[ruby-core:55622] [ruby-trunk - Feature #8553] Bignum#size (and Fixnum#size)

Issue #8553 has been updated by akr (Akira Tanaka).

File bignum-size-abs-bytewise.patch added

> In JRuby, Bignum#size is implemented using Java's BigInteger.bitLength().
...
> Returns the number of bits in the minimal two's-complement representation of this BigInteger, excluding a sign bit. For positive BigIntegers, this is equivalent to the number of bits in the ordinary binary representation. (Computes (ceil(log2(this < 0 ? -this : this+1))).

I think two's complement representation excluding a sign bit is not familiar for most people.

Especially Bignum#size is bytewise, unlike Java's BigInteger.bitLength().
So it is not useful for calculating required number of bytes as two's complement signed integers.

> This leads to the deviation you see in RubySpec, where JRuby and Rubinius differ from MRI on various Bignum sizes.

We should ignore RubySpec.

> Perhaps the least impact and most value would be to reimplement MRI's Bignum#size in terms of actual 2's complement bit length, the same way as JRuby (and apparently Rubinius)?

It seems Rubinius uses absolute value.

  % bin/rbx -ve '
  1.upto(200) {|n|
    v = (1<<n)
    a = [(-v-1).size, (-v).size, (-v+1).size, (v-1).size, v.size, (v+1).size]
    p [n, a] if a.uniq.length != 1
  }'
  rubinius 2.0.0.rc1 (1.8.7 854aef25 yyyy-mm-dd JI) [x86_64-unknown-linux-gnu]
  [64, [9, 9, 8, 8, 9, 9]]
  [72, [10, 10, 9, 9, 10, 10]]
  [80, [11, 11, 10, 10, 11, 11]]
  [88, [12, 12, 11, 11, 12, 12]]
  [96, [13, 13, 12, 12, 13, 13]]
  [104, [14, 14, 13, 13, 14, 14]]
  [112, [15, 15, 14, 14, 15, 15]]
  [120, [16, 16, 15, 15, 16, 16]]
  [128, [17, 17, 16, 16, 17, 17]]
  [136, [18, 18, 17, 17, 18, 18]]
  [144, [19, 19, 18, 18, 19, 19]]
  [152, [20, 20, 19, 19, 20, 20]]
  [160, [21, 21, 20, 20, 21, 21]]
  [168, [22, 22, 21, 21, 22, 22]]
  [176, [23, 23, 22, 22, 23, 23]]
  [184, [24, 24, 23, 23, 24, 24]]
  [192, [25, 25, 24, 24, 25, 25]]
  [200, [26, 26, 25, 25, 26, 26]]

It seems (-(1 << n)).size == (1 << n).size for all n.

I implemented a patch as my first proposal (absolute value).

How do you think, matz or nurse?

> RE: Fixnum#size... I think Ruby should introduce a new constant for native word size, and encourage people to use it.

It is another issue.
However I guess several developers may oppose new Fixnum dependent features
as https://bugs.ruby-lang.org/issues/7517


----------------------------------------
Feature #8553: Bignum#size (and Fixnum#size)
https://bugs.ruby-lang.org/issues/8553#change-40111

Author: akr (Akira Tanaka)
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: 


How about changing Bignum#size to a well defined behavior?

Recently I changed Bignum implementation to use 128 bit integer type
if the type is available.  (r41379)

After that, rubyspec fails with Bignum#size as follows:

| 1)
| Bignum#size returns the number of bytes in the machine representation in multiples of four FAILED
| Expected 16
|  to equal 12
| 
http://a.mrkn.jp/~mrkn/chkbuild/mavericks/ruby-trunk-m64-o3/log/20130620T231101Z.log.html.gz

I think this failure itself is not a problem.
This is just an over-specification of rubyspec.
Actually the test also fails before r41379 on platforms which doesn't support 64 bit integer type.

It is because the Bignum#size returns multiples of sizeof(BDIGIT), not 4.
sizeof(BDIGIT) can be 2 or 4 before r41379 or 8 since r41379.
The test only considers platforms sizeof(BDIGIT) is 4.

However this test failure reminds me that I always felt that current
Bignum#size (and Fixnum#size) behavior is not useful.

I guess almost all people doesn't interest sizeof(BDIGIT).

So I'd like to change Bignum#size to return number of bytes.
It means val.size returns n if val is a Bignum and
256**n <= abs(val) < 256**(n+1).

One exception is that val.size returns 0 if val is Bignum zero.

My considerations:

* The above Bignum#size behavior is based on absolute values, abs(n).
  This is compatible to current behavior and easy to understand.
  There are other possibilities: exception on negative values and
  definition on 2's complement value.
  I think exception is too different from current behavior.
  A definition based on 2's complement is difficult to treat sign bits.

* Bignum#size returns number of bytes, not bits.
  I think some method which returns number of bits may be useful but
  it is different from current Bignum#size.
  It is better to name another one, such as bitsize and it is not this issue.

* Fixnum#size is difficult to change.
  Currently Fixnum#size returns sizeof(long).
  Some programs, mspec for example, uses it to obtain a word size. 
  So it is difficult to change because compatibility.
  However I doubt such use is correct.
  If a program want to detect 64bit platform, Fixnum#size should not be used
  because it returns 4 on LLP64 platforms (64bit Windows).
  It is better to use [0].pack("l!").size for sizof(long) and
  [nil].pack("p").size for sizeof(char *).

  However some people may hope Fixnum and Bignum consistent.
  For that purpose, Fixnum#size should also be changed same way.
  It will breaks some applications, though.

Any opinion?



-- 
http://bugs.ruby-lang.org/