On Sat, Mar 7, 2020 at 1:23 PM Sudarshan S Chawathe <xxxxxx@eip10.org> wrote:

There seems to be a bug in the SRFI-95 sample implementation of the
sorted? procedure when given a vector argument with duplicates.

The sample run below illustrates the problem and suggests a simple fix
as well. (The sorted? procedure definition is verbatim from the
sample implementation.)

I apologize for the hackish definitions of some of the dependencies,
but I don't think they change the behavior in the illustrated case. (I
had some dependency trouble getting the official SLIB installed on my
machine.)

I believe this bug also exists in the current SLIB (slib-3b6).

$ cat w.scm
(import (scheme base)
(scheme write))

(define array? vector?)

(define array-ref vector-ref)

(define (array-dimensions arr)
(list (vector-length arr)))

(define identity values)

(define (sorted? seq less? . opt-key)
(define key (if (null? opt-key) identity (car opt-key)))
(cond ((null? seq) #t)
((array? seq)
(let ((dimax (+ -1 (car (array-dimensions seq)))))
(or (<= dimax 1)
(let loop ((idx (+ -1 dimax))
(last (key (array-ref seq dimax))))
(or (negative? idx)
(let ((nxt (key (array-ref seq idx))))
(and (less? nxt last)
(loop (+ -1 idx) nxt))))))))
((null? (cdr seq)) #t)
(else
(let loop ((last (key (car seq)))
(next (cdr seq)))
(or (null? next)
(let ((nxt (key (car next))))
(and (not (less? nxt last))
(loop nxt (cdr next)))))))))

(write (sorted? '(1 2 2 3) <))
(newline)
(write (sorted? #(1 2 2 3) <))
(newline)
$ chibi-scheme w.scm
#t
#f
$ diff -u w.scm w2.scm
--- w.scm 2020-03-07 16:02:00.395402293 -0500
+++ w2.scm 2020-03-07 16:02:00.395402293 -0500
@@ -20,7 +20,7 @@
(last (key (array-ref seq dimax))))
(or (negative? idx)
(let ((nxt (key (array-ref seq idx))))
- (and (less? nxt last)
+ (and (not (less? last nxt last))
(loop (+ -1 idx) nxt))))))))
((null? (cdr seq)) #t)
(else
$ chibi-scheme w2.scm
#t
#t
$

Regards,

-chaw