[Features] project_isometric and PolarNewton (#67)

Jutho · lkdvos · web-flow · commit d082c7d4005c · 2025-10-08T00:04:06.000Z
* Add `hermitianpart!` and `antihermitianpart!`

* Add `ishermitian` and `isantihermitian`

* blocked (anti)hermitian

* rename to project_

* also change includes

* add blocked ishermitian

* add PolarNewton

* add more tests and project_isometric

* fix/restore matrixproperties

* move polar alg definition

* rename `svdalg` to `svd_alg`

* remove `opnorm` from imports

---------

Co-authored-by: Lukas Devos &lt;ldevos98@gmail.com&gt;
diff --git a/src/MatrixAlgebraKit.jl b/src/MatrixAlgebraKit.jl
@@ -3,16 +3,16 @@ module MatrixAlgebraKit
 using LinearAlgebra: LinearAlgebra
 using LinearAlgebra: norm # TODO: eleminate if we use VectorInterface.jl?
 using LinearAlgebra: mul!, rmul!, lmul!, adjoint!, rdiv!, ldiv!
-using LinearAlgebra: sylvester
+using LinearAlgebra: sylvester, lu!
 using LinearAlgebra: isposdef, issymmetric
 using LinearAlgebra: Diagonal, diag, diagind, isdiag
 using LinearAlgebra: UpperTriangular, LowerTriangular
 using LinearAlgebra: BlasFloat, BlasReal, BlasComplex, BlasInt
 
 export isisometry, isunitary, ishermitian, isantihermitian
 
-export project_hermitian, project_antihermitian
-export project_hermitian!, project_antihermitian!
+export project_hermitian, project_antihermitian, project_isometric
+export project_hermitian!, project_antihermitian!, project_isometric!
 export qr_compact, qr_full, qr_null, lq_compact, lq_full, lq_null
 export qr_compact!, qr_full!, qr_null!, lq_compact!, lq_full!, lq_null!
 export svd_compact, svd_full, svd_vals, svd_trunc
@@ -34,6 +34,7 @@ export LAPACK_HouseholderQR, LAPACK_HouseholderLQ, LAPACK_Simple, LAPACK_Expert,
     LAPACK_QRIteration, LAPACK_Bisection, LAPACK_MultipleRelativelyRobustRepresentations,
     LAPACK_DivideAndConquer, LAPACK_Jacobi
 export LQViaTransposedQR
+export PolarViaSVD, PolarNewton
 export DiagonalAlgorithm
 export NativeBlocked
 export CUSOLVER_Simple, CUSOLVER_HouseholderQR, CUSOLVER_QRIteration, CUSOLVER_SVDPolar,
diff --git a/src/implementations/polar.jl b/src/implementations/polar.jl
@@ -11,7 +11,7 @@ function check_input(::typeof(left_polar!), A::AbstractMatrix, WP, ::AbstractAlg
     @assert W isa AbstractMatrix && P isa AbstractMatrix
     @check_size(W, (m, n))
     @check_scalar(W, A)
-    @check_size(P, (n, n))
+    isempty(P) || @check_size(P, (n, n))
     @check_scalar(P, A)
     return nothing
 end
@@ -21,7 +21,7 @@ function check_input(::typeof(right_polar!), A::AbstractMatrix, PWᴴ, ::Abstrac
     n >= m ||
         throw(ArgumentError("input matrix needs at least as many columns as rows"))
     @assert P isa AbstractMatrix && Wᴴ isa AbstractMatrix
-    @check_size(P, (m, m))
+    isempty(P) || @check_size(P, (m, m))
     @check_scalar(P, A)
     @check_size(Wᴴ, (m, n))
     @check_scalar(Wᴴ, A)
@@ -43,25 +43,154 @@ function initialize_output(::typeof(right_polar!), A::AbstractMatrix, ::Abstract
     return (P, Wᴴ)
 end
 
