# This file is a part of Julia. License is MIT: https://julialang.org/license

module TestTridiagonal

using Test, LinearAlgebra, SparseArrays, Random

include("testutils.jl") # test_approx_eq_modphase

#Test equivalence of eigenvectors/singular vectors taking into account possible phase (sign) differences
function test_approx_eq_vecs(a::StridedVecOrMat{S}, b::StridedVecOrMat{T}, error=nothing) where {S<:Real,T<:Real}
    n = size(a, 1)
    @test n==size(b,1) && size(a,2)==size(b,2)
    error===nothing && (error=n^3*(eps(S)+eps(T)))
    for i=1:n
        ev1, ev2 = a[:,i], b[:,i]
        deviation = min(abs(norm(ev1-ev2)),abs(norm(ev1+ev2)))
        if !isnan(deviation)
            @test deviation ≈ 0.0 atol=error
        end
    end
end

@testset for elty in (Float32, Float64, ComplexF32, ComplexF64, Int)
    n = 12 #Size of matrix problem to test
    Random.seed!(123)
    if elty == Int
        Random.seed!(61516384)
        d = rand(1:100, n)
        dl = -rand(0:10, n-1)
        du = -rand(0:10, n-1)
        v = rand(1:100, n)
        B = rand(1:100, n, 2)
        a = rand(1:100, n-1)
        b = rand(1:100, n)
        c = rand(1:100, n-1)
    else
        d = convert(Vector{elty}, 1 .+ randn(n))
        dl = convert(Vector{elty}, randn(n - 1))
        du = convert(Vector{elty}, randn(n - 1))
        v = convert(Vector{elty}, randn(n))
        B = convert(Matrix{elty}, randn(n, 2))
        a = convert(Vector{elty}, randn(n - 1))
        b = convert(Vector{elty}, randn(n))
        c = convert(Vector{elty}, randn(n - 1))
        if elty <: Complex
            a += im*convert(Vector{elty}, randn(n - 1))
            b += im*convert(Vector{elty}, randn(n))
            c += im*convert(Vector{elty}, randn(n - 1))
        end
    end
    @test_throws DimensionMismatch SymTridiagonal(dl, fill(elty(1), n+1))
    @test_throws ArgumentError SymTridiagonal(rand(n, n))
    @test_throws ArgumentError Tridiagonal(dl, dl, dl)
    @test_throws ArgumentError convert(SymTridiagonal{elty}, Tridiagonal(dl, d, du))

    if elty != Int
        @testset "issue #1490" begin
            @test det(fill(elty(1),3,3)) ≈ zero(elty) atol=3*eps(real(one(elty)))
            @test det(SymTridiagonal(elty[],elty[])) == one(elty)
        end
    end

