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/*
* HDLC encode/decode — AX.25 bit order (LSB first), FCS-CCITT.
* SPDX-License-Identifier: GPL-3.0-or-later
*/
#include "hdlc.h"
uint16_t hdlc_fcs_update(uint16_t fcs, uint8_t byte)
{
fcs ^= (uint16_t)byte;
for (int i = 0; i < 8; i++) {
if (fcs & 1u) {
fcs = (uint16_t)((fcs >> 1) ^ 0x8408u);
} else {
fcs >>= 1;
}
}
return fcs;
}
uint16_t hdlc_fcs_final(uint16_t fcs)
{
return (uint16_t)~fcs;
}
static bool push_bit(uint8_t *bits, size_t bits_cap, size_t *n, bool v)
{
if (*n >= bits_cap) {
return false;
}
bits[*n] = v ? 1u : 0u;
(*n)++;
return true;
}
static bool stuff_bit(uint8_t *bits, size_t bits_cap, size_t *n,
unsigned *ones, bool bit)
{
if (!push_bit(bits, bits_cap, n, bit)) {
return false;
}
if (bit) {
(*ones)++;
if (*ones == 5u) {
if (!push_bit(bits, bits_cap, n, false)) {
return false;
}
*ones = 0;
}
} else {
*ones = 0;
}
return true;
}
static bool emit_raw_byte(uint8_t *bits, size_t bits_cap, size_t *n, uint8_t b)
{
for (int i = 0; i < 8; i++) {
if (!push_bit(bits, bits_cap, n, (b >> i) & 1u)) {
return false;
}
}
return true;
}
static bool emit_stuffed_byte(uint8_t *bits, size_t bits_cap, size_t *n,
unsigned *ones, uint8_t b)
{
for (int i = 0; i < 8; i++) {
if (!stuff_bit(bits, bits_cap, n, ones, (b >> i) & 1u)) {
return false;
}
}
return true;
}
size_t hdlc_encode_bits(const uint8_t *info, size_t info_len,
uint8_t *bits, size_t bits_cap,
unsigned trailing_flags)
{
if (!info || !bits || info_len > HDLC_MAX_INFO) {
return 0;
}
size_t n = 0;
unsigned ones = 0;
if (!emit_raw_byte(bits, bits_cap, &n, HDLC_FLAG)) {
return 0;
}
uint16_t fcs = HDLC_FCS_INIT;
for (size_t i = 0; i < info_len; i++) {
fcs = hdlc_fcs_update(fcs, info[i]);
if (!emit_stuffed_byte(bits, bits_cap, &n, &ones, info[i])) {
return 0;
}
}
fcs = hdlc_fcs_final(fcs);
if (!emit_stuffed_byte(bits, bits_cap, &n, &ones, (uint8_t)(fcs & 0xFFu))) {
return 0;
}
if (!emit_stuffed_byte(bits, bits_cap, &n, &ones, (uint8_t)((fcs >> 8) & 0xFFu))) {
return 0;
}
ones = 0;
unsigned flags = trailing_flags ? trailing_flags : 1u;
for (unsigned f = 0; f < flags; f++) {
if (!emit_raw_byte(bits, bits_cap, &n, HDLC_FLAG)) {
return 0;
}
}
return n;
}
void hdlc_rx_init(hdlc_rx_t *rx)
{
rx->len = 0;
rx->ones = 0;
rx->in_frame = false;
rx->stuffing = false;
rx->byte_acc = 0;
rx->bit_count = 0;
}
bool hdlc_rx_bit(hdlc_rx_t *rx, bool bit, uint8_t *out, size_t *out_len)
{
if (bit) {
rx->ones++;
} else {
if (rx->ones == 5u) {
/* stuffed 0 — discard */
rx->ones = 0;
return false;
}
if (rx->ones == 6u) {
/* flag */
if (rx->in_frame && rx->len >= 3) {
uint16_t fcs = HDLC_FCS_INIT;
for (size_t i = 0; i < rx->len; i++) {
fcs = hdlc_fcs_update(fcs, rx->buf[i]);
}
if (fcs == HDLC_FCS_GOOD) {
size_t info_len = rx->len - 2;
for (size_t i = 0; i < info_len; i++) {
out[i] = rx->buf[i];
}
*out_len = info_len;
rx->len = 0;
rx->bit_count = 0;
rx->byte_acc = 0;
rx->ones = 0;
rx->in_frame = true; /* abutting frames */
return true;
}
}
rx->len = 0;
rx->bit_count = 0;
rx->byte_acc = 0;
rx->ones = 0;
rx->in_frame = true;
return false;
}
if (rx->ones > 6u) {
/* abort */
rx->in_frame = false;
rx->len = 0;
rx->bit_count = 0;
rx->byte_acc = 0;
rx->ones = 0;
return false;
}
rx->ones = 0;
}
if (!rx->in_frame) {
return false;
}
rx->byte_acc |= (uint8_t)((bit ? 1u : 0u) << rx->bit_count);
rx->bit_count++;
if (rx->bit_count >= 8) {
if (rx->len < sizeof(rx->buf)) {
rx->buf[rx->len++] = rx->byte_acc;
} else {
rx->in_frame = false;
rx->len = 0;
}
rx->byte_acc = 0;
rx->bit_count = 0;
}
return false;
}
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