Module: BigMath

Defined in:
lib/bigdecimal.rb,
lib/bigdecimal/math.rb

Overview

– Contents:

sqrt(x, prec)
sin (x, prec)
cos (x, prec)
tan (x, prec)
atan(x, prec)
PI  (prec)
E   (prec) == exp(1.0,prec)

where:

x    ... BigDecimal number to be computed.
prec ... Number of digits to be obtained.

++

Provides mathematical functions.

Example:

require "bigdecimal/math"

include BigMath

a = BigDecimal((PI(49)/2).to_s)
puts sin(a,100) # => 0.9999999999...9999999986e0

Class Method Summary collapse

Class Method Details

.atan(x, prec) ⇒ Object

call-seq:

atan(decimal, numeric) -> BigDecimal

Computes the arctangent of decimal to the specified number of digits of precision, numeric.

If decimal is NaN, returns NaN.

BigMath.atan(BigDecimal('-1'), 32).to_s
#=> "-0.78539816339744830961566084581988e0"



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# File 'lib/bigdecimal/math.rb', line 162

def atan(x, prec)
  prec = BigDecimal::Internal.coerce_validate_prec(prec, :atan)
  x = BigDecimal::Internal.coerce_to_bigdecimal(x, prec, :atan)
  return BigDecimal::Internal.nan_computation_result if x.nan?
  n = prec + BigDecimal.double_fig
  pi = PI(n)
  x = -x if neg = x < 0
  return pi.div(neg ? -2 : 2, prec) if x.infinite?
  return pi.div(neg ? -4 : 4, prec) if x.round(prec) == 1
  x = BigDecimal("1").div(x, n) if inv = x > 1
  x = (-1 + sqrt(1 + x.mult(x, n), n)).div(x, n) if dbl = x > 0.5
  y = x
  d = y
  t = x
  r = BigDecimal("3")
  x2 = x.mult(x,n)
  while d.nonzero? && ((m = n - (y.exponent - d.exponent).abs) > 0)
    m = BigDecimal.double_fig if m < BigDecimal.double_fig
    t = -t.mult(x2,n)
    d = t.div(r,m)
    y += d
    r += 2
  end
  y *= 2 if dbl
  y = pi / 2 - y if inv
  y = -y if neg
  y.mult(1, prec)
end

.cos(x, prec) ⇒ Object

call-seq:

cos(decimal, numeric) -> BigDecimal

Computes the cosine of decimal to the specified number of digits of precision, numeric.

If decimal is Infinity or NaN, returns NaN.

BigMath.cos(BigMath.PI(16), 32).to_s
#=> "-0.99999999999999999999999999999997e0"



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# File 'lib/bigdecimal/math.rb', line 124

def cos(x, prec)
  prec = BigDecimal::Internal.coerce_validate_prec(prec, :cos)
  x = BigDecimal::Internal.coerce_to_bigdecimal(x, prec, :cos)
  return BigDecimal::Internal.nan_computation_result if x.infinite? || x.nan?
  sign, x = _sin_periodic_reduction(x, prec + BigDecimal.double_fig, add_half_pi: true)
  sign * sin(x, prec)
end

.E(prec) ⇒ Object

call-seq:

E(numeric) -> BigDecimal

Computes e (the base of natural logarithms) to the specified number of digits of precision, numeric.

BigMath.E(32).to_s
#=> "0.27182818284590452353602874713527e1"



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# File 'lib/bigdecimal/math.rb', line 245

def E(prec)
  prec = BigDecimal::Internal.coerce_validate_prec(prec, :E)
  BigMath.exp(1, prec)
end

.exp(x, prec) ⇒ Object

call-seq:

BigMath.exp(decimal, numeric)    -> BigDecimal

Computes the value of e (the base of natural logarithms) raised to the power of decimal, to the specified number of digits of precision.

If decimal is infinity, returns Infinity.

If decimal is NaN, returns NaN.