    @testset "constructor" begin
        for (x, y) in ((d, dl), (GenericArray(d), GenericArray(dl)))
            ST = (SymTridiagonal(x, y))::SymTridiagonal{elty, typeof(x)}
            @test ST == Matrix(ST)
            @test ST.dv === x
            @test ST.ev === y
            TT = (Tridiagonal(y, x, y))::Tridiagonal{elty, typeof(x)}
            @test TT == Matrix(TT)
            @test TT.dl === y
            @test TT.d  === x
            @test TT.du === y
        end
        ST = SymTridiagonal{elty}([1,2,3,4], [1,2,3])
        @test eltype(ST) == elty
        TT = Tridiagonal{elty}([1,2,3], [1,2,3,4], [1,2,3])
        @test eltype(TT) == elty
        ST = SymTridiagonal{elty,Vector{elty}}(d, GenericArray(dl))
        @test isa(ST, SymTridiagonal{elty,Vector{elty}})
        TT = Tridiagonal{elty,Vector{elty}}(GenericArray(dl), d, GenericArray(dl))
        @test isa(TT, Tridiagonal{elty,Vector{elty}})
        @test_throws MethodError SymTridiagonal(d, GenericArray(dl))
        @test_throws MethodError SymTridiagonal(GenericArray(d), dl)
        @test_throws MethodError Tridiagonal(GenericArray(dl), d, GenericArray(dl))
        @test_throws MethodError Tridiagonal(dl, GenericArray(d), dl)
        @test_throws MethodError SymTridiagonal{elty}(d, GenericArray(dl))
        @test_throws MethodError Tridiagonal{elty}(GenericArray(dl), d,GenericArray(dl))
        STI = SymTridiagonal([1,2,3,4], [1,2,3])
        TTI = Tridiagonal([1,2,3], [1,2,3,4], [1,2,3])
        TTI2 = Tridiagonal([1,2,3], [1,2,3,4], [1,2,3], [1,2])
        @test SymTridiagonal(STI) === STI
        @test Tridiagonal(TTI)    === TTI
        @test Tridiagonal(TTI2)   === TTI2
        @test isa(SymTridiagonal{elty}(STI), SymTridiagonal{elty})
        @test isa(Tridiagonal{elty}(TTI), Tridiagonal{elty})
        TTI2y = Tridiagonal{elty}(TTI2)
        @test isa(TTI2y, Tridiagonal{elty})
        @test TTI2y.du2 == convert(Vector{elty}, [1,2])
    end
    @testset "interconversion of Tridiagonal and SymTridiagonal" begin
        @test Tridiagonal(dl, d, dl) == SymTridiagonal(d, dl)
        @test SymTridiagonal(d, dl) == Tridiagonal(dl, d, dl)
        @test Tridiagonal(dl, d, du) + Tridiagonal(du, d, dl) == SymTridiagonal(2d, dl+du)
        @test SymTridiagonal(d, dl) + Tridiagonal(dl, d, du) == Tridiagonal(dl + dl, d+d, dl+du)
        @test convert(SymTridiagonal,Tridiagonal(SymTridiagonal(d, dl))) == SymTridiagonal(d, dl)
        @test Array(convert(SymTridiagonal{ComplexF32},Tridiagonal(SymTridiagonal(d, dl)))) == convert(Matrix{ComplexF32}, SymTridiagonal(d, dl))
    end
    @testset "tril/triu" begin
        zerosd = fill!(similar(d), 0)
        zerosdl = fill!(similar(dl), 0)
        zerosdu = fill!(similar(du), 0)
        @test_throws ArgumentError tril!(SymTridiagonal(d, dl), -n - 2)
        @test_throws ArgumentError tril!(SymTridiagonal(d, dl), n)
        @test_throws ArgumentError tril!(Tridiagonal(dl, d, du), -n - 2)
        @test_throws ArgumentError tril!(Tridiagonal(dl, d, du), n)
        @test tril(SymTridiagonal(d,dl))    == Tridiagonal(dl,d,zerosdl)
        @test tril(SymTridiagonal(d,dl),1)  == Tridiagonal(dl,d,dl)
        @test tril(SymTridiagonal(d,dl),-1) == Tridiagonal(dl,zerosd,zerosdl)
        @test tril(SymTridiagonal(d,dl),-2) == Tridiagonal(zerosdl,zerosd,zerosdl)
        @test tril(Tridiagonal(dl,d,du))    == Tridiagonal(dl,d,zerosdu)
        @test tril(Tridiagonal(dl,d,du),1)  == Tridiagonal(dl,d,du)
        @test tril(Tridiagonal(dl,d,du),-1) == Tridiagonal(dl,zerosd,zerosdu)
        @test tril(Tridiagonal(dl,d,du),-2) == Tridiagonal(zerosdl,zerosd,zerosdu)

