broadcast with 0

Author: dlfivefifty

src/broadcast.jl

@@ -229,6 +229,30 @@ function blockbanded_axpy!(a, X::AbstractMatrix, Y::AbstractMatrix)
     Y
 end
 
+for op in (:+, :-)
+    @eval begin
+        function similar(bc::Broadcasted{<:BlockBandedStyle, <:Any, typeof($op), <:Tuple{<:AbstractMatrix,<:AbstractMatrix}}, ::Type{T}) where T
+            A,B = bc.args
+            A_size = blocksizes(A).block_sizes
+            A_size == blocksizes(B).block_sizes || throw(DimensionMismatch())
+            Al,Au = blockbandwidths(A)
+            Bl,Bu = blockbandwidths(B)
+            BlockBandedMatrix{T}(undef, A_size, (max(Al,Bl), max(Au,Bu)))
+        end
+
+        function similar(bc::Broadcasted{<:BandedBlockBandedStyle, <:Any, typeof($op), <:Tuple{<:AbstractMatrix,<:AbstractMatrix}}, ::Type{T}) where T
+            A,B = bc.args
+            A_size = blocksizes(A).block_sizes
+            A_size == blocksizes(B).block_sizes || throw(DimensionMismatch())
+            Al,Au = blockbandwidths(A)
+            Bl,Bu = blockbandwidths(B)
+            Aλ,Aμ = subblockbandwidths(A)
+            Bλ,Bμ = subblockbandwidths(B)
+
+            BandedBlockBandedMatrix{T}(undef, A_size, (max(Al,Bl), max(Au,Bu)), (max(Aλ,Bλ), max(Aμ,Bμ)))
+        end
+    end
+end
 
 function copyto!(dest::AbstractArray{T}, bc::Broadcasted{<:AbstractBlockBandedStyle, <:Any, typeof(+),
                                                             <:Tuple{<:AbstractMatrix,<:AbstractMatrix}}) where T
@@ -244,29 +268,23 @@ function copyto!(dest::AbstractArray{T}, bc::Broadcasted{<:AbstractBlockBandedSt
     end
 end
 
-function similar(bc::Broadcasted{<:BlockBandedStyle, <:Any, typeof(+), <:Tuple{<:AbstractMatrix,<:AbstractMatrix}}, ::Type{T}) where T
-    A,B = bc.args
-    A_size = blocksizes(A).block_sizes
-    A_size == blocksizes(B).block_sizes || throw(DimensionMismatch())
-    Al,Au = blockbandwidths(A)
-    Bl,Bu = blockbandwidths(B)
-    BlockBandedMatrix{T}(undef, A_size, (max(Al,Bl), max(Au,Bu)))
-end
-
-function similar(bc::Broadcasted{<:BandedBlockBandedStyle, <:Any, typeof(+), <:Tuple{<:AbstractMatrix,<:AbstractMatrix}}, ::Type{T}) where T
+function copyto!(dest::AbstractArray{T}, bc::Broadcasted{<:AbstractBlockBandedStyle, <:Any, typeof(-),
+                                                            <:Tuple{<:AbstractMatrix,<:AbstractMatrix}}) where T
     A,B = bc.args
-    A_size = blocksizes(A).block_sizes
-    A_size == blocksizes(B).block_sizes || throw(DimensionMismatch())
-    Al,Au = blockbandwidths(A)
-    Bl,Bu = blockbandwidths(B)
-    Aλ,Aμ = subblockbandwidths(A)
-    Bλ,Bμ = subblockbandwidths(B)
 
-    BandedBlockBandedMatrix{T}(undef, A_size, (max(Al,Bl), max(Au,Bu)), (max(Aλ,Bλ), max(Aμ,Bμ)))
+    if dest ≡ B
+        lmul!(-one(T), B)
+        blockbanded_axpy!(one(T), A, dest)
+    elseif dest ≡ A
+        blockbanded_axpy!(-one(T), B, dest)
+    else
+        blockbanded_copyto!(dest, B)
+        lmul!(-one(T), dest)
+        blockbanded_axpy!(one(T), A, dest)
+    end
 end
 
 
-
 function copyto!(dest::AbstractArray{T}, bc::Broadcasted{<:AbstractBlockBandedStyle, <:Any, typeof(+),
                                                         <:Tuple{<:Broadcasted{<:AbstractBlockBandedStyle,<:Any,typeof(*),<:Tuple{<:Number,<:AbstractMatrix}},
                                                         <:AbstractMatrix}}) where T