On Tue, 11 Jul 2006, Linus Torvalds wrote:
>
> - for low bits: CM (compression method):
>
> "This identifies the compression method used in the file. CM = 8
> denotes the "deflate" compression method with a window size up
> to 32K. This is the method used by gzip and PNG (see
> references [1] and [2] in Chapter 3, below, for the reference
> documents). CM = 15 is reserved. It might be used in a future
> version of this specification to indicate the presence of an
> extra field before the compressed data."
>
> - four high bits are CINFO:
>
> "For CM = 8, CINFO is the base-2 logarithm of the LZ77 window
> size, minus eight (CINFO=7 indicates a 32K window size). Values
> of CINFO above 7 are not allowed in this version of the
> specification. CINFO is not defined in this specification for
> CM not equal to 8."
>
> so 0x78 means "deflate with 32kB window size", but I don't see anything
> guaranteeing that we might not see something else for an object that
> cannot be compressed, for example.
Ahh. Looking at the zlib sources, I see
/* Write the zlib header */
if (s->status == INIT_STATE) {
uInt header = (Z_DEFLATED + ((s->w_bits-8)<<4)) << 8;
uInt level_flags = (s->level-1) >> 1;
if (level_flags > 3) level_flags = 3;
header |= (level_flags << 6);
if (s->strstart != 0) header |= PRESET_DICT;
header += 31 - (header % 31);
s->status = BUSY_STATE;
putShortMSB(s, header);
(which is that first 16-bit word, MSB first). So we'll always have the
Z-DEFLATED (8) there in the low four bits, but the high nybble will be
"s->w_bits-8" where w_bits comes from windowBits, and I think we can
depend on it beign 15:
"The windowBits parameter is the base two logarithm of the window size
(the size of the history buffer). It should be in the range 8..15 for this
version of the library. Larger values of this parameter result in better
compression at the expense of memory usage. The default value is 15 if
deflateInit is used instead."
so since we use deflateInit(), we know the window will be 15.
So I guess we _can_ depend on the first byte being 0x78 for our use.
Goodie.
Linus
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