From: "Ara.T.Howard" <ahoward@...>
Date: 2004-07-22T13:32:03+09:00
Subject: Re: [PATCH] was - Re: substring by range parameter (bug?)

On Thu, 22 Jul 2004, Yukihiro Matsumoto wrote:

> Hi,
>
> In message "[PATCH] was - Re: substring by range parameter (bug?)"
>    on 04/07/22, "Ara.T.Howard" <ahoward@noaa.gov> writes:
>
> |if you look at the source, it is clear that rb_range_beg_len intends to fail
> |with ranges like -1...-42.  i think it is a bug that it does not.  this patch
> |should fix it:
>
> I think I understand you, but not sure.  Can you show me an example of
> wrong behavior?
>
> 							matz.

note the second case:

   wrong:

     ~ > irb
     irb(main):001:0> 'foobar'[-1..-1]
     => "r"
     irb(main):002:0> 'foobar'[-1..-2]
     => ""
     irb(main):003:0> 'foobar'[-1..-3]
     => nil

   right (patch applied):

     ~ > irb
     irb(main):001:0> 'foobar'[-1..-1]
     => "r"
     irb(main):002:0> 'foobar'[-1..-2]
     => nil
     irb(main):003:0> 'foobar'[-1..-3]
     => nil


it seems like a simple off by one error cause by mapping

   -n,...,-3,-2,-1 => 0,...,n-3,n-2,n-1


my interpretation of rb_range_beg_len was that the test

   if (len < 0)

is to catch errors made by specifying ranges whose ends are before their
beginings, like in

   'foobar'[5..2] # => nil

however this breaks down in the case of

   'foobar'[-1..-2] # => ''

because

   end++;

occurs AFTER neg ranges are mapped to pos ranges but before

   if (!EXCL(range)...

i think it should always be the case that

   beg <= end

after beg and end have been mapped positive but BEFORE end is incremented to
include end if it's exclusive.  else we have and off by one problem...

this is, i think, what testing

   len < 0

was supposed to catch - but this fails when

   beg = -n
   end = -n - 1

as is the case with -1..-2

maybe i've misinterpreted this?  if not here is the patch again - please note
my first one was wrong!  sorry.


diff -u range.c.org range.c
--- range.c.org 2004-07-21 15:56:02.000000000 -0600
+++ range.c     2004-07-21 16:54:48.000000000 -0600
@@ -22,6 +22,7 @@
             end = len;
      }
      if (end < 0) end += len;
+    if (end < beg) goto out_of_range; /* b4 including end point require end >= beg */
      if (!EXCL(range)) end++;   /* include end point */
      if (end < 0) goto out_of_range;
      len = end - beg;


kind regards.

-a
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