Class: Flt::Support::Rationalizer

Inherits:
Object
  • Object
show all
Extended by:
AuxiliarFunctions
Includes:
AuxiliarFunctions
Defined in:
lib/flt/support/rationalizer.rb,
lib/flt/support/rationalizer_extra.rb

Overview

This class provides efficient conversion of fraction (as approximate floating point numbers) to rational numbers.

Defined Under Namespace

Modules: AuxiliarFunctions

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(tol = Tolerance(:epsilon)) ⇒ Rationalizer

Create Rationalizator with given tolerance.



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# File 'lib/flt/support/rationalizer.rb', line 113

def initialize(tol=Tolerance(:epsilon))
  @tol = tol
end

Class Method Details

.[](*args) ⇒ Object 



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# File 'lib/flt/support/rationalizer.rb', line 117

def self.[](*args)
  new(*args)
end

.max_denominator(f, max_den = 1000000000, num_class = nil) ⇒ Object

Best fraction given maximum denominator Algorithm Copyright © 1991 by Joseph K. Horn.

The implementation of this method uses floating point arithmetic which limits the magnitude and precision of the results, specially using Float values.



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# File 'lib/flt/support/rationalizer.rb', line 242

def self.max_denominator(f, max_den=1000000000, num_class=nil)
  return rationalize_special(f) if special?(f)
  return nil if max_den < 1
  num_class ||= f.class
  context = num_class.context
  return ip(f),1 if fp(f) == 0

  cast = lambda{|x| context.Num(x)}

  one = cast[1]

   sign = f < 0
   f = -f if sign

   a,b,c = 0,1,f
   while b < max_den && c != 0
     cc = one/c
     a,b,c = b, ip(cc)*b+a, fp(cc)
   end

   if b>max_den
     b -= a*ceil(cast[b-max_den]/a)
   end

   f1,f2 = [a,b].collect{|x| abs(cast[rnd(x*f)]/x-f)}

   a = f1 > f2 ? b : a

   num,den = rnd(a*f).to_i,a
   den = 1 if abs(den) < 1

   num = -num if sign

  return num,den
end

.rationalize_special(x) ⇒ Object 



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# File 'lib/flt/support/rationalizer.rb', line 278

def self.rationalize_special(x)
  if x.nan?
    [0, 0]
  else
    [sign(x), 0]
  end
end

.to_r(x) ⇒ Object

Exact conversion to rational. Ruby provides this method for all numeric types since version 1.9.1, but before that it wasn’t available for Float or BigDecimal. This methods supports old Ruby versions.



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# File 'lib/flt/support/rationalizer.rb', line 10

def self.to_r(x)
  if x.respond_to?(:to_r)
    x.to_r
  else
    case x
    when Float
      # Float did not had a #to_r method until Ruby 1.9.1
      return Rational(x.to_i, 1) if x.modulo(1) == 0
      if !x.finite?
        return Rational(0, 0) if x.nan?
        return x < 0 ? Rational(-1, 0) : Rational(1, 0)
      end

      f, e = Math.frexp(x)

      if e < Float::MIN_EXP
         bits = e + Float::MANT_DIG - Float::MIN_EXP
      else
         bits = [Float::MANT_DIG,e].max
         # return Rational(x.to_i, 1) if bits < e
      end
        p = Math.ldexp(f, bits)
        e = bits - e
        if e < Float::MAX_EXP
          q = Math.ldexp(1, e)
        else
          q = Float::RADIX**e
        end
      Rational(p.to_i, q.to_i)

    when BigDecimal
      # BigDecimal probably didn't have #to_r at some point
      s, f, b, e = x.split
      p = f.to_i
      p = -p if s < 0
      e = f.size - e
      if e < 0
        p *= b**(-e)
        e = 0
      end
      q = b**(e)
      Rational(p,q)

    else
      x.to_r
    end
  end
end

Instance Method Details

#exact_binary_rationalization(x) ⇒ Object

An exact rationalization method for binary floating point that yields smallest fractions when possible and is not too slow



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# File 'lib/flt/support/rationalizer_extra.rb', line 147

def exact_binary_rationalization(x)
  p, q = x, 1
  while p.modulo(1) != 0
    p *= 2.0
    q <<= 1 # q *= 2
  end
  Rational(p.to_i, q)
end

#exact_float_rationalization(x) ⇒ Object

An a here’s a shorter implementation relying on the semantics of the power operator, but which is somewhat slow:



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# File 'lib/flt/support/rationalizer_extra.rb', line 158

def exact_float_rationalization(x)
  f,e = Math.frexp(x)
  f = Math.ldexp(f, Float::MANT_DIG)
  e -= Float::MANT_DIG
  return Rational(f.to_i*(Float::RADIX**e.to_i), 1)
end

#rationalize(x) ⇒ Object

Rationalization method that finds the fraction with smallest denominator fraction within the tolerance distance of an approximate (floating point) number.



