[r6rs-discuss] [Formal] right shift formula and bit-field signedness

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This message is a formal comment which was submitted to formal-comment_at_r6rs.org, following the requirements described at: http://www.r6rs.org/process.html
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submitter's name: Daniel Villeneuve
submitter's email address: daniel_villeneuve_at_sympatico.ca
type of issue: Defect
priority: Minor
R6RS component: Exact bitwise arithmetic
version of the report: 5.92 lib
SUMMARY
Several fixes to the bitwise library.
DESCRIPTION
This comment covers several (9) items, not all with the same importance.
1) I assume that the intent of all bitwise procedures is to return exact
   integers.  If this is so, the formula used to define
   bitwise-arithmetic-shift does not cover the case of negative ei2.  A
   possible one would be:
  (->exact (floor (* ei1 (expt 2 ei2))))
  This is akin to sign-extending right shift, so
  (bitwise-arithmetic-shift -6 -1) => -3
  (bitwise-arithmetic-shift -5 -1) => -2
  (bitwise-arithmetic-shift -4 -1) => -2
  (bitwise-arithmetic-shift -3 -1) => -2
  (bitwise-arithmetic-shift -2 -1) => -1
  (bitwise-arithmetic-shift -1 -1) => -1
2) Bit fields are always defined using a finite number of bits, contrary
   to 2's complement representation which implicitly uses an infinite
   extension of $0$ bits or $1$ bits to the left.
   Either a) we specify that the leftmost value bit of a bit-field is
   understood to be repeated infinitely to the left, or b) that $0$ bits
   are used no matter the value of the leftmost bit. With a), we can have
   negative values as output to bitwise-bit-field but we have problems with
   0-bit and 1-bit bit fields, with shifting and masking, etc. With b), we
   only have positive bit fields.
   I suggest that b) is more useful/intuitive:
   (bitwise-bit-set?
     (bitwise-bit-field #b00101100 0 6) 8) => a) #t; b) #f
   (bitwise-arithmetic-shift-right
     (bitwise-bit-field #b00101100 0 6) 4) => a) -2; b) 2
   (bitwise-arithmetic-shift-right
     (bitwise-bit-field #b00101100 0 7) 4) => a)  2; b) 2
   Either way, a clarification on the signedness of bit-fields should
   be added to the introduction.
3) The domain of bitwise-rotate-bit-field should restrict ei2 and ei3
   as for the other bit-field procedures, and ei4 can be left
   unrestricted (though non-negative), as this is both intuitive and
   supported by the proposed implementation.
4) The primitive bitwise-zero? is used in the implementation of
   bitwise-bit-set? but is not defined.  It should be replaced with zero?.
5) The primitive bitwise-negative? is used in the implementation of
   bitwise-length but is not defined.  It should be replaced with
   negative?.
6) The spec of bitwise-length should start as "Returns ..." instead
   of "These procedures return ...".
7) The spec of bitwise-bit-set? should start as "Returns ..." instead
   of "Otherwise, returns ...".  There seems to be an extra newline after
   the domain.
8) General: replace "The XXX procedure returns ..." by "This procedure
   returns ..." when unambiguous, for homogeneity.
9) In the spec of bitwise-reverse-bit-field, replace "from the ei1"
   by "from ei1".
--
Daniel Villeneuve