilities previous-word)
    (probability-list->guess (hash-table-ref probabilities
                                             previous-word)
                             (random))))

(define make-table-from-file
  (lambda (filename)
    (occurences->probabilities
     (collect-occurences filename))))

(define guess-sentence
  (lambda (start-word table number)
    (letrec ((collector
              (lambda (start-word number collected)
                (if (<= number 0)
                    (reverse collected)
                    (let ((name (generate-next-word table start-word)))
                      (collector name (- number 1) (cons name
                                                         collected)))))))
      (collector start-word number (cons start-word '())))))