optimize parsing common timestamps

This commit adds a dedicated function for parsing the two most common
timestamps used by sqlite3.

```
goos: darwin
goarch: arm64
pkg: github.com/charlievieth/go-sqlite3
cpu: Apple M4 Pro
                            │ base.10.txt  │             new.10.txt              │
                            │    sec/op    │   sec/op     vs base                │
Suite/BenchmarkParseTime-14   11.347µ ± 0%   8.044µ ± 0%  -29.11% (p=0.000 n=10)

                            │ base.10.txt  │            new.10.txt            │
                            │     B/op     │     B/op      vs base            │
Suite/BenchmarkParseTime-14   1.713Ki ± 0%   1.713Ki ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal

                            │ base.10.txt │           new.10.txt           │
                            │  allocs/op  │ allocs/op   vs base            │
Suite/BenchmarkParseTime-14    64.00 ± 0%   64.00 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal
```

timefmt benchmarks:
```
goos: darwin
goarch: arm64
pkg: github.com/charlievieth/go-sqlite3/internal/timefmt
cpu: Apple M4 Pro
BenchmarkParse/Stdlib-14         	11050214	       103.5 ns/op	       0 B/op	       0 allocs/op
BenchmarkParse/Timefmt-14        	34134667	        34.45 ns/op	       0 B/op	       0 allocs/op
BenchmarkParse/Timefmt_T-14      	33790362	        33.97 ns/op	       0 B/op	       0 allocs/op
PASS
ok  	github.com/charlievieth/go-sqlite3/internal/timefmt	3.814s
```

Author: charlievieth

internal/timefmt/timefmt.go

@@ -1,6 +1,10 @@
 package timefmt
 
-import "time"
+import (
+	"errors"
+	"strconv"
+	"time"
+)
 
 const digits = "0123456789"
 
@@ -120,3 +124,166 @@ func Format(t time.Time) []byte {
 
 	return b
 }
+
+func isLeap(year int) bool {
+	return year%4 == 0 && (year%100 != 0 || year%400 == 0)
+}
+
+// daysBefore[m] counts the number of days in a non-leap year
+// before month m begins. There is an entry for m=12, counting
+// the number of days before January of next year (365).
+var daysBefore = [...]int32{
+	0,
+	31,
+	31 + 28,
+	31 + 28 + 31,
+	31 + 28 + 31 + 30,
+	31 + 28 + 31 + 30 + 31,
+	31 + 28 + 31 + 30 + 31 + 30,
+	31 + 28 + 31 + 30 + 31 + 30 + 31,
+	31 + 28 + 31 + 30 + 31 + 30 + 31 + 31,
+	31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30,
+	31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31,
+	31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30,
+	31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30 + 31,
+}
+
+func daysIn(m time.Month, year int) int {
+	if m == time.February && isLeap(year) {
+		return 29
+	}
+	return int(daysBefore[m] - daysBefore[m-1])
+}
+
+// isDigit reports whether s[i] is in range and is a decimal digit.
+func isDigit(s string, i int) bool {
+	if len(s) <= i {
+		return false
+	}
+	c := s[i]
+	return '0' <= c && c <= '9'
+}
+
+func commaOrPeriod(b byte) bool {
+	return b == '.' || b == ','
+}
+
+var errBad = errors.New("bad value for field") // placeholder not passed to user
+
+func parseNanoseconds(value string, nbytes int) (ns int, rangeErrString string, err error) {
+	if !commaOrPeriod(value[0]) {
+		err = errBad
+		return
+	}
+	if nbytes > 10 {
+		value = value[:10]
+		nbytes = 10
+	}
+	if ns, err = strconv.Atoi(value[1:nbytes]); err != nil {
+		return
+	}
+	if ns < 0 {
+		rangeErrString = "fractional second"
+		return
+	}
+	// We need nanoseconds, which means scaling by the number
+	// of missing digits in the format, maximum length 10.
+	scaleDigits := 10 - nbytes
+	for i := 0; i < scaleDigits; i++ {
+		ns *= 10
+	}
+	return
+}
+
+func parse(s string, local *time.Location) (time.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.
+	// "2006-01-02 15:04:05.999999999-07:00"
+	if len(s) < len("2006-01-02 15:04:05") {
+		return time.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(time.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] == ' ' || s[10] == 'T') && s[13] == ':' && s[16] == ':') {
+		return time.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 := time.Date(year, time.Month(month), day, hour, min, sec, nsec, time.UTC)
+	if len(s) != 1 || s[0] != 'Z' {
+		if len(s) != len("-07:00") {
+			return time.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.Time{}, false
+		}
+		zoneOffset := (hr*60 + mm) * 60
+		if s[0] == '-' {
+			zoneOffset *= -1
+		}
+		t = t.Add(-(time.Duration(zoneOffset) * time.Second))
+
+		// Use local zone with the given offset if possible.
+		t2 := t.In(local)
+		_, offset := t2.Zone()
+		if offset == zoneOffset {
+			t = t2
+		} else {
+			t = t.In(time.FixedZone("", zoneOffset))
+		}
+	}
+
+	return t, true
+}
+
+// Parse is an specialized version of time.Parse that is optimized for
+// the below two timestamps:
+//
+//   - "2006-01-02 15:04:05.999999999-07:00"
+//   - "2006-01-02T15:04:05.999999999-07:00"
+func Parse(s string, local *time.Location) (time.Time, error) {
+	if t, ok := parse(s, local); ok {
+		return t, nil
+	}
+	if len(s) > 10 && s[10] == 'T' {
+		return time.Parse("2006-01-02T15:04:05.999999999-07:00", s)
+	}
+	return time.Parse("2006-01-02 15:04:05.999999999-07:00", s)
+}

