[FFmpeg-devel] zlib decoder

[Reimar Doeffinger]

Hello,
On Mon, Jul 02, 2007 at 10:30:16PM +0100, M?ns Rullg?rd wrote:
> Here, at long last, is my highly anticipated zlib decoder.

Yay!

> static void
> copy_bytes(uint8_t *dst, uint8_t *src, unsigned int len)
> {
>     while (len--)
>         *dst++ = *src++;
> }
> 
> static int
> copy_offset(AVInflateContext *ctx, uint8_t *p, uint8_t *start, uint8_t *end,
>             unsigned int offset, unsigned int len)
> {
>     unsigned int outlen = FFMIN(len, end - p);

looks quite similar to the LZO code which is benchmarked a bit on at
least x86, maybe you can reuse some parts of it?

>     bitpos = get_bits_count(&ctx->gb);
>     ctx->skip = bitpos & 7;
>     bytepos = bitpos / 8;
> 
>     if (needbits || ctx->skip) {
>         unsigned isize;
>         uint8_t *in;
> 
>         if (tailbits) {
>            isize = (tailbits - bitpos + 7) / 8;

if (tailbits > bitpos) seems safer to me, even if it should be true
anyway (didn't check so closely).
I also can't help but wonder: is that tail stuff only for really
handling gzip streams?
Do we actually have those and not only gzip blocks where we wouldn't
need such extra handling??

>    /* TODO: don't copy to buffer if last byte has been written */
>    if (*outsize >= sizeof(ctx->buf)) {
>        ctx->bufsize = sizeof(ctx->buf);
>        memcpy(ctx->buf, out - ctx->bufsize, ctx->bufsize);
>    } else if (ctx->bufsize + *outsize > sizeof(ctx->buf)) {
>        unsigned drop = ctx->bufsize + *outsize - sizeof(ctx->buf);
>        unsigned bsize = ctx->bufsize - drop;
>        memmove(ctx->buf, ctx->buf + drop, bsize);
>        memcpy(ctx->buf + bsize, outbuf, *outsize);
>        ctx->bufsize = sizeof(ctx->buf);
>    } else {
>        memcpy(ctx->buf + ctx->bufsize, outbuf, *outsize);
>        ctx->bufsize += *outsize;
>    }

If I didn't mess up the following seems simple (although potentially
slower):

> int cpsize = FFMIN(*outsize, sizeof(ctx->buf));
> int drop = ctx->bufsize + cpsize - sizeof(ctx->buf);
> if (drop > 0) {
>   ctx->bufsize -= drop;
>   memmove(ctx->buf, ctx->buf + drop, ctx->bufsize);
> }
> memcpy(ctx->buf + ctx->bufsize, outbuf, cpsize);
> ctx->bufsize += cpsize;

And maybe extending and using using fifo.h is possible and acceptable speed-wise?
Probably not worth it though...

Greetings,
Reimar Doeffinger