Re: [Qemu-devel] [PATCH 3/6] json: Make lexer's "character consumed" logic less confusing

[Eric Blake <eblake@redhat.com>]

On 08/27/2018 02:00 AM, Markus Armbruster wrote:
> The lexer uses macro TERMINAL_NEEDED_LOOKAHEAD() to decide whether a
> state transition consumes the input character.  It returns true when
> the state transition is defined with the TERMINAL() macro.  To detect
> that, it checks whether input '\0' would have resulted in the same
> state transition, and the new state is not IN_ERROR.
> 
> Why does that even work?  For all states, the new state on input '\0'
> is either IN_ERROR or defined with TERMINAL().  If the state
> transition equals the one we'd get for input '\0', it goes to IN_ERROR
> or to the argument of TERMINAL().  We never use TERMINAL(IN_ERROR),
> because it makes no sense.  Thus, if it doesn't go to IN_ERROR, it
> must be defined with TERMINAL().
> 
> Since this isn't quite confusing enough, we negate the result to get
> @char_consumed, and ignore it when @flush is true.
> 
> Instead of deriving the lookahead bit from the state transition, make
> it explicit.  This is easier to understand, and a bit more flexible,
> too.
> 
> Signed-off-by: Markus Armbruster <armbru@redhat.com>
> ---
>   qobject/json-lexer.c      | 27 ++++++++++++++++-----------
>   qobject/json-parser-int.h |  1 +
>   2 files changed, 17 insertions(+), 11 deletions(-)
> 

Good description of the convoluted logic. And yes, I find the new 
version more legible, even if more lines of code.

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
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