[FFmpeg-devel] [PATCH] avcodec/webvttdec: strip classes

[Andreas Rheinhardt]

Leon Grutters:
> On 3/20/25 9:02 AM, Andreas Rheinhardt wrote:
>> Leon Grutters:
>>> If a supported tag has a class, e.g "<i.bold>" it is ignored entirely;
>>> so for example "<i.bold>Hello</i>" would be converted to "Hello{\i0}"
>>> instead of the intended "{\i1}Hello{\i0}".
>>>
>>> Signed-off-by: Leon Grutters<gruttersleonbot2 at gmail.com>
>>> ---
>>>   libavcodec/webvttdec.c | 51 +++++++++++++++++++++++++++++++++---------
>>>   1 file changed, 40 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/libavcodec/webvttdec.c b/libavcodec/webvttdec.c
>>> index 35bdbe805d..4111d138c4 100644
>>> --- a/libavcodec/webvttdec.c
>>> +++ b/libavcodec/webvttdec.c
>>> @@ -29,25 +29,53 @@
>>>   #include "ass.h"
>>>   #include "codec_internal.h"
>>>   #include "libavutil/bprint.h"
>>> +#include "libavutil/mem.h"
>>>     static const struct {
>>>       const char *from;
>>>       const char *to;
>>>   } webvtt_tag_replace[] = {
>>> -    {"<i>", "{\\i1}"}, {"</i>", "{\\i0}"},
>>> -    {"<b>", "{\\b1}"}, {"</b>", "{\\b0}"},
>>> -    {"<u>", "{\\u1}"}, {"</u>", "{\\u0}"},
>>>       {"{", "\\{{}"}, {"\\", "\\\xe2\x81\xa0"}, // escape to avoid
>>> ASS markup conflicts
>>>       {">", ">"}, {"<", "<"},
>>>       {"‎", "\xe2\x80\x8e"}, {"‏", "\xe2\x80\x8f"},
>>>       {"&", "&"}, {" ", "\\h"},
>>>   };
>>> +static const struct {
>>> +    const char *from;
>>> +    const char *to;
>>> +} webvtt_valid_tags[] = {
>>> +    {"i", "{\\i1}"}, {"/i", "{\\i0}"},
>>> +    {"b", "{\\b1}"}, {"/b", "{\\b0}"},
>>> +    {"u", "{\\u1}"}, {"/u", "{\\u0}"},
>>> +};
>> These strings are so small that one can avoid the relocations by using
>> fixes-size buffers.
> What do you mean exactly? I'm not sure I understand.

A fixed size buffer in the struct instead of a pointer to a string
somewhere else: char from[3]; char to[6]; should work.

>>>     static int webvtt_event_to_ass(AVBPrint *buf, const char *p)
>>>   {
>>> -    int i, again = 0, skip = 0;
>>> +    int i, again = 0/*, skip = 0*/;
>>>         while (*p) {
>>> +        if (*p == '<') {
>>> +            const char *tag_end = strchr(p, '>');
>>> +            char *tag_body, *tag_name, *saveptr = NULL;
>>> +            ptrdiff_t len;
>>> +            if (!tag_end)
>>> +                break;
>>> +            len = tag_end - p + 1;
>>> +            tag_body = av_strndup(p + 1, len - 2);
>>> +            if (!tag_body)
>>> +                return AVERROR(ENOMEM);
>> This allocation seems unnecessary. You can inspect the string without
>> modifying it by using strncmp() below and by using a maximum field width
>> for the %s directive in a parameter.
> My thought process here was that if it's done this way, you have saveptr
> for possibly getting the classes (and tag annotation) later on.

You can always save a pointer to start and end of the current component
manually.

>>> +            tag_name = av_strtok(tag_body, ".", &saveptr);
>>> +            for (i = 0; i < FF_ARRAY_ELEMS(webvtt_valid_tags); i++) {
>>> +                const char *from = webvtt_valid_tags[i].from;
>>> +                if(!strcmp(tag_name, from)) {
>>> +                    av_bprintf(buf, "%s", webvtt_valid_tags[i].to);
>>> +                    break;
>>> +                }
>>> +            }
>>> +            p += len;
>>> +            again = 1;
>>> +            av_freep(&tag_body);
>>> +        }
>>>             for (i = 0; i < FF_ARRAY_ELEMS(webvtt_tag_replace); i++) {
>>>               const char *from = webvtt_tag_replace[i].from;
>>> @@ -59,21 +87,22 @@ static int webvtt_event_to_ass(AVBPrint *buf,
>>> const char *p)
>>>                   break;
>>>               }
>>>           }
>>> +
>>>           if (!*p)
>>>               break;
>>>             if (again) {
>>>               again = 0;
>>> -            skip = 0;
>>> +            // skip = 0;
>>>               continue;
>>>           }
>>> -        if (*p == '<')
>>> -            skip = 1;
>>> -        else if (*p == '>')
>>> -            skip = 0;
>>> -        else if (p[0] == '\n' && p[1])
>>> +        // if (*p == '<')
>>> +        //     skip = 1;
>>> +        // else if (*p == '>')
>>> +        //     skip = 0;
>>> +        if (p[0] == '\n' && p[1])
>>>               av_bprintf(buf, "\\N");
>>> -        else if (!skip && *p != '\r')
>>> +        else if (/*!skip && */*p != '\r')
>>>               av_bprint_chars(buf, *p, 1);
>>>           p++;
>>>       }
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