-# Implementation
-# --------------
+# Implementation via SVD
+# -----------------------
 function left_polar!(A::AbstractMatrix, WP, alg::PolarViaSVD)
     check_input(left_polar!, A, WP, alg)
-    U, S, Vᴴ = svd_compact!(A, alg.svdalg)
+    U, S, Vᴴ = svd_compact!(A, alg.svd_alg)
     W, P = WP
     W = mul!(W, U, Vᴴ)
-    S .= sqrt.(S)
-    SsqrtVᴴ = lmul!(S, Vᴴ)
-    P = mul!(P, SsqrtVᴴ', SsqrtVᴴ)
+    if !isempty(P)
+        S .= sqrt.(S)
+        SsqrtVᴴ = lmul!(S, Vᴴ)
+        P = mul!(P, SsqrtVᴴ', SsqrtVᴴ)
+    end
     return (W, P)
 end
 function right_polar!(A::AbstractMatrix, PWᴴ, alg::PolarViaSVD)
     check_input(right_polar!, A, PWᴴ, alg)
-    U, S, Vᴴ = svd_compact!(A, alg.svdalg)
+    U, S, Vᴴ = svd_compact!(A, alg.svd_alg)
     P, Wᴴ = PWᴴ
     Wᴴ = mul!(Wᴴ, U, Vᴴ)
-    S .= sqrt.(S)
-    USsqrt = rmul!(U, S)
-    P = mul!(P, USsqrt, USsqrt')
+    if !isempty(P)
+        S .= sqrt.(S)
+        USsqrt = rmul!(U, S)
+        P = mul!(P, USsqrt, USsqrt')
+    end
     return (P, Wᴴ)
 end
+
+# Implementation via Newton
+# --------------------------
+function left_polar!(A::AbstractMatrix, WP, alg::PolarNewton)
+    check_input(left_polar!, A, WP, alg)
+    W, P = WP
+    if isempty(P)
+        W = _left_polarnewton!(A, W, P; alg.kwargs...)
+        return W, P
+    else
+        W = _left_polarnewton!(copy(A), W, P; alg.kwargs...)
+        # we still need `A` to compute `P`
+        P = project_hermitian!(mul!(P, W', A))
+        return W, P
+    end
+end
+
+function right_polar!(A::AbstractMatrix, PWᴴ, alg::PolarNewton)
+    check_input(right_polar!, A, PWᴴ, alg)
+    P, Wᴴ = PWᴴ
+    if isempty(P)
+        Wᴴ = _right_polarnewton!(A, Wᴴ, P; alg.kwargs...)
+        return P, Wᴴ
+    else
+        Wᴴ = _right_polarnewton!(copy(A), Wᴴ, P; alg.kwargs...)
+        # we still need `A` to compute `P`
+        P = project_hermitian!(mul!(P, A, Wᴴ'))
+        return P, Wᴴ
+    end
+end
+
+# these methods only compute W and destroy A in the process
+function _left_polarnewton!(A::AbstractMatrix, W, P = similar(A, (0, 0)); tol = defaulttol(A), maxiter = 10)
+    m, n = size(A) # we must have m >= n
+    Rᴴinv = isempty(P) ? similar(P, (n, n)) : P # use P as workspace when available
+    if m > n # initial QR
+        Q, R = qr_compact!(A)
+        Rc = view(A, 1:n, 1:n)
+        copy!(Rc, R)
+        Rᴴinv = ldiv!(UpperTriangular(Rc)', one!(Rᴴinv))
+    else # m == n
+        R = A
+        Rc = view(W, 1:n, 1:n)
+        copy!(Rc, R)
+        Rᴴinv = ldiv!(lu!(Rc)', one!(Rᴴinv))
+    end
+    γ = sqrt(norm(Rᴴinv) / norm(R)) # scaling factor
+    rmul!(R, γ)
+    rmul!(Rᴴinv, 1 / γ)
+    R, Rᴴinv = _avgdiff!(R, Rᴴinv)
+    copy!(Rc, R)
+    i = 1
+    conv = norm(Rᴴinv, Inf)
+    while i < maxiter && conv > tol
+        Rᴴinv = ldiv!(lu!(Rc)', one!(Rᴴinv))
+        γ = sqrt(norm(Rᴴinv) / norm(R)) # scaling factor
+        rmul!(R, γ)
+        rmul!(Rᴴinv, 1 / γ)
+        R, Rᴴinv = _avgdiff!(R, Rᴴinv)
+        copy!(Rc, R)
+        conv = norm(Rᴴinv, Inf)
+        i += 1
+    end
+    if conv > tol
+        @warn "`left_polar!` via Newton iteration did not converge within $maxiter iterations (final residual: $conv)"
+    end
+    if m > n
+        return mul!(W, Q, Rc)
+    end
+    return W
+end
+
+function _right_polarnewton!(A::AbstractMatrix, Wᴴ, P = similar(A, (0, 0)); tol = defaulttol(A), maxiter = 10)
+    m, n = size(A) # we must have m <= n
+    Lᴴinv = isempty(P) ? similar(P, (m, m)) : P # use P as workspace when available
+    if m < n # initial QR
+        L, Q = lq_compact!(A)
+        Lc = view(A, 1:m, 1:m)
+        copy!(Lc, L)
+        Lᴴinv = ldiv!(LowerTriangular(Lc)', one!(Lᴴinv))
+    else # m == n
+        L = A
+        Lc = view(Wᴴ, 1:m, 1:m)
+        copy!(Lc, L)
+        Lᴴinv = ldiv!(lu!(Lc)', one!(Lᴴinv))
+    end
+    γ = sqrt(norm(Lᴴinv) / norm(L)) # scaling factor
+    rmul!(L, γ)
+    rmul!(Lᴴinv, 1 / γ)
+    L, Lᴴinv = _avgdiff!(L, Lᴴinv)
+    copy!(Lc, L)
+    i = 1
+    conv = norm(Lᴴinv, Inf)
+    while i < maxiter && conv > tol
+        Lᴴinv = ldiv!(lu!(Lc)', one!(Lᴴinv))
+        γ = sqrt(norm(Lᴴinv) / norm(L)) # scaling factor
+        rmul!(L, γ)
+        rmul!(Lᴴinv, 1 / γ)
+        L, Lᴴinv = _avgdiff!(L, Lᴴinv)
+        copy!(Lc, L)
+        conv = norm(Lᴴinv, Inf)
+        i += 1
+    end
+    if conv > tol
+        @warn "`right_polar!` via Newton iteration did not converge within $maxiter iterations (final residual: $conv)"
+    end
+    if m < n
+        return mul!(Wᴴ, Lc, Q)
+    end
+    return Wᴴ
+end
+
+# in place computation of the average and difference of two arrays
+function _avgdiff!(A::AbstractArray, B::AbstractArray)
+    axes(A) == axes(B) || throw(DimensionMismatch())
+    @simd for I in eachindex(A, B)
+        @inbounds begin
+            a = A[I]
+            b = B[I]
+            A[I] = (a + b) / 2
+            B[I] = b - a
+        end
+    end
+    return A, B
+end
diff --git a/src/implementations/projections.jl b/src/implementations/projections.jl
@@ -5,6 +5,8 @@ function copy_input(::typeof(project_hermitian), A::AbstractMatrix)
 end
 copy_input(::typeof(project_antihermitian), A) = copy_input(project_hermitian, A)
 