        @test_throws ArgumentError triu!(SymTridiagonal(d, dl), -n)
        @test_throws ArgumentError triu!(SymTridiagonal(d, dl), n + 2)
        @test_throws ArgumentError triu!(Tridiagonal(dl, d, du), -n)
        @test_throws ArgumentError triu!(Tridiagonal(dl, d, du), n + 2)
        @test triu(SymTridiagonal(d,dl))    == Tridiagonal(zerosdl,d,dl)
        @test triu(SymTridiagonal(d,dl),-1) == Tridiagonal(dl,d,dl)
        @test triu(SymTridiagonal(d,dl),1)  == Tridiagonal(zerosdl,zerosd,dl)
        @test triu(SymTridiagonal(d,dl),2)  == Tridiagonal(zerosdl,zerosd,zerosdl)
        @test triu(Tridiagonal(dl,d,du))    == Tridiagonal(zerosdl,d,du)
        @test triu(Tridiagonal(dl,d,du),-1) == Tridiagonal(dl,d,du)
        @test triu(Tridiagonal(dl,d,du),1)  == Tridiagonal(zerosdl,zerosd,du)
        @test triu(Tridiagonal(dl,d,du),2)  == Tridiagonal(zerosdl,zerosd,zerosdu)

        @test !istril(SymTridiagonal(d,dl))
        @test istril(SymTridiagonal(d,zerosdl))
        @test !istriu(SymTridiagonal(d,dl))
        @test istriu(SymTridiagonal(d,zerosdl))
        @test istriu(Tridiagonal(zerosdl,d,du))
        @test !istriu(Tridiagonal(dl,d,zerosdu))
        @test istril(Tridiagonal(dl,d,zerosdu))
        @test !istril(Tridiagonal(zerosdl,d,du))

        @test isdiag(SymTridiagonal(d,zerosdl))
        @test !isdiag(SymTridiagonal(d,dl))
        @test isdiag(Tridiagonal(zerosdl,d,zerosdu))
        @test !isdiag(Tridiagonal(dl,d,zerosdu))
        @test !isdiag(Tridiagonal(zerosdl,d,du))
        @test !isdiag(Tridiagonal(dl,d,du))
    end

    @testset "iszero and isone" begin
        Tzero = Tridiagonal(zeros(elty, 9), zeros(elty, 10), zeros(elty, 9))
        Tone = Tridiagonal(zeros(elty, 9), ones(elty, 10), zeros(elty, 9))
        Tmix = Tridiagonal(zeros(elty, 9), zeros(elty, 10), zeros(elty, 9))
        Tmix[end, end] = one(elty)

        Szero = SymTridiagonal(zeros(elty, 10), zeros(elty, 9))
        Sone = SymTridiagonal(ones(elty, 10), zeros(elty, 9))
        Smix = SymTridiagonal(zeros(elty, 10), zeros(elty, 9))
        Smix[end, end] = one(elty)

        @test iszero(Tzero)
        @test !isone(Tzero)
        @test !iszero(Tone)
        @test isone(Tone)
        @test !iszero(Tmix)
        @test !isone(Tmix)

        @test iszero(Szero)
        @test !isone(Szero)
        @test !iszero(Sone)
        @test isone(Sone)
        @test !iszero(Smix)
        @test !isone(Smix)
    end

