From: camsight@...
Date: 2005-05-05T01:04:50+09:00
Subject: Fixnum's binary representation

Hi, people!

As far as I know, Ruby's Fixnum is 30-bit signed integer.
One bit is used as a flag for whether it's a direct value.
Another bit is used for non-Fixnum direct values.
I wondered how Fixnum#[] works and tested it.

def show_binary(i)
	result = ''
	32.times do |b|
		result = i[b].to_s + result
		if (b % 8 == 7) and (b != 31)
			result = " " + result
		end
	end
	puts result + ": " + i.to_s
end

show_binary(1)
show_binary(-1)
show_binary(2 ** 31 - 1)
show_binary((2 ** 31 - 1) * (-1))

Result:

00000000 00000000 00000000 00000001: 1
11111111 11111111 11111111 11111111: -1
01111111 11111111 11111111 11111111: 2147483647
10000000 00000000 00000000 00000001: -2147483647

This is exactly how 32-bit signed integer's binary representations look
like.
My guess is that Fixnum#[] works as if it's 32-bit signed integer.
It's not showing its real binary representation.
Is my guess true?

My another question is that even if (2 ** 31 - 1) is not a Fixnum, the
above code works for it (Actually Bignum#[]).
If the number is bigger than that, [] doesn't work.
Fixnum#[] and Bignum#[] are cleverly hiding the internal facts and are
made to simulate 32-bit signed integers?

Thanks.
Sam