You shouldn't need to do any form of lookahead there. Since the behaviour
for the general case is just to skip, why not try to parse the header and
use the error action to skip to the end if it fails?
On Mon, 03 Dec 2012 20:22:20 Solomon Gibbs wrote:
> I have two states; one is a specific instance of the other, more
> general, state. I believe that the right way to avoid entering both
> states simultaneously is to implement lookahead with k>1, but I can't
> find any examples of how to do this.
>
> The Ragle user's guide says:
>
> In both the use of fhold and fexec the user must be cautious of
> combining the resulting machine with another in such a way that the
> transition on which the current position is adjusted is not combined
> with a transition from the other machine.
>
> I'm not entirely sure what this means, except perhaps "don't try to
> read past the end of the current expression".
>
> My machine looks like this:
>
> seglen16 = any{2} >{ swab(p, &len, 2); len = len - 2; };
> action check {len--}
> buffer = (any when check)* %when !check @{ printf("[%d]:%d\n", len,
*p); };
>
> # JPEG Markers
> mk_app0 = 0xFF 0xE0;
> mk_appx = 0xFF (0xE0..0xEF);
> marker = 0xFF ^0x00;
> nonmarker = !marker - zlen;
>
> # JPEG APP Segments
> seg_app0_jfif = mk_app0 seglen16 "JFIF" 0x00 buffer @{ printf("jfif
> app0\n"); }; seg_appx_unk = mk_appx nonmarker* @{ printf("unknown
app
> content\n"); }; seg_app = (seg_app0_jfif | seg_app1_exif |
seg_appx_unk);
>
> # Main Machine
> expr = (mk_soi @lerr(bad) nonmarker* seg_app* nonmarker* mk_eoi);
>
>
> I want to tokenize a JPEG header, skipping unknown segments and
> handling well-known segments like JFIF. The JPEG application segment
> app0 starts with 0xFFE0. If app0 contains JFIF data, the app0 marker
> will be followed by a two-byte length and the string "JFIF\0". This
> means I need 7 bytes of lookahead when identifying application
> segments
>
> Thanks for any pointers.
>
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