Class: Float

Inherits:
Object
  • Object
show all
Defined in:
lib/astro-algo.rb

Instance Method Summary collapse

Instance Method Details

#to_r ⇒ Object

Convert Float to Rational. Algorithm from Dave Burt: blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/142199



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# File 'lib/astro-algo.rb', line 30

def to_r
    return Rational(0, 1) if self == 0.0
    x = self
    negative = false
    if x < 0.0
        x = -x
        negative = true
    end
    f, e = Math.frexp(x)
    # raise unless 0.5 <= f and f < 1.0
    # x = f * 2**e exactly

    # Suck up _chunk_ bits at a time; 28 is enough so that we suck
    # up all bits in 2 iterations for all known binary double-
    # precision formats, and small enough to fit in an int.
    chunk = 28
    top = 0
    # invariant: x = (top + f) * 2**e exactly
    while f > 0.0
        f = Math.ldexp(f, chunk)
        digit = f.to_i
        raise unless digit >> chunk == 0
        top = (top << chunk) | digit
        f -= digit
        # raise unless 0.0 <= f and f < 1.0
        e -= chunk
    end
    # raise if top == 0

    # now x = top * 2**e exactly; fold in 2**e
    r = Rational(top, 1)
    if e > 0
        r *= 2**e
    else
        r /= 2**-e
    end
    negative ? -r : r
end