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# File 'lib/flt/support/rationalizer.rb', line 125

def rationalize(x)
  # Use the algorithm which has been found most efficient, rationalize_Knuth.
  rationalize_Knuth(x)
end

#rationalize_Horn(x) ⇒ Object

This is algorithm PDQ2 by Joe Horn.



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# File 'lib/flt/support/rationalizer.rb', line 165

def rationalize_Horn(x)
  rationalization(x) do |z, t|
    a,b = num_den(t)
    n0,d0 = (n,d = num_den(z))
    cn,x,pn,cd,y,pd,lo,hi,mid,q,r = 1,1,0,0,0,1,0,1,1,0,0
    begin
      q,r = n.divmod(d)
      x = q*cn+pn
      y = q*cd+pd
      pn = cn
      cn = x
      pd = cd
      cd = y
      n,d = d,r
    end until b*(n0*y-d0*x).abs <= a*d0*y

    if q > 1
      hi = q
      begin
        mid = (lo + hi).div(2)
        x = cn - pn*mid
        y = cd - pd*mid
        if b*(n0*y - d0*x).abs <= a*d0*y
          lo = mid
        else
          hi = mid
        end
      end until hi - lo <= 1
      x = cn - pn*lo
      y = cd - pd*lo
    end
    [x, y]
  end
end

#rationalize_Horn_Hutchins(value) ⇒ Object

Smallest denominator rationalization procedure by Joe Horn and Tony Hutchins; this is the most efficient method as implemented in RPL. Tony Hutchins has come up with PDR6, an improvement over PDQ2; though benchmarking does not show any speed improvement under Ruby.



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# File 'lib/flt/support/rationalizer_extra.rb', line 50

def rationalize_Horn_Hutchins(value)
  rationalization(value) do |x, dx|
    a,b = num_den(dx)
    n,d = num_den(x)
    pc,ce = n,-d
    pc,cd = 1,0
    t = a*b
    begin
      tt = (-pe).div(ce)
      pd,cd = cd,pd+tt*cd
      pe,ce = ce,pe+tt*ce
    end until b*ce.abs <= t*cd
    tt = t * (pe<0 ? -1 : (pe>0 ? +1 : 0))
    tt = (tt*d+b*ce).div(tt*pd+b*pe)
    [(n*cd-ce-(n*pd-pe)*tt)/d, tt/(cd-tt*pd)]
  end
end

#rationalize_Horn_simple(x, smallest_denominator = false) ⇒ Object

Simple Rationalization by Joe Horn



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# File 'lib/flt/support/rationalizer_extra.rb', line 9

def rationalize_Horn_simple(x, smallest_denominator = false)
  rationalization(x) do |z, t|
    a,b = num_den(t)
    n0,d0 = (n,d = z.nio_xr.nio_num_den)
    cn,x,pn,cd,y,pd,lo,hi,mid,q,r = 1,1,0,0,0,1,0,1,1,0,0
    begin
      q,r = n.divmod(d)
      x = q*cn+pn
      y = q*cd+pd
      pn = cn
      cn = x
      pd = cd
      cd = y
      n,d = d,r
    end until b*(n0*y-d0*x).abs <= a*d0*y
    if smallest_denominator
      if q>1
        hi = q
        begin
          mid = (lo+hi).div(2)
          x = cn-pn*mid
          y = cd-pd*mid
          if b*(n0*y-d0*x).abs <= a*d0*y
            lo = mid
          else
            hi = mid
          end
        end until hi-lo <= 1
        x = cn - pn*lo
        y = cd - pd*lo
      end
    end
    [x, y]
  end
end

#rationalize_HornHutchins(x) ⇒ Object

This is from a RPL program by Tony Hutchins (PDR6).