+copy_input(::typeof(project_isometric), A) = copy_input(left_polar, A)
+
 function check_input(::typeof(project_hermitian!), A::AbstractMatrix, B::AbstractMatrix, ::AbstractAlgorithm)
     LinearAlgebra.checksquare(A)
     n = Base.require_one_based_indexing(A)
@@ -18,6 +20,16 @@ function check_input(::typeof(project_antihermitian!), A::AbstractMatrix, B::Abs
     return nothing
 end
 
+function check_input(::typeof(project_isometric!), A::AbstractMatrix, W::AbstractMatrix, ::AbstractAlgorithm)
+    m, n = size(A)
+    m >= n ||
+        throw(ArgumentError("input matrix needs at least as many rows as columns"))
+    @assert W isa AbstractMatrix
+    @check_size(W, (m, n))
+    @check_scalar(W, A)
+    return nothing
+end
+
 # Outputs
 # -------
 function initialize_output(::typeof(project_hermitian!), A::AbstractMatrix, ::NativeBlocked)
@@ -27,15 +39,26 @@ function initialize_output(::typeof(project_antihermitian!), A::AbstractMatrix,
     return A
 end
 
+function initialize_output(::typeof(project_isometric!), A::AbstractMatrix, ::AbstractAlgorithm)
+    return similar(A)
+end
+
 # Implementation
 # --------------
-function project_hermitian!(A::AbstractMatrix, B, alg::NativeBlocked)
-    check_input(project_hermitian!, A, B, alg)
-    return project_hermitian_native!(A, B, Val(false); alg.kwargs...)
+function project_hermitian!(A::AbstractMatrix, Aₕ, alg::NativeBlocked)
+    check_input(project_hermitian!, A, Aₕ, alg)
+    return project_hermitian_native!(A, Aₕ, Val(false); alg.kwargs...)
 end
-function project_antihermitian!(A::AbstractMatrix, B, alg::NativeBlocked)
-    check_input(project_antihermitian!, A, B, alg)
-    return project_hermitian_native!(A, B, Val(true); alg.kwargs...)
+function project_antihermitian!(A::AbstractMatrix, Aₐ, alg::NativeBlocked)
+    check_input(project_antihermitian!, A, Aₐ, alg)
+    return project_hermitian_native!(A, Aₐ, Val(true); alg.kwargs...)
+end
+
+function project_isometric!(A::AbstractMatrix, W, alg::AbstractAlgorithm)
+    check_input(project_isometric!, A, W, alg)
+    noP = similar(W, (0, 0))
+    W, _ = left_polar!(A, (W, noP), alg)
+    return W
 end
 
 function project_hermitian_native!(A::AbstractMatrix, B::AbstractMatrix, anti::Val; blocksize = 32)
diff --git a/src/interface/decompositions.jl b/src/interface/decompositions.jl
@@ -117,6 +117,28 @@ const LAPACK_SVDAlgorithm = Union{
     LAPACK_Jacobi,
 }
 
