fix accuracy of logit

src/basicfuns.jl

@@ -120,7 +120,13 @@ for ``0 < x < 1``.
 
 Its inverse is the [`logistic`](@ref) function.
 """
-logit(x::Real) = log(x / (one(x) - x))
+function logit(x::Real)
+    if 4 * x < 1
+        -log(inv(x) - 1)
+    else
+        2 * atanh(2*x - 1)
+    end
+end
 
 """
 $(SIGNATURES)

test/basicfuns.jl

@@ -86,6 +86,11 @@ end
     @test logistic(+750.0) === 1.0
     @test iszero(logit(0.5))
     @test logit(logistic(2)) ≈ 2.0
+    @testset "accuracy of `logit`" begin
+        for t in (Float16, Float32, Float64)
+            @test 2 * ulp_error_maximum(logit, range(start = t(0), stop = t(1), length = 500)) < 3
+        end
+    end
 end
 
 @testset "logcosh and logabssinh" begin

test/common/ULPError.jl

@@ -0,0 +1,51 @@
+module ULPError
+
+export ulp_error, ulp_error_maximum
+
+function ulp_error(accurate::AbstractFloat, approximate::AbstractFloat)
+    # the ULP error is usually not required to great accuracy, so `Float32` should be precise enough
+    zero_return = 0f0
+    inf_return = Inf32
+    # handle floating-point edge cases
+    if !(isfinite(accurate) && isfinite(approximate))
+        accur_is_nan = isnan(accurate)
+        approx_is_nan = isnan(approximate)
+        if accur_is_nan || approx_is_nan
+            return if accur_is_nan === approx_is_nan
+                zero_return
+            else
+                inf_return
+            end
+        end
+        if isinf(approximate)
+            return if isinf(accurate) && (signbit(accurate) == signbit(approximate))
+                zero_return
+            else
+                inf_return
+            end
+        end
+    end
+    acc = if accurate isa Union{Float16, Float32}
+        # widen for better accuracy when doing so does not impact performance too much
+        widen(accurate)
+    else
+        accurate
+    end
+    abs(Float32((approximate - acc) / eps(approximate))::Float32)
+end
+
+function ulp_error(accurate, approximate, x::AbstractFloat)
+    acc = accurate(x)
+    app = approximate(x)
+    ulp_error(acc, app)
+end
+
+function ulp_error(func::Func, x::AbstractFloat) where {Func}
+    ulp_error(func ∘ BigFloat, func, x)
+end
+
+function ulp_error_maximum(func::Func, iterator) where {Func}
+    maximum(Base.Fix1(ulp_error, func), iterator)
+end
+
+end

test/runtests.jl

@@ -10,6 +10,9 @@ using OffsetArrays
 using Random
 using Test
 
+include("common/ULPError.jl")
+using .ULPError
+
 Random.seed!(1234)
 
 include("basicfuns.jl")