;; The first three lines of this file were inserted by DrScheme. They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "htdp-intermediate-reader.ss" "lang")((modname 18.1.14-3) (read-case-sensitive #t) (teachpacks ((lib "draw.ss" "teachpack" "htdp"))) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ((lib "draw.ss" "teachpack" "htdp")))))
;A web-page (wp) is a structure:
;(make-wp header doc) where
;header is a symbol and doc is
;a web-document.
;
;A web-document (doc) is either
;1. empty,
;2. (cons sym doc) where sym is a symbol and
;doc is a web-document, or
;3. (cons wp doc) where wp is a webpage and
;doc is a web-document.

(define-struct wp (header doc))

;find-for-wp : wp symbol -> list-of-symbols or false. Given a-wp and a-symbol, return the headers of the web-pages that are accessed on the way to locating a-symbol within documents within a-wp. Returns false if a-symbol never occurs.  If there are 2 or more occurrences of a-symbol, find-for-wp returns a list-of-headers from only a single  occurrence by following the path that gives preference to documents before embedded web pages.

(define (find-for-wp a-wp a-symbol) 
  (cond 
    [(or (cons? (find-for-doc (wp-doc a-wp) a-symbol))
         (empty? (find-for-doc (wp-doc a-wp) a-symbol))) (cons (wp-header a-wp) (find-for-doc (wp-doc a-wp) a-symbol))]
    [else false]))

;find-for-doc : doc symbol -> list-of-symbols or false
;Given a-doc and a-symbol, return the headers of the web-pages that are accessed on the way to locating a-symbol within documents within a-doc (a-doc inclusive). Returns false if a-symbol never occurs. Return empty if a-doc contains a-symbol.  If there are 2 or more occurrences of a-symbol, find-for-doc returns the list-of-headers from only a single  occurrence by following the path that gives preference to documents before embedded web pages.

(define (find-for-doc a-doc a-symbol)
  (cond
    [(empty? a-doc) false]
    [else (local ((define firs )
                  (define res ))
            (cond    
              [(symbol? (first a-doc))
               (cond
                 [(symbol=? a-symbol (first a-doc)) empty]
                 [else (find-for-doc (rest a-doc) a-symbol)])]
              [(or (cons? (find-for-doc (rest a-doc) a-symbol))
                   (empty? (find-for-doc (rest a-doc) a-symbol))) (find-for-doc (rest a-doc) a-symbol)]
              [(or (cons? (find-for-wp (first a-doc) a-symbol))
                   (empty? (find-for-wp (first a-doc) a-symbol))) (find-for-wp (first a-doc) a-symbol)]))]))



(define DOC7 (cons 'ImADoc empty))
(define DOC6 (cons 'ImADoc empty))
(define WP3 (make-wp 'ImAPage empty))
(define DOC5 (cons WP3 DOC6))
(define WP2 (make-wp 'AmIAPageToo? DOC5))
(define DOC4 (cons 'MaybeADoc DOC7))
(define DOC3 (cons WP2 DOC4))
(define DOC2 (cons 'ADoctor? DOC3))
(define DOC1 (cons 'OrADocument? DOC2))
(define WP1 (make-wp 'NoAWebPage DOC1))

(find-for-wp WP1 'ImADoc)

;find : wp symbol -> list-of-symbols or false.
;Given a-wp and a-symbol, return the headers of the web-pages that are accessed on the way to locating a-symbol within documents within a-wp. Returns false if a-symbol never occurs.  If there are 2 or more occurrences of a-symbol, find-for-wp returns a list-of-headers from only a single  occurrence by following the path that gives preference to documents before embedded web pages.

(define (find a-wp a-symbol)
  (local 
    ((define (find-for-wp a-wp a-symbol)
       (local ((define r (find-for-doc (wp-doc a-wp) a-symbol)))
         (cond
           [(or (cons? r)
                (empty? r)) (cons (wp-header a-wp) r)]
           [else false])))
     
     (define (find-for-doc a-doc a-symbol)
       (local ((define firs (find-for-wp (first a-doc) a-symbol))
               (define res (find-for-doc (rest a-doc) a-symbol)))
         (cond
           [(empty? a-doc) false]
           [(symbol? (first a-doc))
            (cond
              [(symbol=? a-symbol (first a-doc)) empty]
              [else res])]
           [(wp? (first a-doc))
            (cond
              [(or (cons? res)
                   (empty? res)) res]
              [(or (cons? firs)
                   (empty? firs)) firs])]))))
    (find-for-wp a-wp a-symbol)))