mat64: fix shadow boundary case

Also add more detailed explanation of rectangleOverlap.

mat64/shadow.go

@@ -191,15 +191,41 @@ func (v *Vector) checkOverlap(a blas64.Vector) bool {
 // rectanglesOverlap returns whether the strided rectangles a and b overlap
 // when b is offset by off elements after a but has at least one element before
 // the end of a. a and b have aCols and bCols respectively.
+//
+// rectanglesOverlap works by shifting both matrices left such that the left
+// column of a is at 0. The column indexes are flattened by obtaining the shifted
+// relative left and right column positions modulo the common stride. This allows
+// direct comparison of the column offsets when the matrix backing data slices
+// are known to overlap.
 func rectanglesOverlap(off, aCols, bCols, stride int) bool {
 	if stride == 1 {
+		// Unit stride means overlapping data
+		// slices must overlap as matrices.
 		return true
 	}
+
+	// Flatten the shifted matrix column positions
+	// so a starts at 0, modulo the common stride.
+	const aFrom = 0
 	aTo := aCols
 	bFrom := off % stride
 	bTo := (bFrom + bCols) % stride
+
+	// Compare the case where the b matrix is
+	// not wrapped.
 	if bFrom < bTo {
 		return aTo > bFrom
 	}
-	return bTo > 0
+
+	// Compare the case where the right column
+	// of b aligns with the stride of the backing
+	// data slice.
+	if bTo == 0 {
+		return bFrom < aTo
+	}
+
+	// Compare the case where the right column of
+	// b wraps around to the left again, modulo
+	// stride.
+	return bTo > aFrom
 }

mat64/shadow_test.go

@@ -89,3 +89,29 @@ type interval struct{ from, to int }
 func intervalsOverlap(a, b interval) bool {
 	return a.to > b.from && b.to > a.from
 }
+
+// See https://github.com/gonum/matrix/issues/359 for details.
+func TestIssue359(t *testing.T) {
+	for xi := 0; xi < 2; xi++ {
+		for xj := 0; xj < 2; xj++ {
+			for yi := 0; yi < 2; yi++ {
+				for yj := 0; yj < 2; yj++ {
+					a := NewDense(3, 3, []float64{
+						1, 2, 3,
+						4, 5, 6,
+						7, 8, 9,
+					})
+					x := a.View(xi, xj, 2, 2).(*Dense)
+					y := a.View(yi, yj, 2, 2).(*Dense)
+
+					panicked, _ := panics(func() { x.checkOverlap(y.mat) })
+					if !panicked {
+						t.Errorf("expected panic for aliased with offsets x(%d,%d) y(%d,%d):\nx:\n%v\ny:\n%v",
+							xi, xj, yi, yj, Formatted(x), Formatted(y),
+						)
+					}
+				}
+			}
+		}
+	}
+}