Add sort function helper library.

common/data/sortby/sortby.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 provides a succinct way to generate correctly-behaved Less
+// funcions for use with the stdlib 'sort' package.
+package sortby
+
+// LessFn is the type of the function which compares element i with element j of
+// a given slice. Unlike the stdlib sort interpretation of this function,
+// a LessFn in sortby should only compare a single field in your datastructure's
+// elements. Multiple LessFns can be composed with This and ThenBy to create

[Vadim Sh., 2017/01/25 03:43:34]

what This and ThenBy?

[iannucci, 2017/01/25 03:47:24]

Removed them (generated extra unnecessary indirection+syntax), will update
comment

+// a composite Less implementation to pass to sort.
+type LessFn func(i, j int) bool
+
+// Chain is a list of LessFns, each of which sorts a single aspect of your
+// object. Nil LessFns will be ignored.
+type Chain []LessFn
+
+// Use is a sort-compatible LessFn that actually executes the full chain of
+// comparisons.
+func (c Chain) Use(i, j int) bool {
+        for _, less := range c {
+                if less == nil {
+                        continue
+                }
+                if less(i, j) {
+                        return true
+                } else if less(j, i) {
+                        return false
+                }
+        }
+        return false
+}