2020-05-18-sps-am.rkt

#lang racket

(define last-str #f)


(define (b-after-a? tree)
  (match tree
    [(? string? s)
     (cond [(and (equal? s "b")
                 (equal? last-str "a"))
            #t]
           [else
            (begin
              (set! last-str s)
              #f)])]
    [else
     (or (b-after-a? (car tree))
         (b-after-a? (cdr tree)))]))


(require rackunit)

(check-equal? (b-after-a?
               (cons (cons "a" "f")
                     (cons (cons "g" "z")
                           (cons "y" "b"))))
              #f)

(check-equal? (b-after-a?
               (cons (cons "a" "f")
                     (cons (cons "g" "a")
                           (cons "b" "b"))))
              #t)



(define (b-after-a?/sps tree last-str)
  (match tree
    [(? string? s)
     (cond [(and (equal? s "b")
                 (equal? last-str "a"))
            (list #t "b")]
           [else
            (list #f s)])]
    [else
     (match (b-after-a?/sps (car tree) last-str)
       [(list success last-str-2)
        (if success
            (list success last-str-2)
            (b-after-a?/sps (cdr tree) last-str-2))])]))


(require rackunit)

(check-equal? (b-after-a?/sps
               (cons (cons "a" "f")
                     (cons (cons "g" "z")
                           (cons "y" "b")))
               #f)
              (list #f "b"))

(check-equal? (b-after-a?/sps
               (cons (cons "a" "f")
                     (cons (cons "g" "a")
                           (cons "b" "b")))
               #f)
              (list #t "b"))