    @testset for mat_type in (Tridiagonal, SymTridiagonal)
        A = mat_type == Tridiagonal ? mat_type(dl, d, du) : mat_type(d, dl)
        fA = map(elty <: Complex ? ComplexF64 : Float64, Array(A))
        @testset "similar, size, and copyto!" begin
            B = similar(A)
            @test size(B) == size(A)
            if mat_type == Tridiagonal # doesn't work for SymTridiagonal yet
                copyto!(B, A)
                @test B == A
            end
            @test isa(similar(A), mat_type{elty})
            @test isa(similar(A, Int), mat_type{Int})
            @test isa(similar(A, (3, 2)), SparseMatrixCSC)
            @test isa(similar(A, Int, (3, 2)), SparseMatrixCSC{Int})
            @test size(A, 3) == 1
            @test size(A, 1) == n
            @test size(A) == (n, n)
            @test_throws ArgumentError size(A, 0)
        end
        @testset "getindex" begin
            @test_throws BoundsError A[n + 1, 1]
            @test_throws BoundsError A[1, n + 1]
            @test A[1, n] == convert(elty, 0.0)
            @test A[1, 1] == d[1]
        end
        @testset "setindex!" begin
            @test_throws BoundsError A[n + 1, 1] = 0 # test bounds check
            @test_throws BoundsError A[1, n + 1] = 0 # test bounds check
            @test_throws ArgumentError A[1, 3]   = 1 # test assignment off the main/sub/super diagonal
            if mat_type == Tridiagonal
                @test (A[3, 3] = A[3, 3]; A == fA) # test assignment on the main diagonal
                @test (A[3, 2] = A[3, 2]; A == fA) # test assignment on the subdiagonal
                @test (A[2, 3] = A[2, 3]; A == fA) # test assignment on the superdiagonal
                @test ((A[1, 3] = 0) == 0; A == fA) # test zero assignment off the main/sub/super diagonal
            else # mat_type is SymTridiagonal
                @test ((A[3, 3] = A[3, 3]) == A[3, 3]; A == fA) # test assignment on the main diagonal
                @test_throws ArgumentError A[3, 2] = 1 # test assignment on the subdiagonal
                @test_throws ArgumentError A[2, 3] = 1 # test assignment on the superdiagonal
            end
        end
        @testset "diag" begin
            @test (@inferred diag(A))::typeof(d) == d
            @test (@inferred diag(A, 0))::typeof(d) == d
            @test (@inferred diag(A, 1))::typeof(d) == (mat_type == Tridiagonal ? du : dl)
            @test (@inferred diag(A, -1))::typeof(d) == dl
            @test (@inferred diag(A, n-1))::typeof(d) == zeros(elty, 1)
            @test_throws ArgumentError diag(A, -n - 1)
            @test_throws ArgumentError diag(A, n + 1)
            GA = mat_type == Tridiagonal ? mat_type(GenericArray.((dl, d, du))...) : mat_type(GenericArray.((d, dl))...)
            @test (@inferred diag(GA))::typeof(GenericArray(d)) == GenericArray(d)
            @test (@inferred diag(GA, -1))::typeof(GenericArray(d)) == GenericArray(dl)
        end
        @testset "Idempotent tests" begin
            for func in (conj, transpose, adjoint)
                @test func(func(A)) == A
            end
        end
        if elty != Int
            @testset "Simple unary functions" begin
                for func in (det, inv)
                    @test func(A) ≈ func(fA) atol=n^2*sqrt(eps(real(one(elty))))
                end
            end
        end
        ds = mat_type == Tridiagonal ? (dl, d, du) : (d, dl)
        for f in (real, imag)
            @test f(A)::mat_type == mat_type(map(f, ds)...)
        end
        if elty <: Real
            for f in (round, trunc, floor, ceil)
                fds = [f.(d) for d in ds]
                @test f.(A)::mat_type == mat_type(fds...)
                @test f.(Int, A)::mat_type == f.(Int, fA)
            end
        end
        fds = [abs.(d) for d in ds]
        @test abs.(A)::mat_type == mat_type(fds...)
        @testset "Multiplication with strided matrix/vector" begin
            @test (x = fill(1.,n); A*x ≈ Array(A)*x)
            @test (X = fill(1.,n,2); A*X ≈ Array(A)*X)
        end
        @testset "Binary operations" begin
            B = mat_type == Tridiagonal ? mat_type(a, b, c) : mat_type(b, a)
            fB = map(elty <: Complex ? ComplexF64 : Float64, Array(B))
            for op in (+, -, *)
                @test Array(op(A, B)) ≈ op(fA, fB)
            end
            α = rand(elty)
            @test Array(α*A) ≈ α*Array(A)
            @test Array(A*α) ≈ Array(A)*α
            @test Array(A/α) ≈ Array(A)/α

