Bit syntax frustrations, again

James Hague <>
Wed Sep 25 19:31:02 CEST 2002


I've been writing an Erlang module to decode swf (Flash) files.  The swf
format is documented at http://www.openswf.org.  The problem I've been
having is that Erlang starts decoding bits with the most significant bit,
whereas the swf format--which is very bit-oriented--starts with the least
significant bit.  As best I can tell, the "little" qualifier in Erlang's bit
syntax refers to bytes in multi-byte values, not bits.

To give an example, suppose the next byte in the input stream contains
16#fe, and I need to grab a 3-bit field followed by a 5-bit field.  The
obvious match of:

	<<Field1:3, Field2:5, _/binary>> = Binary

results in:

	Field1 = 2#111
	Field2 = 2#11110

In the swf format, though, the correct answer is:

	Field1 = 2#110
	Field2 = 2#11111

In this simple case there's an easy enough fix, but not for the general case
when bit fields cross byte boundaries.  I don't see a nice fix in that case.
I'm guessing I'll have to put aside the bit syntax for most of it and do bit
manipulations directly.  Bleah.  Any thoughts we could appreciated.



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