Re: [Qemu-devel] [PATCH 6/6] json: Eliminate lexer state IN_WHITESPACE, pseudo-token JSON_SKIP

[Markus Armbruster]

Eric Blake <address@hidden> writes:

> On 08/27/2018 02:00 AM, Markus Armbruster wrote:
>> The lexer ignores whitespace like this:
>>
>>           on whitespace      on non-ws   spontaneously
>>      IN_START --> IN_WHITESPACE --> JSON_SKIP --> IN_START
>>                      ^    |
>>                       \__/  on whitespace
>>
>> This accumulates a whitespace token in state IN_WHITESPACE, only to
>> throw it away on the transition via JSON_SKIP to the start state.
>> Wasteful.  Go from IN_START to IN_START on whitspace directly,
>
> s/whitspace/whitespace/

Will fix.

>> dropping the whitespace character.
>>
>> Signed-off-by: Markus Armbruster <address@hidden>
>> ---
>>   qobject/json-lexer.c      | 22 +++++-----------------
>>   qobject/json-parser-int.h |  1 -
>>   2 files changed, 5 insertions(+), 18 deletions(-)
>>
>> @@ -263,10 +253,10 @@ static const uint8_t json_lexer[][256] =  {
>>           [','] = JSON_COMMA,
>>           [':'] = JSON_COLON,
>>           ['a' ... 'z'] = IN_KEYWORD,
>> -        [' '] = IN_WHITESPACE,
>> -        ['\t'] = IN_WHITESPACE,
>> -        ['\r'] = IN_WHITESPACE,
>> -        ['\n'] = IN_WHITESPACE,
>> +        [' '] = IN_START,
>> +        ['\t'] = IN_START,
>> +        ['\r'] = IN_START,
>> +        ['\n'] = IN_START,
>>       },
>>       [IN_START_INTERP]['%'] = IN_INTERP,
>
> Don't you need to set [IN_START_INTERP][' '] to IN_START_INTERP,
> rather than IN_START?  Otherwise, the presence of skipped whitespace
> would change whether interpolation happens.  (At least, that's what
> you had in an earlier version of this patch).
>
>>   };
>> @@ -323,10 +313,8 @@ static void json_lexer_feed_char(JSONLexer *lexer, char 
>> ch, bool flush)
>>               json_message_process_token(lexer, lexer->token, new_state,
>>                                          lexer->x, lexer->y);
>>               /* fall through */
>> -        case JSON_SKIP:
>> -            g_string_truncate(lexer->token, 0);
>> -            /* fall through */
>>           case IN_START:
>> +            g_string_truncate(lexer->token, 0);
>>               new_state = lexer->start_state;
>
> Oh, I see. We are magically reverting to the correct start state if
> the requested transition reports IN_START, rather than blindly using
> IN_START.

Correct.  Less repetitive this way.

> Reviewed-by: Eric Blake <address@hidden>

Thanks!