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# File 'lib/bigdecimal.rb', line 328

def self.exp(x, prec)
  prec = BigDecimal::Internal.coerce_validate_prec(prec, :exp)
  x = BigDecimal::Internal.coerce_to_bigdecimal(x, prec, :exp)
  return BigDecimal::Internal.nan_computation_result if x.nan?
  return x.positive? ? BigDecimal::Internal.infinity_computation_result : BigDecimal(0) if x.infinite?
  return BigDecimal(1) if x.zero?

  # exp(x * 10**cnt) = exp(x)**(10**cnt)
  cnt = x < -1 || x > 1 ? x.exponent : 0
  prec2 = prec + BigDecimal.double_fig + cnt
  x = x._decimal_shift(-cnt)

  # Calculation of exp(small_prec) is fast because calculation of x**n is fast
  # Calculation of exp(small_abs) converges fast.
  # exp(x) = exp(small_prec_part + small_abs_part) = exp(small_prec_part) * exp(small_abs_part)
  x_small_prec = x.round(Integer.sqrt(prec2))
  y = _exp_taylor(x_small_prec, prec2).mult(_exp_taylor(x.sub(x_small_prec, prec2), prec2), prec2)

  # calculate exp(x * 10**cnt) from exp(x)
  # exp(x * 10**k) = exp(x * 10**(k - 1)) ** 10
  cnt.times do
    y2 = y.mult(y, prec2)
    y5 = y2.mult(y2, prec2).mult(y, prec2)
    y = y5.mult(y5, prec2)
  end

  y.mult(1, prec)
end

.log(x, prec) ⇒ Object

call-seq:

BigMath.log(decimal, numeric)    -> BigDecimal

Computes the natural logarithm of decimal to the specified number of digits of precision, numeric.

If decimal is zero or negative, raises Math::DomainError.

If decimal is positive infinity, returns Infinity.

If decimal is NaN, returns NaN.

Raises:

  • (Math::DomainError) 


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# File 'lib/bigdecimal.rb', line 251

def self.log(x, prec)
  prec = BigDecimal::Internal.coerce_validate_prec(prec, :log)
  raise Math::DomainError, 'Complex argument for BigMath.log' if Complex === x

  x = BigDecimal::Internal.coerce_to_bigdecimal(x, prec, :log)
  return BigDecimal::Internal.nan_computation_result if x.nan?
  raise Math::DomainError, 'Negative argument for log' if x < 0
  return -BigDecimal::Internal.infinity_computation_result if x.zero?
  return BigDecimal::Internal.infinity_computation_result if x.infinite?
  return BigDecimal(0) if x == 1

  prec2 = prec + BigDecimal.double_fig
  BigDecimal.save_limit do
    BigDecimal.limit(0)
    if x > 10 || x < 0.1
      log10 = log(BigDecimal(10), prec2)
      exponent = x.exponent
      x = x._decimal_shift(-exponent)
      if x < 0.3
        x *= 10
        exponent -= 1
      end
      return (log10 * exponent).add(log(x, prec2), prec)
    end

    x_minus_one_exponent = (x - 1).exponent

    # log(x) = log(sqrt(sqrt(sqrt(sqrt(x))))) * 2**sqrt_steps
    sqrt_steps = [Integer.sqrt(prec2) + 3 * x_minus_one_exponent, 0].max

    lg2 = 0.3010299956639812
    sqrt_prec = prec2 + [-x_minus_one_exponent, 0].max + (sqrt_steps * lg2).ceil

    sqrt_steps.times do
      x = x.sqrt(sqrt_prec)
    end

    # Taylor series for log(x) around 1
    # log(x) = -log((1 + X) / (1 - X)) where X = (x - 1) / (x + 1)
    # log(x) = 2 * (X + X**3 / 3 + X**5 / 5 + X**7 / 7 + ...)
    x = (x - 1).div(x + 1, sqrt_prec)
    y = x
    x2 = x.mult(x, prec2)
    1.step do |i|
      n = prec2 + x.exponent - y.exponent + x2.exponent
      break if n <= 0 || x.zero?
      x = x.mult(x2.round(n - x2.exponent), n)
      y = y.add(x.div(2 * i + 1, n), prec2)
    end

    y.mult(2 ** (sqrt_steps + 1), prec)
  end
end

.PI(prec) ⇒ Object

call-seq:

PI(numeric) -> BigDecimal

Computes the value of pi to the specified number of digits of precision, numeric.

