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Resolve NaN in hypot near zero #669

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Resolve NaN in hypot near zero #669

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9 changes: 8 additions & 1 deletion src/dual.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -587,11 +587,18 @@ end
#-------#

@inline function calc_hypot(x, y, z, ::Type{T}) where T
pm1(x) = signbit(x) ? -1 : 1

vx = value(x)
vy = value(y)
vz = value(z)
h = hypot(vx, vy, vz)
p = (vx / h) * partials(x) + (vy / h) * partials(y) + (vz / h) * partials(z)
p = if iszero(h)
hp = sqrt(sum(abs2, partials(x)) + sum(abs2, partials(y)) + sum(abs2, partials(z)))
pm1(vx) * partials(x) / hp + pm1(vy) * partials(y) / hp + pm1(vz) * partials(z) / hp
else
(vx / h) * partials(x) + (vy / h) * partials(y) + (vz / h) * partials(z)
end
return Dual{T}(h, p)
end

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3 changes: 3 additions & 0 deletions test/MiscTest.jl
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Expand Up @@ -140,6 +140,9 @@ let i, j
i != j && @test h[i, j] ≈ 0.0
end
end
hypotx(x) = hypot(x, 0.0, 0.0)
@test ForwardDiff.derivative(hypotx, 0.0) ≈ 1
@test ForwardDiff.derivative(hypotx, -0.0) ≈ -1

########
# misc #
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