time: use names for beginning and end of zone transition times

No functional changes, just more readable code.

LGTM=r
R=golang-codereviews, gobot, r
CC=golang-codereviews
https://golang.org/cl/59240043

src/pkg/time/zoneinfo.go

@@ -45,6 +45,13 @@ type zoneTrans struct {
 	isstd, isutc bool  // ignored - no idea what these mean
 }
 
+// alpha and omega are the beginning and end of time for zone
+// transitions.
+const (
+	alpha = -1 << 63  // math.MinInt64
+	omega = 1<<63 - 1 // math.MaxInt64
+)
+
 // UTC represents Universal Coordinated Time (UTC).
 var UTC *Location = &utcLoc
 
@@ -83,9 +90,9 @@ func FixedZone(name string, offset int) *Location {
 	l := &Location{
 		name:       name,
 		zone:       []zone{{name, offset, false}},
-		tx:         []zoneTrans{{-1 << 63, 0, false, false}},
-		cacheStart: -1 << 63,
-		cacheEnd:   1<<63 - 1,
+		tx:         []zoneTrans{{alpha, 0, false, false}},
+		cacheStart: alpha,
+		cacheEnd:   omega,
 	}
 	l.cacheZone = &l.zone[0]
 	return l
@@ -105,8 +112,8 @@ func (l *Location) lookup(sec int64) (name string, offset int, isDST bool, start
 		name = "UTC"
 		offset = 0
 		isDST = false
-		start = -1 << 63
-		end = 1<<63 - 1
+		start = alpha
+		end = omega
 		return
 	}
 
@@ -124,11 +131,11 @@ func (l *Location) lookup(sec int64) (name string, offset int, isDST bool, start
 		name = zone.name
 		offset = zone.offset
 		isDST = zone.isDST
-		start = -1 << 63
+		start = alpha
 		if len(l.tx) > 0 {
 			end = l.tx[0].when
 		} else {
-			end = 1<<63 - 1
+			end = omega
 		}
 		return
 	}
@@ -136,7 +143,7 @@ func (l *Location) lookup(sec int64) (name string, offset int, isDST bool, start
 	// Binary search for entry with largest time <= sec.
 	// Not using sort.Search to avoid dependencies.
 	tx := l.tx
-	end = 1<<63 - 1
+	end = omega
 	lo := 0
 	hi := len(tx)
 	for hi-lo > 1 {

src/pkg/time/zoneinfo_plan9.go

@@ -100,7 +100,7 @@ func loadZoneDataPlan9(s string) (l *Location, err error) {
 	for i := range tx {
 		if tx[i].when <= sec && (i+1 == len(tx) || sec < tx[i+1].when) {
 			l.cacheStart = tx[i].when
-			l.cacheEnd = 1<<63 - 1
+			l.cacheEnd = omega
 			if i+1 < len(tx) {
 				l.cacheEnd = tx[i+1].when
 			}

src/pkg/time/zoneinfo_read.go

@@ -173,7 +173,7 @@ func loadZoneData(bytes []byte) (l *Location, err error) {
 	if len(tx) == 0 {
 		// Build fake transition to cover all time.
 		// This happens in fixed locations like "Etc/GMT0".
-		tx = append(tx, zoneTrans{when: -1 << 63, index: 0})
+		tx = append(tx, zoneTrans{when: alpha, index: 0})
 	}
 
 	// Committed to succeed.
@@ -185,7 +185,7 @@ func loadZoneData(bytes []byte) (l *Location, err error) {
 	for i := range tx {
 		if tx[i].when <= sec && (i+1 == len(tx) || sec < tx[i+1].when) {
 			l.cacheStart = tx[i].when
-			l.cacheEnd = 1<<63 - 1
+			l.cacheEnd = omega
 			if i+1 < len(tx) {
 				l.cacheEnd = tx[i+1].when
 			}

src/pkg/time/zoneinfo_windows.go

@@ -165,8 +165,8 @@ func initLocalFromTZI(i *syscall.Timezoneinformation) {
 	if nzone == 1 {
 		// No daylight savings.
 		std.offset = -int(i.Bias) * 60
-		l.cacheStart = -1 << 63
-		l.cacheEnd = 1<<63 - 1
+		l.cacheStart = alpha
+		l.cacheEnd = omega
 		l.cacheZone = std
 		l.tx = make([]zoneTrans, 1)
 		l.tx[0].when = l.cacheStart