tstime: hand-implement parseInt for specific needs of rfc3339 parsing.

Makes parsing 4.6x faster.

name         old time/op  new time/op  delta
ParseInt-12  32.1ns ± 1%   6.9ns ± 2%  -78.55%  (p=0.000 n=10+9)

Signed-off-by: David Anderson <danderson@tailscale.com>

tstime/tstime.go

@@ -8,7 +8,6 @@ package tstime
 import (
 	"errors"
 	"fmt"
-	"strconv"
 	"strings"
 	"sync"
 	"time"
@@ -105,10 +104,19 @@ func Parse3339(s string) (time.Time, error) {
 }
 
 func parseInt(s string, dst *int) bool {
-	n, err := strconv.ParseInt(s, 10, 0)
-	if err != nil || n < 0 {
+	if len(s) == 0 || len(s) > len("999999999") {
+		*dst = 0
 		return false
 	}
-	*dst = int(n)
+	n := 0
+	for i := 0; i < len(s); i++ {
+		d := s[i] - '0'
+		if d > 9 {
+			*dst = 0
+			return false
+		}
+		n = n*10 + int(d)
+	}
+	*dst = n
 	return true
 }

tstime/tstime_test.go

@@ -76,6 +76,30 @@ func TestZoneOf(t *testing.T) {
 	}
 }
 
+func TestParseInt(t *testing.T) {
+	tests := []struct {
+		in   string
+		want int
+		ret  bool
+	}{
+		{"", 0, false},
+		{"1", 1, true},
+		{"999999999", 999999999, true},
+		{"123456789", 123456789, true},
+		{"000000", 0, true},
+		{"bork", 0, false},
+		{"123bork", 0, false},
+	}
+
+	for _, tt := range tests {
+		var got int
+		gotRet := parseInt(tt.in, &got)
+		if gotRet != tt.ret || got != tt.want {
+			t.Errorf("parseInt(%q) = %v, %d; want %v, %d", tt.in, gotRet, got, tt.ret, tt.want)
+		}
+	}
+}
+
 func BenchmarkGoParse3339(b *testing.B) {
 	run := func(in string) func(*testing.B) {
 		return func(b *testing.B) {
@@ -113,6 +137,7 @@ func BenchmarkGoParse3339InLocation(b *testing.B) {
 		b.Fatalf("times don't stringify the same: %q vs %q", s1, s2)
 	}
 
+	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
 		_, err := time.ParseInLocation(time.RFC3339Nano, in, loc)
 		if err != nil {
@@ -153,3 +178,10 @@ func BenchmarkParse3339(b *testing.B) {
 	b.Run("Z", run("2020-04-05T15:56:00.148487491Z"))
 	b.Run("TZ", run("2020-04-05T15:56:00.148487491+08:00"))
 }
+
+func BenchmarkParseInt(b *testing.B) {
+	var out int
+	for i := 0; i < b.N; i++ {
+		parseInt("148487491", &out)
+	}
+}