60 lines
1.8 KiB
C
60 lines
1.8 KiB
C
// Copyright 2020 Nicholas J. Kain <njkain at gmail dot com>
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// SPDX-License-Identifier: MIT
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#ifndef NCMLIB_NET_CHECKSUM16_H
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#define NCMLIB_NET_CHECKSUM16_H
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// RFC 1071 is still a good reference.
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#include <stdint.h>
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// When summing ones-complement 16-bit values using a 32-bit unsigned
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// representation, fold the carry bits that have spilled into the upper
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// 16-bits of the 32-bit unsigned value back into the 16-bit ones-complement
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// binary value.
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static inline uint16_t net_checksum16_foldcarry(uint32_t v)
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{
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v = (v >> 16) + (v & 0xffff);
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v += v >> 16;
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return v;
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}
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// Produces the correct result on little endian in the sense that
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// the binary value returned, when stored to memory, will match
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// the result on big endian; if the numeric value returned
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// must match big endian results, then call ntohs() on the result.
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static uint16_t net_checksum16(const void *buf, size_t size)
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{
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const char *b = (const char *)buf;
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const char *bend = b + size;
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uint32_t sum = 0, t = 0;
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uint8_t z[4] = { 0 };
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switch (size & 3) {
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case 3: z[2] = (uint8_t)*--bend;
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case 2: z[1] = (uint8_t)*--bend;
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case 1: z[0] = (uint8_t)*--bend;
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default: break;
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}
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memcpy(&t, z, 4);
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sum += t & 0xffffu;
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sum += (t >> 16);
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for (; b < bend; b += 4) {
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memcpy(&t, b, 4);
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sum += t & 0xffffu;
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sum += (t >> 16);
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}
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return ~net_checksum16_foldcarry(sum);
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}
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// For two sequences of bytes A and B that return checksums CS(A) and CS(B),
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// this function will calculate the checksum CS(AB) of the concatenated value
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// AB given the checksums of the individual parts CS(A) and CS(B).
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static inline uint16_t net_checksum16_add(uint16_t a, uint16_t b)
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{
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const uint32_t A = a;
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const uint32_t B = b;
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return ~net_checksum16_foldcarry((~A & 0xffffu) + (~B & 0xffffu));
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}
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#endif
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