time: correctly handle timezone before first transition time

LGTM=r
R=golang-codereviews, r, arnehormann, bradfitz
CC=golang-codereviews
https://golang.org/cl/58450043

src/pkg/time/zoneinfo.go

@@ -101,7 +101,7 @@ func FixedZone(name string, offset int) *Location {
 func (l *Location) lookup(sec int64) (name string, offset int, isDST bool, start, end int64) {
 	l = l.get()
 
-	if len(l.tx) == 0 {
+	if len(l.zone) == 0 {
 		name = "UTC"
 		offset = 0
 		isDST = false
@@ -119,6 +119,20 @@ func (l *Location) lookup(sec int64) (name string, offset int, isDST bool, start
 		return
 	}
 
+	if len(l.tx) == 0 || sec < l.tx[0].when {
+		zone := &l.zone[l.lookupFirstZone()]
+		name = zone.name
+		offset = zone.offset
+		isDST = zone.isDST
+		start = -1 << 63
+		if len(l.tx) > 0 {
+			end = l.tx[0].when
+		} else {
+			end = 1<<63 - 1
+		}
+		return
+	}
+
 	// Binary search for entry with largest time <= sec.
 	// Not using sort.Search to avoid dependencies.
 	tx := l.tx
@@ -144,6 +158,58 @@ func (l *Location) lookup(sec int64) (name string, offset int, isDST bool, start
 	return
 }
 
+// lookupFirstZone returns the index of the time zone to use for times
+// before the first transition time, or when there are no transition
+// times.
+//
+// The reference implementation in localtime.c from
+// http://www.iana.org/time-zones/repository/releases/tzcode2013g.tar.gz
+// implements the following algorithm for these cases:
+// 1) If the first zone is unused by the transitions, use it.
+// 2) Otherwise, if there are transition times, and the first
+//    transition is to a zone in daylight time, find the first
+//    non-daylight-time zone before and closest to the first transition
+//    zone.
+// 3) Otherwise, use the first zone that is not daylight time, if
+//    there is one.
+// 4) Otherwise, use the first zone.
+func (l *Location) lookupFirstZone() int {
+	// Case 1.
+	if !l.firstZoneUsed() {
+		return 0
+	}
+
+	// Case 2.
+	if len(l.tx) > 0 && l.zone[l.tx[0].index].isDST {
+		for zi := int(l.tx[0].index) - 1; zi >= 0; zi-- {
+			if !l.zone[zi].isDST {
+				return zi
+			}
+		}
+	}
+
+	// Case 3.
+	for zi := range l.zone {
+		if !l.zone[zi].isDST {
+			return zi
+		}
+	}
+
+	// Case 4.
+	return 0
+}
+
+// firstZoneUsed returns whether the first zone is used by some
+// transition.
+func (l *Location) firstZoneUsed() bool {
+	for _, tx := range l.tx {
+		if tx.index == 0 {
+			return true
+		}
+	}
+	return false
+}
+
 // lookupName returns information about the time zone with
 // the given name (such as "EST") at the given pseudo-Unix time
 // (what the given time of day would be in UTC).

src/pkg/time/zoneinfo_test.go

+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package time_test
+
+import (
+	"testing"
+	"time"
+)
+
+// Test that we get the correct results for times before the first
+// transition time.  To do this we explicitly check early dates in a
+// couple of specific timezones.
+func TestFirstZone(t *testing.T) {
+	time.ForceZipFileForTesting(true)
+	defer time.ForceZipFileForTesting(false)
+
+	const format = "Mon, 02 Jan 2006 15:04:05 -0700 (MST)"
+	var tests = []struct {
+		zone  string
+		unix  int64
+		want1 string
+		want2 string
+	}{
+		{
+			"PST8PDT",
+			-1633269601,
+			"Sun, 31 Mar 1918 01:59:59 -0800 (PST)",
+			"Sun, 31 Mar 1918 03:00:00 -0700 (PDT)",
+		},
+		{
+			"Pacific/Fakaofo",
+			1325242799,
+			"Thu, 29 Dec 2011 23:59:59 -1100 (TKT)",
+			"Sat, 31 Dec 2011 00:00:00 +1300 (TKT)",
+		},
+	}
+
+	for _, test := range tests {
+		z, err := time.LoadLocation(test.zone)
+		if err != nil {
+			t.Fatal(err)
+		}
+		s := time.Unix(test.unix, 0).In(z).Format(format)
+		if s != test.want1 {
+			t.Errorf("for %s %d got %q want %q", test.zone, test.unix, s, test.want1)
+		}
+		s = time.Unix(test.unix+1, 0).In(z).Format(format)
+		if s != test.want2 {
+			t.Errorf("for %s %d got %q want %q", test.zone, test.unix, s, test.want2)
+		}
+	}
+}