internal/timefmt/timefmt_test.go

@@ -54,6 +54,61 @@ func TestFormatTimeAllocs(t *testing.T) {
 	}
 }
 
+func TestParse(t *testing.T) {
+	rr := rand.New(rand.NewSource(time.Now().UnixNano()))
+	locs := make([]*time.Location, 1000)
+	for i := range locs {
+		offset := rr.Intn(60 * 60 * 14) // 14 hours
+		if rr.Int()&1 != 0 {
+			offset = -offset
+		}
+		locs[i] = time.FixedZone(strconv.Itoa(offset), offset)
+	}
+	// Append some standard locations
+	locs = append(locs, time.Local, time.UTC)
+
+	times := []time.Time{
+		{},
+		time.Now(),
+		time.Date(0, 0, 0, 0, 0, 0, 0, time.UTC),
+		time.Date(1, 1, 1, 1, 1, 1, 1, time.UTC),
+		time.Date(20_000, 1, 1, 1, 1, 1, 1, time.UTC),
+		time.Date(-1, 0, 0, 0, 0, 0, 0, time.UTC),
+		time.Date(2028, 2, 29, 0, 0, 0, 0, time.UTC),   // Leap day
+		time.Date(2028, 2, 29, 1, 1, 1, 1, time.Local), // Leap day
+	}
+	for i := 0; i < 100; i++ {
+		times = append(times, time.Now().Add(time.Duration(rr.Int63n(int64(time.Hour*24*365)))))
+	}
+
+	passed := 0
+	for _, loc := range locs {
+		for _, tt := range times {
+			tt = tt.In(loc)
+			for _, format := range sqlite3.SQLiteTimestampFormats[:2] {
+				s := tt.Format(format)
+				want, err := time.ParseInLocation(format, s, loc)
+				if err != nil {
+					continue
+				}
+				got, err := timefmt.Parse(s, loc)
+				if err != nil {
+					t.Error(err)
+					continue
+				}
+				if !got.Equal(want) {
+					t.Errorf("timefmt.Parse(%q) = %s; want: %s", s, got, want)
+					continue
+				}
+				passed++
+			}
+		}
+	}
+	if passed == 0 {
+		t.Fatal("No tests passed")
+	}
+}
+
 func BenchmarkFormat(b *testing.B) {
 	loc, err := time.LoadLocation("America/New_York")
 	if err != nil {
@@ -64,3 +119,38 @@ func BenchmarkFormat(b *testing.B) {
 		_ = timefmt.Format(ts)
 	}
 }
+
+func BenchmarkParse(b *testing.B) {
+	layout := sqlite3.SQLiteTimestampFormats[0]
+	loc, err := time.LoadLocation("America/New_York")
+	if err != nil {
+		b.Fatal(err)
+	}
+	ts := time.Date(2024, 1, 2, 15, 4, 5, 123456789, loc).Format(layout)
+
+	b.Run("Stdlib", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			_, err := time.Parse(layout, ts)
+			if err != nil {
+				b.Fatal(err)
+			}
+		}
+	})
+	b.Run("Timefmt", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			_, err := timefmt.Parse(ts, time.Local)
+			if err != nil {
+				b.Fatal(err)
+			}
+		}
+	})
+	b.Run("Timefmt_T", func(b *testing.B) {
+		ts := time.Date(2024, 1, 2, 15, 4, 5, 123456789, loc).Format(sqlite3.SQLiteTimestampFormats[1])
+		for i := 0; i < b.N; i++ {
+			_, err := timefmt.Parse(ts, time.Local)
+			if err != nil {
+				b.Fatal(err)
+			}
+		}
+	})
+}