            @testset "Matmul with Triangular types" begin
                @test A*LinearAlgebra.UnitUpperTriangular(Matrix(1.0I, n, n)) ≈ fA
                @test A*LinearAlgebra.UnitLowerTriangular(Matrix(1.0I, n, n)) ≈ fA
                @test A*UpperTriangular(Matrix(1.0I, n, n)) ≈ fA
                @test A*LowerTriangular(Matrix(1.0I, n, n)) ≈ fA
            end
            @testset "mul! errors" begin
                Cnn, Cnm, Cmn = Matrix{elty}.(undef, ((n,n), (n,n+1), (n+1,n)))
                @test_throws DimensionMismatch LinearAlgebra.mul!(Cnn,A,Cnm)
                @test_throws DimensionMismatch LinearAlgebra.mul!(Cnn,A,Cmn)
                @test_throws DimensionMismatch LinearAlgebra.mul!(Cnn,B,Cmn)
                @test_throws DimensionMismatch LinearAlgebra.mul!(Cmn,B,Cnn)
                @test_throws DimensionMismatch LinearAlgebra.mul!(Cnm,B,Cnn)
            end
        end
        if mat_type == SymTridiagonal
            @testset "Tridiagonal/SymTridiagonal mixing ops" begin
                B = convert(Tridiagonal{elty}, A)
                @test B == A
                @test B + A == A + B
                @test B - A == A - B
            end
            if elty <: LinearAlgebra.BlasReal
                @testset "Eigensystems" begin
                    zero, infinity = convert(elty, 0), convert(elty, Inf)
                    @testset "stebz! and stein!" begin
                        w, iblock, isplit = LAPACK.stebz!('V', 'B', -infinity, infinity, 0, 0, zero, b, a)
                        evecs = LAPACK.stein!(b, a, w)

                        (e, v) = eigen(SymTridiagonal(b, a))
                        @test e ≈ w
                        test_approx_eq_vecs(v, evecs)
                    end
                    @testset "stein! call using iblock and isplit" begin
                        w, iblock, isplit = LAPACK.stebz!('V', 'B', -infinity, infinity, 0, 0, zero, b, a)
                        evecs = LAPACK.stein!(b, a, w, iblock, isplit)
                        test_approx_eq_vecs(v, evecs)
                    end
                    @testset "stegr! call with index range" begin
                        F = eigen(SymTridiagonal(b, a),1:2)
                        fF = eigen(Symmetric(Array(SymTridiagonal(b, a))),1:2)
                        test_approx_eq_modphase(F.vectors, fF.vectors)
                        @test F.values ≈ fF.values
                    end
                    @testset "stegr! call with value range" begin
                        F = eigen(SymTridiagonal(b, a),0.0,1.0)
                        fF = eigen(Symmetric(Array(SymTridiagonal(b, a))),0.0,1.0)
                        test_approx_eq_modphase(F.vectors, fF.vectors)
                        @test F.values ≈ fF.values
                    end
                    @testset "eigenvalues/eigenvectors of symmetric tridiagonal" begin
                        if elty === Float32 || elty === Float64
                            DT, VT = @inferred eigen(A)
                            @inferred eigen(A, 2:4)
                            @inferred eigen(A, 1.0, 2.0)
                            D, Vecs = eigen(fA)
                            @test DT ≈ D
                            @test abs.(VT'Vecs) ≈ Matrix(elty(1)I, n, n)
                            test_approx_eq_modphase(eigvecs(A), eigvecs(fA))
                            #call to LAPACK.stein here
                            test_approx_eq_modphase(eigvecs(A,eigvals(A)),eigvecs(A))
                        elseif elty != Int
                            # check that undef is determined accurately even if type inference
                            # bails out due to the number of try/catch blocks in this code.
                            @test_throws UndefVarError fA
                        end
                    end
                end
            end
            if elty <: Real
                Ts = SymTridiagonal(d, dl)
                Fs = Array(Ts)
                Tldlt = factorize(Ts)
                @testset "symmetric tridiagonal" begin
                    @test_throws DimensionMismatch Tldlt\rand(elty,n+1)
                    @test size(Tldlt) == size(Ts)
                    if elty <: AbstractFloat
                        @test LinearAlgebra.LDLt{elty,SymTridiagonal{elty,Vector{elty}}}(Tldlt) === Tldlt
                        @test LinearAlgebra.LDLt{elty}(Tldlt) === Tldlt
                        @test typeof(convert(LinearAlgebra.LDLt{Float32,Matrix{Float32}},Tldlt)) ==
                            LinearAlgebra.LDLt{Float32,Matrix{Float32}}
                        @test typeof(convert(LinearAlgebra.LDLt{Float32},Tldlt)) ==
                            LinearAlgebra.LDLt{Float32,SymTridiagonal{Float32,Vector{Float32}}}
                    end
                    for vv in (copy(v), view(v, 1:n))
                        invFsv = Fs\vv
                        x = Ts\vv
                        @test x ≈ invFsv
                        @test Array(Tldlt) ≈ Fs
                    end

