fix negative integer powers (#95)

Author: stevengj

src/DecFP.jl

@@ -494,6 +494,13 @@ function Base.sqrt(z::Complex{T}) where {T<:DecimalFloatingPoint}
     Complex(ξ,η)
 end
 
+# see issue #92 — the fallback power_by_squaring fails for p < 0, and this is more accurate:
+Base.:^(x::DecimalFloatingPoint, p::Integer) = x^oftype(x, p)
+@inline Base.literal_pow(::typeof(^), x::DecimalFloatingPoint, ::Val{0}) = one(x)
+@inline Base.literal_pow(::typeof(^), x::DecimalFloatingPoint, ::Val{1}) = x
+@inline Base.literal_pow(::typeof(^), x::DecimalFloatingPoint, ::Val{2}) = x*x
+@inline Base.literal_pow(::typeof(^), x::DecimalFloatingPoint, ::Val{3}) = x*x*x
+
 # used for next/prevfloat:
 const pinf128 = _parse(Dec128, "+Inf")
 const minf128 = _parse(Dec128, "-Inf")

test/runtests.jl

@@ -147,6 +147,16 @@ for T in (Dec32, Dec64, Dec128)
         @test xd == Tf(x) == T(Tf(x)) == Tf(xd)
     end
 
+    # issue #92
+    p = -2
+    @test T(2)^-2 == parse(T, "0.25") == T(2)^p
+
+    # exercise literal_pow
+    @test T(2)^0 === T(1)
+    @test T(2)^1 === T(2)
+    @test T(2)^2 === T(4)
+    @test T(2)^3 === T(8)
+
     @test trunc(T(2.7)) === floor(T(2.7)) === round(T(2.7), RoundDown) === round(T(2.7), RoundToZero) === T(2)
     @test ceil(T(2.3)) === round(T(2.3), RoundUp) === round(T(2.3), RoundFromZero) === T(3)
     @test round(T(1.5)) === round(T(2.5)) === round(T(1.5), RoundNearest) === round(T(2.5), RoundNearest) === T(2)