sqlite3.go

@@ -2756,9 +2756,11 @@ func (rc *SQLiteRows) nextSyncLocked(dest []driver.Value) error {
 				s = strings.TrimSuffix(s, "Z")
 				var err error
 				var t time.Time
-				for _, format := range SQLiteTimestampFormats {
-					if t, err = time.ParseInLocation(format, s, time.UTC); err == nil {
-						break
+				if t, err = timefmt.Parse(s, time.UTC); err != nil {
+					for _, format := range SQLiteTimestampFormats[2:] {
+						if t, err = time.ParseInLocation(format, s, time.UTC); err == nil {
+							break
+						}
 					}
 				}
 				if err != nil {

sqlite3_test.go

@@ -3468,7 +3468,17 @@ func benchmarkParseTime(b *testing.B) {
 		b.Fatal(err)
 	}
 	defer db.Close()
-	if _, err := db.Exec(`CREATE TABLE time_bench (ts DATETIME NOT NULL);`); err != nil {
+	const createTableStmt = `
+	CREATE TABLE time_bench (
+		ts1 DATETIME NOT NULL,
+		ts2 DATETIME NOT NULL,
+		ts3 DATETIME NOT NULL,
+		ts4 DATETIME NOT NULL,
+		ts5 DATETIME NOT NULL,
+		ts6 DATETIME NOT NULL
+	);`
+	// if _, err := db.Exec(`CREATE TABLE time_bench (ts DATETIME NOT NULL);`); err != nil {
+	if _, err := db.Exec(createTableStmt); err != nil {
 		b.Fatal(err)
 	}
 	// t := time.Date(year, month, day, hour, min, sec, nsec, loc)
@@ -3478,12 +3488,15 @@ func benchmarkParseTime(b *testing.B) {
 	}
 	ts := time.Date(2024, 1, 2, 15, 4, 5, 123456789, loc)
 	for i := 0; i < 8; i++ {
-		_, err := db.Exec(`INSERT INTO time_bench VALUES(?)`, ts)
+		_, err := db.Exec(`INSERT INTO time_bench VALUES(?, ?, ?, ?, ?, ?)`,
+			ts, ts, ts, ts, ts, ts)
 		if err != nil {
 			b.Fatal(err)
 		}
 	}
-	stmt, err := db.Prepare(`SELECT ts FROM time_bench LIMIT 1;`)
+
+	// stmt, err := db.Prepare(`SELECT ts1, ts2, ts3, ts4, ts5, ts6 FROM time_bench LIMIT 1;`)
+	stmt, err := db.Prepare(`SELECT ts1, ts2, ts3, ts4, ts5, ts6 FROM time_bench;`)
 	if err != nil {
 		b.Fatal(err)
 	}
@@ -3495,7 +3508,7 @@ func benchmarkParseTime(b *testing.B) {
 		}
 		var t time.Time
 		for rows.Next() {
-			if err := rows.Scan(&t); err != nil {
+			if err := rows.Scan(&t, &t, &t, &t, &t, &t); err != nil {
 				b.Fatal(err)
 			}
 		}