simplify complex atanh and remove singularity perturbation (JuliaLang#55268)

fixes JuliaLang#55266, and use `inv(z)`
rather than `1/z` and use `muladd` in a couple places.

---------

Co-authored-by: Mosè Giordano <[email protected]>

Author: 2 people

base/complex.jl

@@ -1037,24 +1037,22 @@ end
 function atanh(z::Complex{T}) where T
     z = float(z)
     Tf = float(T)
-    Ω = prevfloat(typemax(Tf))
-    θ = sqrt(Ω)/4
-    ρ = 1/θ
     x, y = reim(z)
     ax = abs(x)
     ay = abs(y)
+    θ = sqrt(floatmax(Tf))/4
     if ax > θ || ay > θ #Prevent overflow
         if isnan(y)
             if isinf(x)
                 return Complex(copysign(zero(x),x), y)
             else
-                return Complex(real(1/z), y)
+                return Complex(real(inv(z)), y)
             end
         end
         if isinf(y)
             return Complex(copysign(zero(x),x), copysign(oftype(y,pi)/2, y))
         end
-        return Complex(real(1/z), copysign(oftype(y,pi)/2, y))
+        return Complex(real(inv(z)), copysign(oftype(y,pi)/2, y))
     end
     β = copysign(one(Tf), x)
     z *= β
@@ -1064,16 +1062,15 @@ function atanh(z::Complex{T}) where T
             ξ = oftype(x, Inf)
             η = y
         else
-            ym = ay+ρ
-            ξ = log(sqrt(sqrt(4+y*y))/sqrt(ym))
-            η = copysign(oftype(y,pi)/2 + atan(ym/2), y)/2
+            ξ = log(sqrt(sqrt(muladd(y, y, 4)))/sqrt(ay))
+            η = copysign(oftype(y,pi)/2 + atan(ay/2), y)/2
         end
     else #Normal case
-        ysq = (ay+ρ)^2
+        ysq = ay^2
         if x == 0
             ξ = x
         else
-            ξ = log1p(4x/((1-x)^2 + ysq))/4
+            ξ = log1p(4x/(muladd(1-x, 1-x, ysq)))/4
         end
         η = angle(Complex((1-x)*(1+x)-ysq, 2y))/2
     end

test/complex.jl

@@ -1215,3 +1215,9 @@ end
     @test !iseven(7+0im) && isodd(7+0im)
     @test !iseven(6+1im) && !isodd(7+1im)
 end
+
+@testset "issue #55266" begin
+    for T in (Float16, Float32, Float64)
+        @test isapprox(atanh(1+im*floatmin(T)), Complex{T}(atanh(1+im*big(floatmin(T)))))
+    end
+end