Migrate clashtcpip to gVisor tcpip copied

internal/gtcpip/checksum/checksum.go

@@ -0,0 +1,68 @@
+// Copyright 2018 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package checksum provides the implementation of the encoding and decoding of
+// network protocol headers.
+package checksum
+
+import (
+	"encoding/binary"
+)
+
+// Size is the size of a checksum.
+//
+// The checksum is held in a uint16 which is 2 bytes.
+const Size = 2
+
+// Put puts the checksum in the provided byte slice.
+func Put(b []byte, xsum uint16) {
+	binary.BigEndian.PutUint16(b, xsum)
+}
+
+// Checksum calculates the checksum (as defined in RFC 1071) of the bytes in the
+// given byte array. This function uses an optimized version of the checksum
+// algorithm.
+//
+// The initial checksum must have been computed on an even number of bytes.
+func Checksum(buf []byte, initial uint16) uint16 {
+	s, _ := calculateChecksum(buf, false, initial)
+	return s
+}
+
+// Checksumer calculates checksum defined in RFC 1071.
+type Checksumer struct {
+	sum uint16
+	odd bool
+}
+
+// Add adds b to checksum.
+func (c *Checksumer) Add(b []byte) {
+	if len(b) > 0 {
+		c.sum, c.odd = calculateChecksum(b, c.odd, c.sum)
+	}
+}
+
+// Checksum returns the latest checksum value.
+func (c *Checksumer) Checksum() uint16 {
+	return c.sum
+}
+
+// Combine combines the two uint16 to form their checksum. This is done
+// by adding them and the carry.
+//
+// Note that checksum a must have been computed on an even number of bytes.
+func Combine(a, b uint16) uint16 {
+	v := uint32(a) + uint32(b)
+	return uint16(v + v>>16)
+}