                    @testset "similar" begin
                        @test isa(similar(Ts), SymTridiagonal{elty})
                        @test isa(similar(Ts, Int), SymTridiagonal{Int})
                        @test isa(similar(Ts, (3, 2)), SparseMatrixCSC)
                        @test isa(similar(Ts, Int, (3, 2)), SparseMatrixCSC{Int})
                    end

                    @test first(logabsdet(Tldlt)) ≈ first(logabsdet(Fs))
                    @test last(logabsdet(Tldlt))  ≈ last(logabsdet(Fs))
                    # just test that the det method exists. The numerical value of the
                    # determinant is unreliable
                    det(Tldlt)
                end
            end
        else # mat_type is Tridiagonal
            @testset "tridiagonal linear algebra" begin
                for (BB, vv) in ((copy(B), copy(v)), (view(B, 1:n, 1), view(v, 1:n)))
                    @test A*vv ≈ fA*vv
                    invFv = fA\vv
                    @test A\vv ≈ invFv
                    # @test Base.solve(T,v) ≈ invFv
                    # @test Base.solve(T, B) ≈ F\B
                    Tlu = factorize(A)
                    x = Tlu\vv
                    @test x ≈ invFv
                end
            end
        end
        @testset "generalized dot" begin
            x = fill(convert(elty, 1), n)
            y = fill(convert(elty, 1), n)
            @test dot(x, A, y) ≈ dot(A'x, y)
        end
    end
end

@testset "SymTridiagonal block matrix" begin
    M = [1 2; 2 4]
    n = 5
    A = SymTridiagonal(fill(M, n), fill(M, n-1))
    @test @inferred A[1,1] == Symmetric(M)
    @test @inferred A[1,2] == M
    @test @inferred A[2,1] == transpose(M)
    @test @inferred diag(A, 1) == fill(M, n-1)
    @test @inferred diag(A, 0) == fill(Symmetric(M), n)
    @test @inferred diag(A, -1) == fill(transpose(M), n-1)
    @test_throws ArgumentError diag(A, -2)
    @test_throws ArgumentError diag(A, 2)
    @test_throws ArgumentError diag(A, n+1)
    @test_throws ArgumentError diag(A, -n-1)
end

@testset "Issue 12068" begin
    @test SymTridiagonal([1, 2], [0])^3 == [1 0; 0 8]
end

@testset "convert for SymTridiagonal" begin
    STF32 = SymTridiagonal{Float32}(fill(1f0, 5), fill(1f0, 4))
    @test convert(SymTridiagonal{Float64}, STF32)::SymTridiagonal{Float64} == STF32
    @test convert(AbstractMatrix{Float64}, STF32)::SymTridiagonal{Float64} == STF32
end

