Skip to content

Fix inaccurate calculation (last digit) and add a workaround for add/sub hang bug #465

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
tompng merged 1 commit into from
Dec 11, 2025
Merged

Fix inaccurate calculation (last digit) and add a workaround for add/sub hang bug #465

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
31 changes: 23 additions & 8 deletions lib/bigdecimal/math.rb
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -563,13 +563,23 @@ def expm1(x, prec)
if x < -1
# log10(exp(x)) = x * log10(e)
lg_e = 0.4342944819032518
exp_prec = prec + (lg_e * x).ceil + 2
exp_prec = prec + (lg_e * x).ceil + BigDecimal.double_fig
elsif x < 1
exp_prec = prec - x.exponent + 2
exp_prec = prec - x.exponent + BigDecimal.double_fig
else
exp_prec = prec
end
exp_prec > 0 ? exp(x, exp_prec).sub(1, prec) : BigDecimal(-1)

return BigDecimal(-1) if exp_prec <= 0

exp = exp(x, exp_prec)

if exp.exponent > prec + BigDecimal.double_fig
# Workaroudn for https://github.com/ruby/bigdecimal/issues/464
exp
else
exp.sub(1, prec)
end
end

# erf(decimal, numeric) -> BigDecimal
Expand All @@ -596,7 +606,12 @@ def erf(x, prec)
log10_erfc = -xf ** 2 / Math.log(10) - Math.log10(xf * Math::PI ** 0.5)
erfc_prec = [prec + log10_erfc.ceil, 1].max
erfc = _erfc_asymptotic(x, erfc_prec)
return BigDecimal(1).sub(erfc, prec) if erfc
if erfc
# Workaround for https://github.com/ruby/bigdecimal/issues/464
return BigDecimal(1) if erfc.exponent < -prec - BigDecimal.double_fig

return BigDecimal(1).sub(erfc, prec)
end
end

prec2 = prec + BigDecimal.double_fig
Expand All @@ -623,7 +638,7 @@ def erfc(x, prec)
x = BigDecimal::Internal.coerce_to_bigdecimal(x, prec, :erfc)
return BigDecimal::Internal.nan_computation_result if x.nan?
return BigDecimal(1 - x.infinite?) if x.infinite?
return BigDecimal(1).sub(erf(x, prec), prec) if x < 0
return BigDecimal(1).sub(erf(x, prec + BigDecimal.double_fig), prec) if x < 0
return BigDecimal(0) if x > 5000000000 # erfc(5000000000) < 1e-10000000000000000000 (underflow)

if x >= 8
Expand Down Expand Up @@ -714,12 +729,12 @@ def gamma(x, prec)
x = BigDecimal::Internal.coerce_to_bigdecimal(x, prec, :gamma)
prec2 = prec + BigDecimal.double_fig
if x < 0.5
raise Math::DomainError 'Numerical argument is out of domain - gamma' if x.frac.zero?
raise Math::DomainError, 'Numerical argument is out of domain - gamma' if x.frac.zero?

# Euler's reflection formula: gamma(z) * gamma(1-z) = pi/sin(pi*z)
pi = PI(prec2)
sin = _sinpix(x, pi, prec2)
return pi.div(gamma(1 - x, prec).mult(sin, prec2), prec)
return pi.div(gamma(1 - x, prec2).mult(sin, prec2), prec)
elsif x.frac.zero? && x < 1000 * prec
return _gamma_positive_integer(x, prec2).mult(1, prec)
end
Expand Down Expand Up @@ -747,7 +762,7 @@ def lgamma(x, prec)
# Euler's reflection formula: gamma(z) * gamma(1-z) = pi/sin(pi*z)
pi = PI(prec2)
sin = _sinpix(x, pi, prec2)
log_gamma = BigMath.log(pi, prec2).sub(lgamma(1 - x, prec).first + BigMath.log(sin.abs, prec2), prec)
log_gamma = BigMath.log(pi, prec2).sub(lgamma(1 - x, prec2).first + BigMath.log(sin.abs, prec2), prec)
[log_gamma, sin > 0 ? 1 : -1]
elsif x.frac.zero? && x < 1000 * prec
log_gamma = BigMath.log(_gamma_positive_integer(x, prec2), prec)
Expand Down
2 changes: 1 addition & 1 deletion test/bigdecimal/helper.rb
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -53,7 +53,7 @@ def assert_converge_in_precision(&block)
[50, 100, 150].each do |n|
value = yield(n)
assert(value != expected, "Unable to estimate precision for exact value")
assert_in_exact_precision(expected, value, n)
assert_equal(expected.mult(1, n), value)
end
end

Expand Down
3 changes: 3 additions & 0 deletions test/bigdecimal/test_bigmath.rb
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -476,6 +476,7 @@ def test_expm1
assert_in_exact_precision(exp(-3, 100) - 1, expm1(BigDecimal("-3"), 100), 100)
assert_in_exact_precision(exp(BigDecimal("1.23e-10"), 120) - 1, expm1(BigDecimal("1.23e-10"), 100), 100)
assert_in_exact_precision(exp(123, 120) - 1, expm1(BigDecimal("123"), 100), 100)
assert_equal(exp(BigDecimal("1e+12"), N), expm1(BigDecimal("1e+12"), N))
end

def test_erf
Expand All @@ -486,6 +487,7 @@ def test_erf
assert_equal(-1, BigMath.erf(MINF, N))
assert_equal(1, BigMath.erf(BigDecimal(1000), 100))
assert_equal(-1, BigMath.erf(BigDecimal(-1000), 100))
assert_equal(1, BigMath.erf(BigDecimal(10000000), 100))
assert_not_equal(1, BigMath.erf(BigDecimal(10), 45))
assert_not_equal(1, BigMath.erf(BigDecimal(15), 100))
assert_equal(1, BigMath.erf(BigDecimal('1e400'), 10))
Expand Down Expand Up @@ -524,6 +526,7 @@ def test_erfc
BigMath.erfc(BigDecimal("40"), 100)
)
assert_converge_in_precision {|n| BigMath.erfc(BigDecimal(30), n) }
assert_converge_in_precision {|n| BigMath.erfc(BigDecimal(-2), n) }
assert_converge_in_precision {|n| BigMath.erfc(30 * SQRT2, n) }
assert_converge_in_precision {|n| BigMath.erfc(BigDecimal(50), n) }
assert_converge_in_precision {|n| BigMath.erfc(BigDecimal(60000), n) }
Expand Down
Loading