From: Brian Candler <lists@...>
Date: 2012-12-11T05:19:17+09:00
Subject: Re: Binary Fixed-Point in Ruby? (Improved FixedPt?)

Axel Friedrich wrote in post #1088522:
> FixedPt handels fixed-point binaries (not decimals).
>
> For example, this should work:
>
> fi = FixedPt.new(0.1,5,7)
>
> p fi.bin
> gives: "11111"
> but should give something like: "01100"

What are the 5 and 7 meant to signify?

0.1 in decimal has no exact representation in binary. It would be 
0.00011001100...

I don't know of an existing library for working in fixed-point. If the 
number of digits after the binary point is fixed, then you could just 
work with integers with a power-of-two scaling factor - similar to 
working with cents instead of dollars.

class FixBin
  def initialize(value, binbits)
    @value = (value * (1 << binbits)).to_i
    @binbits = binbits
  end
  def to_f
    @value * 1.0 / (1 << @binbits)
  end
  def to_s
    res = "%0#{@binbits+1}b" % @value
    res[-@binbits,0] = "."
    res
  end
end

a = FixBin.new(0.1, 7)
puts a.to_f
puts a.to_s

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