time: move RFC 3339 optimizations to separate file

The optimizations were added in CL 425197 and CL 421877.
Move this functionality to a separate file to keep format.go smaller
and to document the justification for why this optimization exists.

Change-Id: I1e4e1ace19f9d596d8c0cf49ab6062f63a87b5bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/430675
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Auto-Submit: Jenny Rakoczy <jenny@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Jenny Rakoczy <jenny@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Jenny Rakoczy <jenny@golang.org>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>

2 files changed, 131 insertions, 117 deletions

diff --git a/src/time/format.go b/src/time/format.go
index afb130cf94..8051687068 100644
--- a/src/time/format.go
+++ b/src/time/format.go
@@ -797,50 +797,6 @@ func (t Time) appendFormat(b []byte, layout string) []byte {
 	return b
 }
 
-func (t Time) appendFormatRFC3339(b []byte, nanos bool) []byte {
-	_, offset, abs := t.locabs()
-
-	// Format date.
-	year, month, day, _ := absDate(abs, true)
-	b = appendInt(b, year, 4)
-	b = append(b, '-')
-	b = appendInt(b, int(month), 2)
-	b = append(b, '-')
-	b = appendInt(b, day, 2)
-
-	b = append(b, 'T')
-
-	// Format time.
-	hour, min, sec := absClock(abs)
-	b = appendInt(b, hour, 2)
-	b = append(b, ':')
-	b = appendInt(b, min, 2)
-	b = append(b, ':')
-	b = appendInt(b, sec, 2)
-
-	if nanos {
-		std := stdFracSecond(stdFracSecond9, 9, '.')
-		b = formatNano(b, uint(t.Nanosecond()), std)
-	}
-
-	if offset == 0 {
-		return append(b, 'Z')
-	}
-
-	// Format zone.
-	zone := offset / 60 // convert to minutes
-	if zone < 0 {
-		b = append(b, '-')
-		zone = -zone
-	} else {
-		b = append(b, '+')
-	}
-	b = appendInt(b, zone/60, 2)
-	b = append(b, ':')
-	b = appendInt(b, zone%60, 2)
-	return b
-}
-
 var errBad = errors.New("bad value for field") // placeholder not passed to user
 
 // ParseError describes a problem parsing a time string.
@@ -1043,79 +999,6 @@ func ParseInLocation(layout, value string, loc *Location) (Time, error) {
 	return parse(layout, value, loc, loc)
 }
 
