Add sort function helper library.

common/data/sortby/sortby_test.go

Index: common/data/sortby/sortby_test.go
diff --git a/common/data/sortby/sortby_test.go b/common/data/sortby/sortby_test.go
new file mode 100644
index 0000000000000000000000000000000000000000..983dee60e390ff9172730c9fab3fbfa4c85925e1
--- /dev/null
+++ b/common/data/sortby/sortby_test.go
@@ -0,0 +1,78 @@
+// Copyright 2017 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package sortby
+
+import (
+	"sort"
+	"testing"
+
+	. "github.com/smartystreets/goconvey/convey"
+)
+
+type CoolStruct struct {
+	A int
+	B int
+	C int
+}
+
+type CoolStructSlice []CoolStruct
+
+func (s CoolStructSlice) Len() int      { return len(s) }
+func (s CoolStructSlice) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
+
+// these could be defined inline inside of the Less function, too.
+func (s CoolStructSlice) LessA(i, j int) bool { return s[i].A < s[j].A }
+func (s CoolStructSlice) LessB(i, j int) bool { return s[i].B < s[j].B }
+func (s CoolStructSlice) LessC(i, j int) bool { return s[i].C < s[j].C }
+
+func (s CoolStructSlice) Less(i, j int) bool {
+	return Chain{s.LessA, s.LessB, nil, s.LessC}.Use(i, j)
+}
+
+func TestSortBy(t *testing.T) {
+	t.Parallel()
+
+	Convey("sortby", t, func() {
+		s := CoolStructSlice{
+			{194, 771, 222},
+			{209, 28, 300},
+			{413, 639, 772},
+			{14, 761, 759},
+			{866, 821, 447},
+			{447, 373, 817},
+			{132, 510, 149},
+			{778, 513, 156},
+			{713, 831, 596},
+			{288, 83, 898},
+			{679, 688, 903},
+			{864, 100, 199},
+			{229, 975, 648},
+			{381, 290, 156},
+			{447, 290, 156}, // intentionally partially equal
+			{311, 614, 434},
+		}
+
+		sort.Sort(s)
+
+		So(s, ShouldResemble, CoolStructSlice{
+			{14, 761, 759},
+			{132, 510, 149},
+			{194, 771, 222},
+			{209, 28, 300},
+			{229, 975, 648},
+			{288, 83, 898},
+			{311, 614, 434},
+			{381, 290, 156},
+			{413, 639, 772},
+			{447, 290, 156},
+			{447, 373, 817},
+			{679, 688, 903},
+			{713, 831, 596},
+			{778, 513, 156},
+			{864, 100, 199},
+			{866, 821, 447},
+		})
+	})
+}