Substraction of PackageRefLists.

debian/list.go

@@ -421,6 +421,44 @@ func (l *PackageRefList) ForEach(handler func([]byte) error) error {
 	return err
 }
 
+// Substract returns all packages in l that are not in r
+func (l *PackageRefList) Substract(r *PackageRefList) *PackageRefList {
+	result := &PackageRefList{Refs: make([][]byte, 0, 128)}
+
+	// pointer to left and right reflists
+	il, ir := 0, 0
+	// length of reflists
+	ll, lr := l.Len(), r.Len()
+
+	for il < ll || ir < lr {
+		if il == ll {
+			// left list exhausted, we got the result
+			break
+		}
+		if ir == lr {
+			// right list exhausted, append what is left to result
+			result.Refs = append(result.Refs, l.Refs[il:]...)
+			break
+		}
+
+		rel := bytes.Compare(l.Refs[il], r.Refs[ir])
+		if rel == 0 {
+			// r contains entry from l, so we skip it
+			il++
+			ir++
+		} else if rel < 0 {
+			// item il is not in r, append
+			result.Refs = append(result.Refs, l.Refs[il])
+			il++
+		} else {
+			// skip over to next item in r
+			ir++
+		}
+	}
+
+	return result
+}
+
 // PackageDiff is a difference between two packages in a list.
 //
 // If left & right are present, difference is in package version

debian/list_test.go

@@ -326,6 +326,28 @@ func (s *PackageListSuite) TestPackageRefListForeach(c *C) {
 	c.Check(err, Equals, e)
 }
 
+func (s *PackageListSuite) TestSubstract(c *C) {
+	r1 := []byte("r1")
+	r2 := []byte("r2")
+	r3 := []byte("r3")
+	r4 := []byte("r4")
+	r5 := []byte("r5")
+
+	empty := &PackageRefList{Refs: [][]byte{}}
+	l1 := &PackageRefList{Refs: [][]byte{r1, r2, r3, r4}}
+	l2 := &PackageRefList{Refs: [][]byte{r1, r3}}
+	l3 := &PackageRefList{Refs: [][]byte{r2, r4}}
+	l4 := &PackageRefList{Refs: [][]byte{r4, r5}}
+	l5 := &PackageRefList{Refs: [][]byte{r1, r2, r3}}
+
+	c.Check(l1.Substract(empty), DeepEquals, l1)
+	c.Check(l1.Substract(l2), DeepEquals, l3)
+	c.Check(l1.Substract(l3), DeepEquals, l2)
+	c.Check(l1.Substract(l4), DeepEquals, l5)
+	c.Check(empty.Substract(l1), DeepEquals, empty)
+	c.Check(l2.Substract(l3), DeepEquals, l2)
+}
+
 func (s *PackageListSuite) TestDiff(c *C) {
 	db, _ := database.OpenDB(c.MkDir())
 	coll := NewPackageCollection(db)