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# File 'lib/flt/support/rationalizer.rb', line 201

def rationalize_HornHutchins(x)
  rationalization(x) do |z, t|
    a,b = num_den(t)
    n0,d0 = (n,d = num_den(z))
    cn,x,pn,cd,y,pd,lo,hi,mid,q,r = 1,1,0,0,0,1,0,1,1,0,0
    begin
      q,r = n.divmod(d)
      x = q*cn+pn
      y = q*cd+pd
      pn = cn
      cn = x
      pd = cd
      cd = y
      n,d = d,r
    end until b*(n0*y-d0*x).abs <= a*d0*y

    if q > 1
      hi = q
      begin
        mid = (lo + hi).div(2)
        x = cn - pn*mid
        y = cd - pd*mid
        if b*(n0*y - d0*x).abs <= a*d0*y
          lo = mid
        else
          hi = mid
        end
      end until hi - lo <= 1
      x = cn - pn*lo
      y = cd - pd*lo
    end
    [x, y]
  end
end

#rationalize_Knuth(value) ⇒ Object

This algorithm is derived from exercise 39 of 4.5.3 in “The Art of Computer Programming”, by Donald E. Knuth.



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# File 'lib/flt/support/rationalizer.rb', line 132

def rationalize_Knuth(value)
  rationalization(value) do |x, dx|
    x     = to_r(x)
    dx    = to_r(dx)
    xp,xq = num_den(x-dx)
    yp,yq = num_den(x+dx)

    a = []
    fin, odd = false, false
    while !fin && xp != 0 && yp != 0
      odd = !odd
      xp,xq = xq,xp
      ax = xp.div(xq)
      xp -= ax*xq

      yp,yq = yq,yp
      ay = yp.div(yq)
      yp -= ay*yq

      if ax!=ay
        fin = true
        ax,xp,xq = ay,yp,yq if odd
      end
      a << ax # .to_i
    end
    a[-1] += 1 if xp != 0 && a.size > 0
    p,q = 1,0
    (1..a.size).each{|i| p, q = q+p*a[-i], p}
    [q, p]
  end
end

#rationalize_Knuth_Goizueta(value) ⇒ Object

Smallest denominator rationalization based on exercise 39 of cite[S 4.5.3]Knuth. This has been found the most efficient method (except for large tolerances) as implemented in Ruby. Here’s the rationalization procedure based on the exercise by Knuth. We need first to calculate the limits (x-dx, x+dx) of the range where we’ll look for the rational number. If we compute them using floating point and then convert then to fractions this method is always more efficient than the other procedures implemented here, but it may be less accurate. We can achieve perfect accuracy as the other methods by doing the substraction and addition with rationals, but then this method becomes less efficient than the others for a low number of iterations (low precision required).



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# File 'lib/flt/support/rationalizer_extra.rb', line 79

def rationalize_Knuth_Goizueta(value)
  rationalization(value) do |x, dx|
    x = to_r(x)
    dx = to_r(dx)
    xp,xq = num_den(x-dx)
    yp,yq = num_den(x+dx)

    a = []
    fin,odd = false,false
    while !fin && xp!=0 && yp!=0
      odd = !odd
      xp,xq = xq,xp
      ax = xp.div(xq)
      xp -= ax*xq

      yp,yq = yq,yp
      ay = yp.div(yq)
      yp -= ay*yq

      if ax!=ay
        fin = true
        ax,xp,xq = ay,yp,yq if odd
      end
      a << ax # .to_i
    end
    a[-1] += 1 if xp!=0 && a.size>0
    p,q = 1,0
    (1..a.size).each{|i| p,q=q+p*a[-i],p}
    [q, p]
  end
end

#rationalize_Knuth_Goizueta_b(value) ⇒ Object

La siguiente variante realiza una iteración menos si xq<xp y una iteración más si xq>xp.



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# File 'lib/flt/support/rationalizer_extra.rb', line 113

def rationalize_Knuth_Goizueta_b(value)
  rationalization(value) do |x, dx|
    x = to_r(x)
    dx = to_r(dx)
    xq,xp = num_den(x-dx)
    yq,yp = num_den(x+dx)

    a = []
    fin,odd = false,false
    while !fin && xp!=0 && yp!=0
      odd = !odd
      xp,xq = xq,xp
      ax = xp.div(xq)
      xp -= ax*xq

      yp,yq = yq,yp
      ay = yp.div(yq)
      yp -= ay*yq

      if ax!=ay
        fin = true
        ax,xp,xq = ay,yp,yq if odd
      end
      a << ax # .to_i
    end
    a[-1] += 1 if xp!=0 && a.size>0
    p,q = 1,0
    (1..a.size).each{|i| p,q=q+p*a[-i],p}
    [p, q]
  end
end