util: Add PathSlice type that satisfies sort.Interface

This commit adds a []string{} type alias named PathSlice, and the
Len(), Swap(), and Less() methods required to satisfy sort.Interface.
Now you can do `sort.Sort(util.PathSlice(foo))` where foo is a slice
of paths. It will be sorted by depth in alphabetical order.

util/util.go

@@ -420,3 +420,38 @@ func SortedStrSliceCompare(a, b []string) error {
 	}
 	return nil
 }
+
+// PathSlice is a type used to implement sort.Interface on a slice of strings,
+// where each string is a path. This allows you to call sort.Sort() on a list
+// of paths, after casting the []string{} to this type. Paths will be sorted
+// by depth in alphabetical order.
+type PathSlice []string
+
+// Len returns the length of obj. It is required to satisfy sort.Interface.
+func (obj PathSlice) Len() int {
+	return len(obj)
+}
+
+// Swap swaps obj[i] and obj[j]. it is required to satisfy sort.Interface.
+func (obj PathSlice) Swap(i, j int) {
+	obj[i], obj[j] = obj[j], obj[i]
+}
+
+// Less returns whether obj[i] is less than obj[j]. It performs the logic
+// required to satisfy sort.Interface.
+func (obj PathSlice) Less(i, j int) bool {
+	x := PathSplitFullReversed(obj[i])
+	y := PathSplitFullReversed(obj[j])
+	if x[0] != y[0] {
+		return x[0] < y[0]
+	}
+	if len(x) > len(y) {
+		return false
+	}
+	for i := range x {
+		if x[i] > y[i] {
+			return false
+		}
+	}
+	return true
+}

util/util_test.go

@@ -857,3 +857,51 @@ func TestSortedStrSliceCompare3(t *testing.T) {
 		t.Errorf("input slice reordered to: %v", slice1)
 	}
 }
+
+func TestPathSliceSort(t *testing.T) {
+	tests := []struct {
+		in  []string
+		out []string
+	}{
+		{
+			in: []string{
+				"/foo/bar/baz",
+				"/bing/bang/boom",
+				"/1/2/3/",
+				"/foo/bar/raz",
+				"/bing/buzz/",
+				"/foo/",
+				"/",
+				"/1/",
+				"/foo/bar/baz/bam",
+				"/bing/bang/",
+				"/1/2/",
+				"/foo/bar/",
+				"/bing/",
+			},
+			out: []string{
+				"/",
+				"/1/",
+				"/1/2/",
+				"/1/2/3/",
+				"/bing/",
+				"/bing/bang/",
+				"/bing/bang/boom",
+				"/bing/buzz/",
+				"/foo/",
+				"/foo/bar/",
+				"/foo/bar/baz",
+				"/foo/bar/baz/bam",
+				"/foo/bar/raz",
+			},
+		},
+	}
+	for _, tt := range tests {
+		sort.Sort(PathSlice(tt.in))
+		for i := range tt.in {
+			if tt.in[i] != tt.out[i] {
+				t.Errorf("path sort failed: wanted: %s got: %s", tt.out[i], tt.in[i])
+			}
+		}
+	}
+}