over_ring (#450)

* over_ring

* change name; special case factored polynomial

Author: jverzani

Project.toml

@@ -2,7 +2,7 @@ name = "Polynomials"
 uuid = "f27b6e38-b328-58d1-80ce-0feddd5e7a45"
 license = "MIT"
 author = "JuliaMath"
-version = "3.2.0"
+version = "3.2.1"
 
 [deps]
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"

src/common.jl

@@ -465,6 +465,17 @@ function _eltype(P::Type{<:AbstractPolynomial}, p::AbstractPolynomial)
     T = isnothing(T′) ? eltype(p) : T′
     T
 end
+
+"""
+    copy_with_eltype(T, [X], p::AbstractPolynomial)
+
+Copy polynomial `p` changing the underlying element type and optionally the symbol.
+"""
+function copy_with_eltype(::Type{T}, X, p::P) where {T, S, Y, P<:AbstractPolynomial{S, Y}}
+    ⟒(P){T, X}(p.coeffs)
+end
+copy_with_eltype(::Type{T}, p::P) where {T, S, X, P<:AbstractPolynomial{S,X}} = copy_with_eltype(T, X, p)
+
 Base.iszero(p::AbstractPolynomial) = all(iszero, p)
 
 

src/polynomials/factored_polynomial.jl

@@ -119,6 +119,11 @@ function Base.convert(P::Type{<:FactoredPolynomial}, p::Polynomial{T,X}) where {
     ⟒(P)(coeffs(p), X)
 end
 
+function copy_with_eltype(::Type{T}, X, p::P) where {T, S, Y, P<:FactoredPolynomial{S, Y}}
+    d = convert(Dict{T, Int}, p.coeffs)
+    FactoredPolynomial{T, X}(d)
+end
+
 ## ----
 ## apply map to factors and the leading coefficient, not the coefficients
 function Base.map(fn, p::P, args...)  where {T,X,P<:FactoredPolynomial{T,X}}

test/StandardBasis.jl

@@ -794,6 +794,16 @@ end
         end
         @test convert(FactoredPolynomial, p) ≈ p
     end
+
+    # reinterpret coefficients
+    for P in (ImmutablePolynomial, Polynomial, SparsePolynomial, LaurentPolynomial, FactoredPolynomial)
+        for T in (Float64, Rational)
+            xs = [1,2,3]
+            p = fromroots(P,xs)
+            @test Polynomials.copy_with_eltype(T, p) == fromroots(P, T.(xs))
+            @test Polynomials.copy_with_eltype(T, :u, p) == fromroots(P, T.(xs); var=:u)
+        end
+    end
 end
 
 @testset "Roots" begin