Thank you so much for your very quick answer. It seems that the conversion to and from bytes are what I was looking for. With that, it is quite easy to do all the manipulations I want.
Best, Nicolas. On Sat, Oct 8, 2011 at 6:57 PM, Matthew Flatt <mfl...@cs.utah.edu> wrote: > At Sat, 8 Oct 2011 17:41:35 +0100, "nicolas.o...@gmail.com" wrote: > > I am trying to write a small implementation of Protocol Buffers ( > > http://code.google.com/p/protobuf/ ) and I have difficulties > > with bit manipulations, especially to transform a negative integer into a > > series of bytes and back. > > > > I can not find an implementation of a non-arithmetic shift for fixnums, > and > > a way to specify which size of fixnum to construct. > > There aren't different sizes of fixnums. When you want to convert an > exact integer to bytes, then `integer->integer-bytes' lets you specify > the size. > > There also isn't a logical shift operator, if I understand what you're > after. If you work with positive integers, then `arithmetic-shift' > (plus `bitwise-and'?) behaves like a logical shift. If you want > logical-shift effects for finite 2's complement representations, then > you probably don't really want to work on fixnum representations, which > are 63 or 31 bits, but instead some power of 2 via bitwise operations > like `bitwise-ior', `bitwise-and', and arithmetic shifts of negative > numbers. Note also that `integer-bytes->integer' lets you say whether > to interpret a set of bytes as unsigned or as 2's complement. > > > I can't also find an easy way to get the byte representation of flonums. > > `real->floating-point-bytes' > > -- Sent from an IBM Model M, 15 August 1989.
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