+# =========================
+# Polar decompositions
+# =========================
+"""
+    PolarViaSVD(svd_alg)
+
+Algorithm for computing the polar decomposition of a matrix `A` via the singular value
+decomposition (SVD) of `A`. The `svd_alg` argument specifies the SVD algorithm to use.
+"""
+struct PolarViaSVD{SVDAlg} <: AbstractAlgorithm
+    svd_alg::SVDAlg
+end
+
+"""
+    PolarNewton(; maxiter = 10, tol = defaulttol(A))
+
+Algorithm for computing the polar decomposition of a matrix `A` via
+scaled Newton iteration, with a maximum of `maxiter` iterations and
+until convergence up to tolerance `tol`.
+"""
+@algdef PolarNewton
+
 # =========================
 # DIAGONAL ALGORITHMS
 # =========================
diff --git a/src/interface/polar.jl b/src/interface/polar.jl
@@ -39,16 +39,6 @@ See also [`left_polar(!)`](@ref left_polar).
 """
 @functiondef right_polar
 
-"""
-    PolarViaSVD(svdalg)
-    
-Algorithm for computing the polar decomposition of a matrix `A` via the singular value
-decomposition (SVD) of `A`. The `svdalg` argument specifies the SVD algorithm to use.
-"""
-struct PolarViaSVD{SVDAlg} <: AbstractAlgorithm
-    svdalg::SVDAlg
-end
-
 # Algorithm selection
 # -------------------
 default_polar_algorithm(A; kwargs...) = default_polar_algorithm(typeof(A); kwargs...)
diff --git a/src/interface/projections.jl b/src/interface/projections.jl
@@ -1,11 +1,13 @@
 @doc """
     project_hermitian(A; kwargs...)
     project_hermitian(A, alg)
-    project_hermitian!(A; kwargs...)
-    project_hermitian!(A, alg)
+    project_hermitian!(A, [Aₕ]; kwargs...)
+    project_hermitian!(A, [Aₕ], alg)
 
 Compute the hermitian part of a (square) matrix `A`, defined as `(A + A') / 2`.
-For real matrices this corresponds to the symmetric part of `A`.
+For real matrices this corresponds to the symmetric part of `A`. In the bang method,
+the output storage can be provided via the optional argument `Aₕ`; by default it is
+equal to `A` and so the input matrix `A` is replaced by its hermitian projection.
 
 See also [`project_antihermitian`](@ref).
 """
@@ -14,16 +16,36 @@ See also [`project_antihermitian`](@ref).
 @doc """
     project_antihermitian(A; kwargs...)
     project_antihermitian(A, alg)
-    project_antihermitian!(A; kwargs...)
-    project_antihermitian!(A, alg)
+    project_antihermitian!(A, [Aₐ]; kwargs...)
+    project_antihermitian!(A, [Aₐ], alg)
 
 Compute the anti-hermitian part of a (square) matrix `A`, defined as `(A - A') / 2`.
-For real matrices this corresponds to the antisymmetric part of `A`.
+For real matrices this corresponds to the antisymmetric part of `A`. In the bang method,
+the output storage can be provided via the optional argument `Aₐ``; by default it is
+equal to `A` and so the input matrix `A` is replaced by its antihermitian projection.
 
 See also [`project_hermitian`](@ref).
 """
 @functiondef project_antihermitian
 
+@doc """
+    project_isometric(A; kwargs...)
+    project_isometric(A, alg)
+    project_isometric!(A, [W]; kwargs...)
+    project_isometric!(A, [W], alg)
+
+Compute the projection of `A` onto the manifold of isometric matrices, i.e. matrices
+satisfying `A' * A ≈ I`. This projection is computed via the polar decomposition, i.e.
+`W` corresponds to the first return value of `left_polar!`, but avoids computing the
+positive definite factor explicitly.
+
+!!! note
+    The bang method `project_isometric!` optionally accepts the output structure and
+    possibly destroys the input matrix `A`. Always use the return value of the function
+    as it may not always be possible to use the provided `W` as output.
+"""
+@functiondef project_isometric
+
 """
 NativeBlocked(; blocksize = 32)
 
@@ -43,3 +65,6 @@ for f in (:project_hermitian!, :project_antihermitian!)
         return default_hermitian_algorithm(A; kwargs...)
     end
 end
+
+default_algorithm(::typeof(project_isometric!), ::Type{A}; kwargs...) where {A} =
+    default_polar_algorithm(A; kwargs...)
diff --git a/test/polar.jl b/test/polar.jl
diff --git a/test/projections.jl b/test/projections.jl
