33 lines
1.2 KiB
Kotlin
33 lines
1.2 KiB
Kotlin
package tech.xvanturing.freeproxy.vpn.net
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/** RFC 1071 定义的 16 位反码和。 */
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object Checksum {
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/** 对 [length] 字节做反码求和，[initial] 用于把伪头部的和接续进来。 */
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fun compute(data: ByteArray, offset: Int, length: Int, initial: Long = 0L): Int {
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var sum = initial
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var index = offset
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val end = offset + length
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while (index + 1 < end) {
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sum += data.u16(index)
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index += 2
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}
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// 奇数长度时最后一字节按高位对齐补零
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if (index < end) sum += data.u8(index) shl 8
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while ((sum ushr 16) != 0L) sum = (sum and 0xFFFF) + (sum ushr 16)
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return (sum.inv() and 0xFFFF).toInt()
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}
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/** TCP/UDP 校验和覆盖的伪头部：源地址、目的地址、协议号与传输层长度。 */
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fun pseudoHeaderSum(sourceIp: Int, destIp: Int, protocol: Int, transportLength: Int): Long {
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var sum = 0L
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sum += ((sourceIp ushr 16) and 0xFFFF).toLong()
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sum += (sourceIp and 0xFFFF).toLong()
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sum += ((destIp ushr 16) and 0xFFFF).toLong()
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sum += (destIp and 0xFFFF).toLong()
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sum += protocol.toLong()
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sum += transportLength.toLong()
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return sum
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}
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}
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