Avoid undesired promotions in entropy calculations (#925)

* Avoid undesired promotions in `entropy` calculations

* Clean file

Author: devmotion

src/scalarstats.jl

@@ -740,7 +740,13 @@ function entropy(p)
     return -sum(xlogx, p)
 end
 
-entropy(p, b::Real) = entropy(p) / log(b)
+function entropy(p, b::Real)
+    e = entropy(p)
+    # Promote explicitly before applying `log` to avoid undesired promotions
+    # with `log(b)::Float64` arising from `b::Int` (ref: #924)
+    _b = first(promote(b, e))
+    return e / log(_b)
+end
 
 """
     renyientropy(p, α)

test/scalarstats.jl

@@ -260,6 +260,10 @@ it = (xᵢ for xᵢ in x)
 @test @inferred(entropy([1//2, 1//2], 2))    ≈ 1.0
 @test @inferred(entropy([0.2, 0.3, 0.5], 2)) ≈ 1.4854752972273344
 
+# issue #924
+@test @inferred(entropy([0.5f0, 0.5f0], 2)) isa Float32
+@test @inferred(entropy([0.5f0, 0.5f0], MathConstants.e)) isa Float32
+
 @test_throws ArgumentError @inferred(entropy(Float64[]))
 @test_throws ArgumentError @inferred(entropy(Int[]))