cclink.cpp

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// Copyright (C) 2026 Douglas Quigg (dstroy0) <dquigg123@gmail.com>
// SPDX-License-Identifier: AGPL-3.0-or-later
 
/**
 * @file cclink.cpp
 * @brief CC-Link cyclic fieldbus frame codec (see cclink.h).
 */
 
#include "services/cclink/cclink.h"
 
#if DETWS_ENABLE_CCLINK
 
#include <string.h>
 
uint8_t detws_cclink_sum(const uint8_t *bytes, size_t len)
{
    uint8_t sum = 0;
    for (size_t i = 0; i < len; i++)
        sum = (uint8_t)(sum + bytes[i]);
    return sum;
}
 
size_t detws_cclink_build(uint8_t station, uint8_t command, const uint8_t *bits, size_t bit_len, const uint8_t *words,
                          size_t word_len, uint8_t *out, size_t cap)
{
    if (!out || (bit_len && !bits) || (word_len && !words) || station > 63)
        return 0;
    size_t n = 2 + bit_len + word_len + 1;
    if (n > cap)
        return 0;
    size_t i = 0;
    out[i++] = station;
    out[i++] = command;
    if (bit_len)
    {
        memcpy(out + i, bits, bit_len);
        i += bit_len;
    }
    if (word_len)
    {
        memcpy(out + i, words, word_len);
        i += word_len;
    }
    out[i] = detws_cclink_sum(out, i); // checksum over station..last data
    i++;
    return i;
}
 
bool detws_cclink_parse(const uint8_t *frame, size_t len, CcLinkFrame *out)
{
    if (!frame || !out || len < 3) // station + command + checksum
        return false;
    size_t body = len - 1;
    if (detws_cclink_sum(frame, body) != frame[body])
        return false;
    out->station = frame[0];
    out->command = frame[1];
    out->payload = (body > 2) ? (frame + 2) : nullptr;
    out->payload_len = body - 2;
    return true;
}
 
bool detws_cclink_get_bit(const uint8_t *bits, size_t bit_len, size_t index)
{
    if (!bits || index / 8 >= bit_len)
        return false;
    return (bits[index / 8] >> (index % 8)) & 1u;
}
 
void detws_cclink_set_bit(uint8_t *bits, size_t bit_len, size_t index, bool value)
{
    if (!bits || index / 8 >= bit_len)
        return;
    uint8_t mask = (uint8_t)(1u << (index % 8));
    if (value)
        bits[index / 8] |= mask;
    else
        bits[index / 8] &= (uint8_t)~mask;
}
 
uint16_t detws_cclink_get_word(const uint8_t *words, size_t word_len, size_t index)
{
    size_t off = index * 2;
    if (!words || off + 1 >= word_len)
        return 0;
    return (uint16_t)(words[off] | (words[off + 1] << 8)); // little-endian
}
 
#endif // DETWS_ENABLE_CCLINK