+ struct fb_file *F = FB_FILE(f);
+ byte *read_ptr = (f->buffer = FB_BUFFER(f));
+ uns blen = f->bufend - f->buffer, back = F->keep_back_buf ? blen >> 2 : 0, read_len = blen;
+ /* Forward or no seek */
+ if (F->wpos <= f->pos)
+ {
+ sh_off_t diff = f->pos - F->wpos;
+ /* Formula for long forward seeks (prefer lseek()) */
+ if (diff > ((sh_off_t)blen << 2))
+ {
+long_seek:
+ f->bptr = f->buffer + back;
+ f->bstop = f->buffer + blen;
+ goto seek;
+ }
+ /* Short forward seek (prefer read() to skip data )*/
+ else if ((uns)diff >= back)
+ {
+ uns skip = diff - back;
+ F->wpos += skip;
+ while (skip)
+ {
+ int l = read(F->fd, f->buffer, MIN(skip, blen));
+ if (unlikely(l <= 0))
+ if (l < 0)
+ die("Error reading %s: %m", f->name);
+ else
+ {
+ F->wpos -= skip;
+ goto eof;
+ }
+ skip -= l;
+ }
+ }
+ /* Reuse part of the previous window and append new data (also F->wpos == f->pos) */
+ else
+ {
+ uns keep = back - (uns)diff;
+ if (keep >= F->wlen)
+ back = diff + (keep = F->wlen);
+ else
+ memmove(f->buffer, f->buffer + F->wlen - keep, keep);
+ read_len -= keep;
+ read_ptr += keep;
+ }
+ f->bptr = f->buffer + back;
+ f->bstop = f->buffer + blen;
+ }
+ /* Backwards seek */
+ else
+ {
+ sh_off_t diff = F->wpos - f->pos;
+ /* Formula for long backwards seeks (keep smaller backbuffer than for shorter seeks ) */
+ if (diff > ((sh_off_t)blen << 1))
+ {
+ if ((sh_off_t)back > f->pos)
+ back = f->pos;
+ goto long_seek;
+ }
+ /* Seek into previous window (do nothing... for example brewind) */
+ else if ((uns)diff <= F->wlen)
+ {
+ f->bstop = f->buffer + F->wlen;
+ f->bptr = f->bstop - diff;
+ f->pos = F->wpos;
+ return 1;
+ }
+ back *= 3;
+ if ((sh_off_t)back > f->pos)
+ back = f->pos;
+ f->bptr = f->buffer + back;
+ read_len = blen;
+ f->bstop = f->buffer + read_len;
+ /* Reuse part of previous window */
+ if (F->wlen && read_len <= back + diff && read_len > back + diff - F->wlen)
+ {
+ uns keep = read_len + F->wlen - back - diff;
+ memmove(f->buffer + read_len - keep, f->buffer, keep);
+ }
+seek:
+ /* Do lseek() */
+ F->wpos = f->pos + (f->buffer - f->bptr);
+ if (sh_seek(F->fd, F->wpos, SEEK_SET) < 0)
+ die("Error seeking %s: %m", f->name);
+ }
+ /* Read (part of) buffer */
+ do
+ {
+ int l = read(F->fd, read_ptr, read_len);
+ if (unlikely(l < 0))
+ die("Error reading %s: %m", f->name);
+ if (!l)
+ if (unlikely(read_ptr < f->bptr))
+ goto eof;
+ else
+ break; /* Incomplete read because of EOF */
+ read_ptr += l;
+ read_len -= l;
+ F->wpos += l;
+ }
+ while (read_ptr <= f->bptr);
+ if (read_len)
+ f->bstop = read_ptr;
+ f->pos += f->bstop - f->bptr;
+ F->wlen = f->bstop - f->buffer;
+ return f->bstop - f->bptr;
+eof:
+ /* Seeked behind EOF */
+ f->bptr = f->bstop = f->buffer;
+ F->wlen = 0;
+ return 0;