wgengine/wglog: match upstream wireguard-go's code for wireguardGoString

It is a bit faster.

But more importantly, it matches upstream byte-for-byte,
which ensures there'll be no corner cases in which we disagree.

name        old time/op    new time/op    delta
SetPeers-8    3.58µs ± 0%    3.16µs ± 2%  -11.74%  (p=0.016 n=4+5)

name        old alloc/op   new alloc/op   delta
SetPeers-8    2.53kB ± 0%    2.53kB ± 0%     ~     (all equal)

name        old allocs/op  new allocs/op  delta
SetPeers-8      99.0 ± 0%      99.0 ± 0%     ~     (all equal)

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

wgengine/wglog/wglog.go

@@ -6,7 +6,6 @@
 package wglog
 
 import (
-	"encoding/base64"
 	"fmt"
 	"strings"
 	"sync"
@@ -121,17 +120,20 @@ func (x *Logger) SetPeers(peers []wgcfg.Peer) {
 
 // wireguardGoString prints p in the same format used by wireguard-go.
 func wireguardGoString(k wgkey.Key) string {
-	const prefix = "peer("
-	b := make([]byte, len(prefix)+44)
-	copy(b, prefix)
-	r := b[len(prefix):]
-	base64.StdEncoding.Encode(r, k[:])
-	r = r[4:]
-	copy(r, "…")
-	r = r[len("…"):]
-	copy(r, b[len(prefix)+39:len(prefix)+43])
-	r = r[4:]
-	r[0] = ')'
-	r = r[1:]
-	return string(b[:len(b)-len(r)])
+	src := k
+	b64 := func(input byte) byte {
+		return input + 'A' + byte(((25-int(input))>>8)&6) - byte(((51-int(input))>>8)&75) - byte(((61-int(input))>>8)&15) + byte(((62-int(input))>>8)&3)
+	}
+	b := []byte("peer(____…____)")
+	const first = len("peer(")
+	const second = len("peer(____…")
+	b[first+0] = b64((src[0] >> 2) & 63)
+	b[first+1] = b64(((src[0] << 4) | (src[1] >> 4)) & 63)
+	b[first+2] = b64(((src[1] << 2) | (src[2] >> 6)) & 63)
+	b[first+3] = b64(src[2] & 63)
+	b[second+0] = b64(src[29] & 63)
+	b[second+1] = b64((src[30] >> 2) & 63)
+	b[second+2] = b64(((src[30] << 4) | (src[31] >> 4)) & 63)
+	b[second+3] = b64((src[31] << 2) & 63)
+	return string(b)
 }