Overload bandedrowsdata for SubArray (#487)

* Overload bandedrowsdata for SubArray

* fix negative bands and add tests

Author: dlfivefifty

.github/workflows/downstream.yml

@@ -44,6 +44,7 @@ jobs:
           - {repo: ApproxFunBase.jl, group: JuliaApproximation}
           - {repo: ApproxFun.jl, group: JuliaApproximation}
           - {repo: InfiniteLinearAlgebra.jl, group: JuliaLinearAlgebra}
+          - {repo: ClassicalOrthogonalPolynomials.jl, group: JuliaApproximation}
 
     steps:
       - uses: actions/checkout@v5

Project.toml

@@ -1,6 +1,6 @@
 name = "BandedMatrices"
 uuid = "aae01518-5342-5314-be14-df237901396f"
-version = "1.10.0"
+version = "1.10.1"
 
 [deps]
 ArrayLayouts = "4c555306-a7a7-4459-81d9-ec55ddd5c99a"

src/banded/BandedMatrix.jl

@@ -266,13 +266,14 @@ BandedMatrix(A::AbstractMatrix) = _BandedMatrix(MemoryLayout(A), A)
 _BandedMatrix(::BandedColumns, A::AbstractMatrix) = _BandedMatrix(copy(bandeddata(A)), axes(A,1), bandwidths(A)...)
 function _BandedMatrix(::BandedRows, A::AbstractMatrix)
     bdata = bandedrowsdata(A)
-    data = similar(bdata, eltype(bdata), reverse(size(bdata)))
-    u, ℓ = bandwidths(A)
+    m,ν = size(bdata)
+    ℓ,u = bandwidths(A)
+    data = similar(bdata, eltype(bdata), ν, size(A,2))
     n = size(A, 2)
-    for j in axes(A, 1), i in max(1, j - u):min(n, j + ℓ)
-        data[ℓ + 1 + j - i, i] = bdata[j, u+1+i-j]
+    for j in axes(A, 1), i in max(1, j - ℓ):min(n, j + u)
+        data[u + 1 + j - i, i] = bdata[j, ℓ+1+i-j]
     end
-    return _BandedMatrix(data, axes(A, 1), bandwidths(A)...)
+    return _BandedMatrix(data, axes(A, 1), ℓ, u)
 end
 function _BandedMatrix(::DiagonalLayout, A::AbstractMatrix{T}) where T
     m,n = size(A)

src/interfaceimpl.jl

@@ -32,6 +32,7 @@ end
 
 bandedrowsdata(Ac::Transpose) = permutedims(bandeddata(parent(Ac)))
 bandedrowsdata(Ac::Adjoint{<:Real}) = permutedims(bandeddata(parent(Ac)))
+bandedrowsdata(V::SubArray) = permutedims(bandeddata(view(parent(parent(V)), reverse(parentindices(V))...)))
 
 # treat subinds as banded
 sublayout(::DiagonalLayout{L}, inds::Type) where L = sublayout(bandedcolumns(L()), inds)

test/test_banded.jl

@@ -608,7 +608,18 @@ include("mymatrix.jl")
             @test BandedMatrix(B) == B
             @test BandedMatrix(B) !== B
             @test BandedMatrices.bandeddata(BandedMatrix(B)) !== BandedMatrices.bandeddata(A)
+
+            @test BandedMatrix(view(B,2:4, 3:5)) == B[2:4,3:5] == [B[k,j] for k=2:4,j=3:5]
+        end
+
+        for V in (view(BandedMatrix((1 => ones(4),), (10, 10))', 1:5, 1:4),
+                  view(BandedMatrix((1 => ones(4),), (10, 10))', 1:3, 1:5),
+                  view(BandedMatrix((-1 => ones(4),), (10, 10))', 1:5, 1:4),
+                  view(BandedMatrix((-1 => ones(4),), (10, 10))', 1:3, 1:5),
+                  view(BandedMatrix((-1 => ones(4), 1 => ones(9)), (10, 10))', 1:3, 1:5))
+            @test BandedMatrix(V) == V
         end
+
     end
 end