ioptimized decodeArgAppend a bit

Benchmark results on linux/amd64:

name                           old time/op  new time/op  delta
ArgsParse-4                    72.8ns ± 2%  68.0ns ± 2%  -6.59%  (p=0.000 n=10+9)
AppendUnquotedArgFastPath-4    20.4ns ± 2%  21.1ns ± 9%    ~     (p=0.614 n=8+10)
AppendUnquotedArgSlowPath-4    68.9ns ± 3%  70.4ns ± 6%    ~     (p=0.148 n=9+10)
URIParsePath-4                 80.9ns ± 2%  78.7ns ± 2%  -2.80%  (p=0.000 n=10+10)
URIParsePathQueryString-4      88.9ns ± 1%  86.3ns ± 1%  -2.90%  (p=0.000 n=10+8)
URIParsePathQueryStringHash-4  95.7ns ± 8%  91.0ns ± 1%  -4.88%  (p=0.000 n=9+10)
URIParseHostname-4             98.6ns ± 1%  95.4ns ± 1%  -3.24%  (p=0.000 n=10+10)

1 files changed, 7 insertions, 11 deletions

diff --git a/bytesconv.go b/bytesconv.go
index 56d6ca2..d92150f 100644
--- a/bytesconv.go
+++ b/bytesconv.go
@@ -265,8 +265,8 @@ func readHexInt(r *bufio.Reader) (int, error) {
 			}
 			return -1, err
 		}
-		k = hexbyte2int(c)
-		if k < 0 {
+		k = int(hex2intTable[c])
+		if k == 16 {
 			if i == 0 {
 				return -1, errEmptyHexNum
 			}
@@ -324,23 +324,19 @@ func hexCharUpper(c byte) byte {
 var hex2intTable = func() []byte {
 	b := make([]byte, 255)
 	for i := byte(0); i < 255; i++ {
-		c := byte(0)
+		c := byte(16)
 		if i >= '0' && i <= '9' {
-			c = 1 + i - '0'
+			c = i - '0'
 		} else if i >= 'a' && i <= 'f' {
-			c = 1 + i - 'a' + 10
+			c = i - 'a' + 10
 		} else if i >= 'A' && i <= 'F' {
-			c = 1 + i - 'A' + 10
+			c = i - 'A' + 10
 		}
 		b[i] = c
 	}
 	return b
 }()
 
-func hexbyte2int(c byte) int {
-	return int(hex2intTable[c]) - 1
-}
-
 const toLower = 'a' - 'A'
 
 var toLowerTable = func() [256]byte {
@@ -401,7 +397,7 @@ func s2b(s string) []byte {
 //
 // dst may point to src. In this case src will be overwritten.
 func AppendUnquotedArg(dst, src []byte) []byte {
-	return decodeArgAppend(dst, src, true)
+	return decodeArgAppend(dst, src)
 }
 
 // AppendQuotedArg appends url-encoded src to dst and returns appended dst.