BigMath.PI(32).to_s
#=> "0.31415926535897932384626433832795e1"



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# File 'lib/bigdecimal/math.rb', line 200

def PI(prec)
  prec = BigDecimal::Internal.coerce_validate_prec(prec, :PI)
  n      = prec + BigDecimal.double_fig
  zero   = BigDecimal("0")
  one    = BigDecimal("1")
  two    = BigDecimal("2")

  m25    = BigDecimal("-0.04")
  m57121 = BigDecimal("-57121")

  pi     = zero

  d = one
  k = one
  t = BigDecimal("-80")
  while d.nonzero? && ((m = n - (pi.exponent - d.exponent).abs) > 0)
    m = BigDecimal.double_fig if m < BigDecimal.double_fig
    t   = t*m25
    d   = t.div(k,m)
    k   = k+two
    pi  = pi + d
  end

  d = one
  k = one
  t = BigDecimal("956")
  while d.nonzero? && ((m = n - (pi.exponent - d.exponent).abs) > 0)
    m = BigDecimal.double_fig if m < BigDecimal.double_fig
    t   = t.div(m57121,n)
    d   = t.div(k,m)
    pi  = pi + d
    k   = k+two
  end
  pi.mult(1, prec)
end

.sin(x, prec) ⇒ Object

call-seq:

sin(decimal, numeric) -> BigDecimal

Computes the sine of decimal to the specified number of digits of precision, numeric.

If decimal is Infinity or NaN, returns NaN.

BigMath.sin(BigMath.PI(5)/4, 32).to_s
#=> "0.70710807985947359435812921837984e0"



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# File 'lib/bigdecimal/math.rb', line 87

def sin(x, prec)
  prec = BigDecimal::Internal.coerce_validate_prec(prec, :sin)
  x = BigDecimal::Internal.coerce_to_bigdecimal(x, prec, :sin)
  return BigDecimal::Internal.nan_computation_result if x.infinite? || x.nan?
  n    = prec + BigDecimal.double_fig
  one  = BigDecimal("1")
  two  = BigDecimal("2")
  sign, x = _sin_periodic_reduction(x, n)
  x1   = x
  x2   = x.mult(x,n)
  y    = x
  d    = y
  i    = one
  z    = one
  while d.nonzero? && ((m = n - (y.exponent - d.exponent).abs) > 0)
    m = BigDecimal.double_fig if m < BigDecimal.double_fig
    x1  = -x2.mult(x1,n)
    i  += two
    z  *= (i-one) * i
    d   = x1.div(z,m)
    y  += d
  end
  y = BigDecimal("1") if y > 1
  y.mult(sign, prec)
end

.sqrt(x, prec) ⇒ Object

call-seq:

sqrt(decimal, numeric) -> BigDecimal

Computes the square root of decimal to the specified number of digits of precision, numeric.

BigMath.sqrt(BigDecimal('2'), 32).to_s
#=> "0.14142135623730950488016887242097e1"



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# File 'lib/bigdecimal/math.rb', line 43

def sqrt(x, prec)
  prec = BigDecimal::Internal.coerce_validate_prec(prec, :sqrt)
  x = BigDecimal::Internal.coerce_to_bigdecimal(x, prec, :sqrt)
  x.sqrt(prec)
end

.tan(x, prec) ⇒ Object

call-seq:

tan(decimal, numeric) -> BigDecimal

Computes the tangent of decimal to the specified number of digits of precision, numeric.

If decimal is Infinity or NaN, returns NaN.

BigMath.tan(BigDecimal("0.0"), 4).to_s
#=> "0.0"

BigMath.tan(BigMath.PI(24) / 4, 32).to_s
#=> "0.99999999999999999999999830836025e0"



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# File 'lib/bigdecimal/math.rb', line 146

def tan(x, prec)
  prec = BigDecimal::Internal.coerce_validate_prec(prec, :tan)
  sin(x, prec + BigDecimal.double_fig).div(cos(x, prec + BigDecimal.double_fig), prec)
end