fix generic ldiv! for CholeskyPivoted (#32593)

* fix generic ldiv! for CholeskyPivoted

* add back one empty line to make backport easier

* don't interfere with previous tests

(cherry picked from commit d0e11cf)

Author: dkarrasch

stdlib/LinearAlgebra/src/cholesky.jl

@@ -424,21 +424,30 @@ end
 function ldiv!(C::CholeskyPivoted, B::StridedVector)
     if C.uplo == 'L'
         ldiv!(adjoint(LowerTriangular(C.factors)),
-            ldiv!(LowerTriangular(C.factors), B[C.piv]))[invperm(C.piv)]
+            ldiv!(LowerTriangular(C.factors), permute!(B, C.piv)))
     else
         ldiv!(UpperTriangular(C.factors),
-            ldiv!(adjoint(UpperTriangular(C.factors)), B[C.piv]))[invperm(C.piv)]
+            ldiv!(adjoint(UpperTriangular(C.factors)), permute!(B, C.piv)))
     end
+    invpermute!(B, C.piv)
 end
 
 function ldiv!(C::CholeskyPivoted, B::StridedMatrix)
+    n = size(C, 1)
+    for i in 1:size(B, 2)
+        permute!(view(B, 1:n, i), C.piv)
+    end
     if C.uplo == 'L'
         ldiv!(adjoint(LowerTriangular(C.factors)),
-            ldiv!(LowerTriangular(C.factors), B[C.piv,:]))[invperm(C.piv),:]
+            ldiv!(LowerTriangular(C.factors), B))
     else
         ldiv!(UpperTriangular(C.factors),
-            ldiv!(adjoint(UpperTriangular(C.factors)), B[C.piv,:]))[invperm(C.piv),:]
+            ldiv!(adjoint(UpperTriangular(C.factors)), B))
     end
+    for i in 1:size(B, 2)
+        invpermute!(view(B, 1:n, i), C.piv)
+    end
+    B
 end
 
 isposdef(C::Union{Cholesky,CholeskyPivoted}) = C.info == 0

stdlib/LinearAlgebra/test/cholesky.jl

@@ -164,7 +164,31 @@ end
                     lpapd = cholesky(apdhL, Val(true))
                     @test norm(apd * (lpapd\b) - b)/norm(b) <= ε*κ*n # Ad hoc, revisit
                     @test norm(apd * (lpapd\b[1:n]) - b[1:n])/norm(b[1:n]) <= ε*κ*n
+                end
+            end
+        end
+
+        for eltyb in (Float64, ComplexF64)
+            Breal = convert(Matrix{BigFloat}, randn(n,n)/2)
+            Bimg  = convert(Matrix{BigFloat}, randn(n,n)/2)
+            B = (eltya <: Complex || eltyb <: Complex) ? complex.(Breal, Bimg) : Breal
+            εb = eps(abs(float(one(eltyb))))
+            ε = max(εa,εb)
+
+            for B in (B, view(B, 1:n, 1:n)) # Array and SubArray
 
+                # Test error bound on linear solver: LAWNS 14, Theorem 2.1
+                # This is a surprisingly loose bound
+                BB = copy(B)
+                ldiv!(capd, BB)
+                @test norm(apd \ B - BB, 1) / norm(BB, 1) <= (3n^2 + n + n^3*ε)*ε/(1-(n+1)*ε)*κ
+                @test norm(apd * BB - B, 1) / norm(B, 1) <= (3n^2 + n + n^3*ε)*ε/(1-(n+1)*ε)*κ
+                if eltya != BigFloat
+                    cpapd = cholesky(apdh, Val(true))
+                    BB = copy(B)
+                    ldiv!(cpapd, BB)
+                    @test norm(apd \ B - BB, 1) / norm(BB, 1) <= (3n^2 + n + n^3*ε)*ε/(1-(n+1)*ε)*κ
+                    @test norm(apd * BB - B, 1) / norm(B, 1) <= (3n^2 + n + n^3*ε)*ε/(1-(n+1)*ε)*κ
                 end
             end
         end