Add SortLockedProjects helper func

lock.go

@@ -1,5 +1,7 @@
 package gps
 
+import "sort"
+
 // Lock represents data from a lock file (or however the implementing tool
 // chooses to store it) at a particular version that is relevant to the
 // satisfiability solving process.
@@ -159,3 +161,23 @@ func prepLock(l Lock) Lock {
 
 	return rl
 }
+
+// SortLockedProjects sorts a slice of LockedProject in alphabetical order by
+// ProjectRoot.
+func SortLockedProjects(lps []LockedProject) {
+	sort.Stable(lpsorter(lps))
+}
+
+type lpsorter []LockedProject
+
+func (lps lpsorter) Swap(i, j int) {
+	lps[i], lps[j] = lps[j], lps[i]
+}
+
+func (lps lpsorter) Len() int {
+	return len(lps)
+}
+
+func (lps lpsorter) Less(i, j int) bool {
+	return lps[i].pi.ProjectRoot < lps[j].pi.ProjectRoot
+}

lock_test.go

@@ -0,0 +1,26 @@
+package gps
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestLockedProjectSorting(t *testing.T) {
+	// version doesn't matter here
+	lps := []LockedProject{
+		NewLockedProject("github.com/sdboyer/gps", NewVersion("v0.10.0"), "", nil),
+		NewLockedProject("foo", NewVersion("nada"), "", nil),
+		NewLockedProject("bar", NewVersion("zip"), "", nil),
+		NewLockedProject("qux", NewVersion("zilch"), "", nil),
+	}
+	lps2 := make([]LockedProject, len(lps))
+	copy(lps2, lps)
+
+	SortLockedProjects(lps2)
+
+	// only the two should have switched positions
+	lps[0], lps[2] = lps[2], lps[0]
+	if !reflect.DeepEqual(lps, lps2) {
+		t.Errorf("SortLockedProject did not sort as expected:\n\t(GOT) %s\n\t(WNT) %s", lps2, lps)
+	}
+}