Support broadcasting with more bands when result is zero (#259)

* Support broadcasting with more bands when result is zero

* Update Project.toml

* add tests

Author: dlfivefifty

Project.toml

@@ -1,6 +1,6 @@
 name = "BandedMatrices"
 uuid = "aae01518-5342-5314-be14-df237901396f"
-version = "0.17.1"
+version = "0.17.2"
 
 [deps]
 ArrayLayouts = "4c555306-a7a7-4459-81d9-ec55ddd5c99a"

src/generic/broadcast.jl

@@ -96,13 +96,24 @@ function _banded_broadcast!(dest::AbstractMatrix, f, src::AbstractMatrix{T}, _1,
 
     d_l, d_u = bandwidths(dest)
     s_l, s_u = bandwidths(src)
-    (d_l ≥ min(s_l,m-1) && d_u ≥ min(s_u,n-1)) || throw(BandError(dest))
+    if d_l < min(s_l,m-1)
+        # check zeros
+        for j = 1:n, k = max(1,j+d_l+1):min(j+s_l,j+d_l,m)
+            iszero(f(inbands_getindex(src, k, j))) || throw(BandError(dest))
+        end
+    end
+    if d_u < min(s_u,n-1)
+        # check zeros
+        for j = 1:n, k = max(1,j-d_u,j-s_u):min(j-d_u-1,m)
+            iszero(f(inbands_getindex(src, k, j))) || throw(BandError(dest))
+        end
+    end 
 
     for j=1:n
         for k = max(1,j-d_u):min(j-s_u-1,m)
             inbands_setindex!(dest, z, k, j)
         end
-        for k = max(1,j-s_u):min(j+s_l,m)
+        for k = max(1,j-s_u,j-d_u):min(j+s_l,j+d_l,m)
             inbands_setindex!(dest, f(inbands_getindex(src, k, j)), k, j)
         end
         for k = max(1,j+s_l+1):min(j+d_l,m)

test/test_broadcasting.jl

@@ -560,4 +560,13 @@ import BandedMatrices: BandedStyle, BandedRows
         @test_throws DimensionMismatch A .* Ones(3)
         @test_throws DimensionMismatch A .* Ones(3,4)
     end
+
+    @testset "degenerate bands" begin
+        A = BandedMatrix{Float64}(undef, (5, 5), (1,-1)); A.data .= NaN
+        B = BandedMatrix{Float64}(undef, (5, 5), (-1,1)); B.data .= NaN
+        Z = Diagonal(Zeros(5))
+        copyto!(A, Z)
+        copyto!(B, Z)
+        @test A == B == zeros(5,5)
+    end
 end