Make BandMatrixRow callable

Author: sjavis

src/MatrixFields/band_matrix_row.jl

@@ -162,3 +162,12 @@ is_auto_broadcastable(::Type{BMR}) where {BMR <: BandMatrixRow} =
     is_auto_broadcastable(eltype(BMR))
 add_auto_broadcasters(row::BandMatrixRow) = map(add_auto_broadcasters, row)
 drop_auto_broadcasters(row::BandMatrixRow) = map(drop_auto_broadcasters, row)
+
+
+@generated function (row::BandMatrixRow{ld, bw, T})(args::Vararg{Any, N}) where {ld, bw, T, N}
+    N == bw || error(
+        "BandMatrixRow with bandwidth $bw expected $bw arguments, but got $N"
+    )
+    terms = [:(row.entries[$i] * args[$i]) for i in 1:bw]
+    return Expr(:call, :+, terms...)
+end

test/MatrixFields/band_matrix_row.jl

@@ -59,4 +59,11 @@ include("matrix_field_test_utils.jl")
     @test zero(typeof(DiagonalMatrixRow(1))) == DiagonalMatrixRow(0)
     @test eltype(typeof(DiagonalMatrixRow(1))) == Int
     @test inv(DiagonalMatrixRow(1)) == DiagonalMatrixRow(float(1))
+
+    # Test callable behaviour
+    @test DiagonalMatrixRow(2.0)(10.0) == 20.0
+    @test BidiagonalMatrixRow(1.0, 2.0)(10.0, 5.0) == 20.0
+    @test TridiagonalMatrixRow(-1.0, 2.0, -1.0)(10.0, 25.0, 12.0) == 28.0
+    @test_throws ErrorException TridiagonalMatrixRow(1, 2, 3)(10.0, 5.0)
+    @test_throws ErrorException TridiagonalMatrixRow(1, 2, 3)(10.0, 5.0, 1.0, 2.0)
 end