@testset "constructors from matrix" begin
    @test SymTridiagonal([1 2 3; 2 5 6; 0 6 9]) == [1 2 0; 2 5 6; 0 6 9]
    @test Tridiagonal([1 2 3; 4 5 6; 7 8 9]) == [1 2 0; 4 5 6; 0 8 9]
end

@testset "constructors with range and other abstract vectors" begin
    @test SymTridiagonal(1:3, 1:2) == [1 1 0; 1 2 2; 0 2 3]
    @test Tridiagonal(4:5, 1:3, 1:2) == [1 1 0; 4 2 2; 0 5 3]
end

@testset "Issue #26994 (and the empty case)" begin
    T = SymTridiagonal([1.0],[3.0])
    x = ones(1)
    @test T*x == ones(1)
    @test SymTridiagonal(ones(0), ones(0)) * ones(0, 2) == ones(0, 2)
end

@testset "issue #29644" begin
    F = lu(Tridiagonal(sparse(1.0I, 3, 3)))
    @test F.L == Matrix(I, 3, 3)
    @test startswith(sprint(show, MIME("text/plain"), F),
          "LinearAlgebra.LU{Float64,LinearAlgebra.Tridiagonal{Float64,SparseArrays.SparseVector")
end

@testset "Issue 29630" begin
    function central_difference_discretization(N; dfunc = x -> 12x^2 - 2N^2,
                                               dufunc = x -> N^2 + 4N*x,
                                               dlfunc = x -> N^2 - 4N*x,
                                               bfunc = x -> 114ℯ^-x * (1 + 3x),
                                               b0 = 0, bf = 57/ℯ,
                                               x0 = 0, xf = 1)
        h = 1/N
        d, du, dl, b = map(dfunc, (x0+h):h:(xf-h)), map(dufunc, (x0+h):h:(xf-2h)),
                       map(dlfunc, (x0+2h):h:(xf-h)), map(bfunc, (x0+h):h:(xf-h))
        b[1] -= dlfunc(x0)*b0     # subtract the boundary term
        b[end] -= dufunc(xf)*bf   # subtract the boundary term
        Tridiagonal(dl, d, du), b
    end

    A90, b90 = central_difference_discretization(90)

    @test A90\b90 ≈ inv(A90)*b90
end

@testset "singular values of SymTridiag" begin
    @test svdvals(SymTridiagonal([-4,2,3], [0,0])) ≈ [4,3,2]
    @test svdvals(SymTridiagonal(collect(0.:10.), zeros(10))) ≈ reverse(0:10)
    @test svdvals(SymTridiagonal([1,2,1], [1,1])) ≈ [3,1,0]
    # test that dependent methods such as `cond` also work
    @test cond(SymTridiagonal([1,2,3], [0,0])) ≈ 3
end

