language: fix off-by-one error

Regions are encoded starting from 1. However, one of the
region-related tables assumed 0-based indices. This
caused a crash when used with ZZ, the largest region.

Fixes golang/go#43834

Change-Id: Iaed6b9d2683cd50504e6d33c8a6df8b21dd1687d
Reviewed-on: https://go-review.googlesource.com/c/text/+/305469
Trust: Marcel van Lohuizen <mpvl@golang.org>
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Seth Vargo <sethvargo@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

language/gen.go

@@ -150,7 +150,9 @@ func (b *builder) writeMatchData() {
 		regions := strings.Split(g.Contains, " ")
 		regionHierarchy[g.Type] = append(regionHierarchy[g.Type], regions...)
 	}
-	regionToGroups := make([]uint8, language.NumRegions)
+	// Regions start at 1, so the slice must be one larger than the number of
+	// regions.
+	regionToGroups := make([]uint8, language.NumRegions+1)
 
 	idToIndex := map[string]uint8{}
 	for i, mv := range lm[0].MatchVariable {

language/match_test.go

@@ -224,6 +224,20 @@ func (t haveTag) String() string {
 	return fmt.Sprintf("%v:%d:%v:%v-%v|%v", t.tag, t.index, t.conf, t.maxRegion, t.maxScript, t.altScript)
 }
 
+func TestIssue43834(t *testing.T) {
+	matcher := NewMatcher([]Tag{English})
+
+	// ZZ is the largest region code and should not cause overflow.
+	desired, _, err := ParseAcceptLanguage("en-ZZ")
+	if err != nil {
+		t.Error(err)
+	}
+	_, i, _ := matcher.Match(desired...)
+	if i != 0 {
+		t.Errorf("got %v; want 0", i)
+	}
+}
+
 func TestBestMatchAlloc(t *testing.T) {
 	m := NewMatcher(makeTagList("en sr nl"))
 	// Go allocates when creating a list of tags from a single tag!