StrSlice substraction.

utils/list.go

@@ -101,3 +101,40 @@ func StrSliceDeduplicate(s []string) []string {
 
 	return s[:j]
 }
+
+// StrSlicesSubstract finds all the strings which are in l but not in r, both slices shoult be sorted
+func StrSlicesSubstract(l, r []string) []string {
+	result := make([]string, 0)
+
+	// pointer to left and right reflists
+	il, ir := 0, 0
+	// length of reflists
+	ll, lr := len(l), len(r)
+
+	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 = append(result, l[il:]...)
+			break
+		}
+
+		if l[il] == r[ir] {
+			// r contains entry from l, so we skip it
+			il++
+			ir++
+		} else if l[il] < r[ir] {
+			// item il is not in r, append
+			result = append(result, l[il])
+			il++
+		} else {
+			// skip over to next item in r
+			ir++
+		}
+	}
+
+	return result
+}

utils/list_test.go

@@ -56,3 +56,19 @@ func (s *ListSuite) TestStrSliceDeduplicate(c *C) {
 	c.Check(StrSliceDeduplicate([]string{"a", "b", "c", "a", "a", "b"}), DeepEquals, []string{"a", "b", "c"})
 	c.Check(StrSliceDeduplicate([]string{"a", "b", "c", "d", "e", "f"}), DeepEquals, []string{"a", "b", "c", "d", "e", "f"})
 }
+
+func (s *ListSuite) TestStrSlicesSubstract(c *C) {
+	empty := []string{}
+	l1 := []string{"r1", "r2", "r3", "r4"}
+	l2 := []string{"r1", "r3"}
+	l3 := []string{"r2", "r4"}
+	l4 := []string{"r4", "r5"}
+	l5 := []string{"r1", "r2", "r3"}
+
+	c.Check(StrSlicesSubstract(l1, empty), DeepEquals, l1)
+	c.Check(StrSlicesSubstract(l1, l2), DeepEquals, l3)
+	c.Check(StrSlicesSubstract(l1, l3), DeepEquals, l2)
+	c.Check(StrSlicesSubstract(l1, l4), DeepEquals, l5)
+	c.Check(StrSlicesSubstract(empty, l1), DeepEquals, empty)
+	c.Check(StrSlicesSubstract(l2, l3), DeepEquals, l2)
+}