-func parseRFC3339(s string, local *Location) (Time, bool) {
-	// parseUint parses s as an unsigned decimal integer and
-	// verifies that it is within some range.
-	// If it is invalid or out-of-range,
-	// it sets ok to false and returns the min value.
-	ok := true
-	parseUint := func(s string, min, max int) (x int) {
-		for _, c := range []byte(s) {
-			if c < '0' || '9' < c {
-				ok = false
-				return min
-			}
-			x = x*10 + int(c) - '0'
-		}
-		if x < min || max < x {
-			ok = false
-			return min
-		}
-		return x
-	}
-
-	// Parse the date and time.
-	if len(s) < len("2006-01-02T15:04:05") {
-		return Time{}, false
-	}
-	year := parseUint(s[0:4], 0, 9999)                       // e.g., 2006
-	month := parseUint(s[5:7], 1, 12)                        // e.g., 01
-	day := parseUint(s[8:10], 1, daysIn(Month(month), year)) // e.g., 02
-	hour := parseUint(s[11:13], 0, 23)                       // e.g., 15
-	min := parseUint(s[14:16], 0, 59)                        // e.g., 04
-	sec := parseUint(s[17:19], 0, 59)                        // e.g., 05
-	if !ok || !(s[4] == '-' && s[7] == '-' && s[10] == 'T' && s[13] == ':' && s[16] == ':') {
-		return Time{}, false
-	}
-	s = s[19:]
-
-	// Parse the fractional second.
-	var nsec int
-	if len(s) >= 2 && s[0] == '.' && isDigit(s, 1) {
-		n := 2
-		for ; n < len(s) && isDigit(s, n); n++ {
-		}
-		nsec, _, _ = parseNanoseconds(s, n)
-		s = s[n:]
-	}
-
-	// Parse the time zone.
-	t := Date(year, Month(month), day, hour, min, sec, nsec, UTC)
-	if s != "Z" {
-		if len(s) != len("-07:00") {
-			return Time{}, false
-		}
-		hr := parseUint(s[1:3], 0, 23) // e.g., 07
-		mm := parseUint(s[4:6], 0, 59) // e.g., 00
-		if !ok || !((s[0] == '-' || s[0] == '+') && s[3] == ':') {
-			return Time{}, false
-		}
-		zoneOffset := (hr*60 + mm) * 60
-		if s[0] == '-' {
-			zoneOffset *= -1
-		}
-		t.addSec(-int64(zoneOffset))
-
-		// Use local zone with the given offset if possible.
-		if _, offset, _, _, _ := local.lookup(t.unixSec()); offset == zoneOffset {
-			t.setLoc(local)
-		} else {
-			t.setLoc(FixedZone("", zoneOffset))
-		}
-	}
-	return t, true
-}
-
 func parse(layout, value string, defaultLocation, local *Location) (Time, error) {
 	alayout, avalue := layout, value
 	rangeErrString := "" // set if a value is out of range
diff --git a/src/time/format_rfc3339.go b/src/time/format_rfc3339.go
new file mode 100644
index 0000000000..3d0a9ca39f
--- /dev/null
+++ b/src/time/format_rfc3339.go
@@ -0,0 +1,131 @@
+// Copyright 2022 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
+
+// RFC 3339 is the most commonly used format.
+//
+// It is implicitly used by the Time.(Marshal|Unmarshal)(Text|JSON) methods.
+// Also, according to analysis on https://go.dev/issue/52746,
+// RFC 3339 accounts for 57% of all explicitly specified time formats,
+// with the second most popular format only being used 8% of the time.
+// The overwhelming use of RFC 3339 compared to all other formats justifies
+// the addition of logic to optimize formatting and parsing.
+
+func (t Time) appendFormatRFC3339(b []byte, nanos bool) []byte {
+	_, offset, abs := t.locabs()
+
+	// Format date.
+	year, month, day, _ := absDate(abs, true)
+	b = appendInt(b, year, 4)
+	b = append(b, '-')
+	b = appendInt(b, int(month), 2)
+	b = append(b, '-')
+	b = appendInt(b, day, 2)
+
+	b = append(b, 'T')
+
+	// Format time.
+	hour, min, sec := absClock(abs)
+	b = appendInt(b, hour, 2)
+	b = append(b, ':')
+	b = appendInt(b, min, 2)
+	b = append(b, ':')
+	b = appendInt(b, sec, 2)
+
+	if nanos {
+		std := stdFracSecond(stdFracSecond9, 9, '.')
+		b = formatNano(b, uint(t.Nanosecond()), std)
+	}
+
+	if offset == 0 {
+		return append(b, 'Z')
+	}
+
+	// Format zone.
+	zone := offset / 60 // convert to minutes
+	if zone < 0 {
+		b = append(b, '-')
+		zone = -zone
+	} else {
+		b = append(b, '+')
+	}
+	b = appendInt(b, zone/60, 2)
+	b = append(b, ':')
+	b = appendInt(b, zone%60, 2)
+	return b
+}
+
+func parseRFC3339(s string, local *Location) (Time, bool) {
+	// parseUint parses s as an unsigned decimal integer and
+	// verifies that it is within some range.
+	// If it is invalid or out-of-range,
+	// it sets ok to false and returns the min value.
+	ok := true
+	parseUint := func(s string, min, max int) (x int) {
+		for _, c := range []byte(s) {
+			if c < '0' || '9' < c {
+				ok = false
+				return min
+			}
+			x = x*10 + int(c) - '0'
+		}
+		if x < min || max < x {
+			ok = false
+			return min
+		}
+		return x
+	}
+
+	// Parse the date and time.
+	if len(s) < len("2006-01-02T15:04:05") {
+		return Time{}, false
+	}
+	year := parseUint(s[0:4], 0, 9999)                       // e.g., 2006
+	month := parseUint(s[5:7], 1, 12)                        // e.g., 01
+	day := parseUint(s[8:10], 1, daysIn(Month(month), year)) // e.g., 02
+	hour := parseUint(s[11:13], 0, 23)                       // e.g., 15
+	min := parseUint(s[14:16], 0, 59)                        // e.g., 04
+	sec := parseUint(s[17:19], 0, 59)                        // e.g., 05
+	if !ok || !(s[4] == '-' && s[7] == '-' && s[10] == 'T' && s[13] == ':' && s[16] == ':') {
+		return Time{}, false
+	}
+	s = s[19:]
+
+	// Parse the fractional second.
+	var nsec int
+	if len(s) >= 2 && s[0] == '.' && isDigit(s, 1) {
+		n := 2
+		for ; n < len(s) && isDigit(s, n); n++ {
+		}
+		nsec, _, _ = parseNanoseconds(s, n)
+		s = s[n:]
+	}
+
+	// Parse the time zone.
+	t := Date(year, Month(month), day, hour, min, sec, nsec, UTC)
+	if s != "Z" {
+		if len(s) != len("-07:00") {
+			return Time{}, false
+		}
+		hr := parseUint(s[1:3], 0, 23) // e.g., 07
+		mm := parseUint(s[4:6], 0, 59) // e.g., 00
+		if !ok || !((s[0] == '-' || s[0] == '+') && s[3] == ':') {
+			return Time{}, false
+		}
+		zoneOffset := (hr*60 + mm) * 60
+		if s[0] == '-' {
+			zoneOffset *= -1
+		}
+		t.addSec(-int64(zoneOffset))
+
+		// Use local zone with the given offset if possible.
+		if _, offset, _, _, _ := local.lookup(t.unixSec()); offset == zoneOffset {
+			t.setLoc(local)
+		} else {
+			t.setLoc(FixedZone("", zoneOffset))
+		}
+	}
+	return t, true
+}