Add sort function helper library.

common/data/sortby/sortby_test.go

+// 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},
+                })
+        })
+}