@testset "sum, mapreduce" begin
    T = Tridiagonal([1,2], [1,2,3], [7,8])
    Tdense = Matrix(T)
    S = SymTridiagonal([1,2,3], [1,2])
    Sdense = Matrix(S)
    @test sum(T) == 24
    @test sum(S) == 12
    @test_throws ArgumentError sum(T, dims=0)
    @test sum(T, dims=1) == sum(Tdense, dims=1)
    @test sum(T, dims=2) == sum(Tdense, dims=2)
    @test sum(T, dims=3) == sum(Tdense, dims=3)
    @test typeof(sum(T, dims=1)) == typeof(sum(Tdense, dims=1))
    @test mapreduce(one, min, T, dims=1) == mapreduce(one, min, Tdense, dims=1)
    @test mapreduce(one, min, T, dims=2) == mapreduce(one, min, Tdense, dims=2)
    @test mapreduce(one, min, T, dims=3) == mapreduce(one, min, Tdense, dims=3)
    @test typeof(mapreduce(one, min, T, dims=1)) == typeof(mapreduce(one, min, Tdense, dims=1))
    @test mapreduce(zero, max, T, dims=1) == mapreduce(zero, max, Tdense, dims=1)
    @test mapreduce(zero, max, T, dims=2) == mapreduce(zero, max, Tdense, dims=2)
    @test mapreduce(zero, max, T, dims=3) == mapreduce(zero, max, Tdense, dims=3)
    @test typeof(mapreduce(zero, max, T, dims=1)) == typeof(mapreduce(zero, max, Tdense, dims=1))
    @test_throws ArgumentError sum(S, dims=0)
    @test sum(S, dims=1) == sum(Sdense, dims=1)
    @test sum(S, dims=2) == sum(Sdense, dims=2)
    @test sum(S, dims=3) == sum(Sdense, dims=3)
    @test typeof(sum(S, dims=1)) == typeof(sum(Sdense, dims=1))
    @test mapreduce(one, min, S, dims=1) == mapreduce(one, min, Sdense, dims=1)
    @test mapreduce(one, min, S, dims=2) == mapreduce(one, min, Sdense, dims=2)
    @test mapreduce(one, min, S, dims=3) == mapreduce(one, min, Sdense, dims=3)
    @test typeof(mapreduce(one, min, S, dims=1)) == typeof(mapreduce(one, min, Sdense, dims=1))
    @test mapreduce(zero, max, S, dims=1) == mapreduce(zero, max, Sdense, dims=1)
    @test mapreduce(zero, max, S, dims=2) == mapreduce(zero, max, Sdense, dims=2)
    @test mapreduce(zero, max, S, dims=3) == mapreduce(zero, max, Sdense, dims=3)
    @test typeof(mapreduce(zero, max, S, dims=1)) == typeof(mapreduce(zero, max, Sdense, dims=1))

    T = Tridiagonal(Int[], Int[], Int[])
    Tdense = Matrix(T)
    S = SymTridiagonal(Int[], Int[])
    Sdense = Matrix(S)
    @test sum(T) == 0
    @test sum(S) == 0
    @test_throws ArgumentError sum(T, dims=0)
    @test sum(T, dims=1) == sum(Tdense, dims=1)
    @test sum(T, dims=2) == sum(Tdense, dims=2)
    @test sum(T, dims=3) == sum(Tdense, dims=3)
    @test typeof(sum(T, dims=1)) == typeof(sum(Tdense, dims=1))
    @test_throws ArgumentError sum(S, dims=0)
    @test sum(S, dims=1) == sum(Sdense, dims=1)
    @test sum(S, dims=2) == sum(Sdense, dims=2)
    @test sum(S, dims=3) == sum(Sdense, dims=3)
    @test typeof(sum(S, dims=1)) == typeof(sum(Sdense, dims=1))

    T = Tridiagonal(Int[], Int[2], Int[])
    Tdense = Matrix(T)
    S = SymTridiagonal(Int[2], Int[])
    Sdense = Matrix(S)
    @test sum(T) == 2
    @test sum(S) == 2
    @test_throws ArgumentError sum(T, dims=0)
    @test sum(T, dims=1) == sum(Tdense, dims=1)
    @test sum(T, dims=2) == sum(Tdense, dims=2)
    @test sum(T, dims=3) == sum(Tdense, dims=3)
    @test typeof(sum(T, dims=1)) == typeof(sum(Tdense, dims=1))
    @test_throws ArgumentError sum(S, dims=0)
    @test sum(S, dims=1) == sum(Sdense, dims=1)
    @test sum(S, dims=2) == sum(Sdense, dims=2)
    @test sum(S, dims=3) == sum(Sdense, dims=3)
    @test typeof(sum(S, dims=1)) == typeof(sum(Sdense, dims=1))
end

@testset "Issue #28994 (sum of Tridigonal and UniformScaling)" begin
    dl = [1., 1.]
    d = [-2., -2., -2.]
    T = Tridiagonal(dl, d, dl)
    S = SymTridiagonal(T)

    @test diag(T + 2I) == zero(d)
    @test diag(S + 2I) == zero(d)
end

end # module TestTridiagonal