hart.cpp

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// Copyright (C) 2026 Douglas Quigg (dstroy0) <dquigg123@gmail.com>
// SPDX-License-Identifier: AGPL-3.0-or-later
 
/**
 * @file hart.cpp
 * @brief HART / HART-IP protocol codec (see hart.h).
 */
 
#include "services/hart/hart.h"
 
#if DETWS_ENABLE_HART
 
#include <string.h>
 
uint8_t detws_hart_checksum(const uint8_t *bytes, size_t len)
{
    uint8_t x = 0;
    for (size_t i = 0; i < len; i++)
        x ^= bytes[i];
    return x;
}
 
size_t detws_hart_build(uint8_t delimiter, const uint8_t *addr, size_t addr_len, uint8_t command, const uint8_t *data,
                        size_t data_len, uint8_t *out, size_t cap)
{
    if (addr_len != 1 && addr_len != 5)
        return 0;
    if (!addr || (data_len && !data))
        return 0;
    // delimiter + addr + command + byte-count + data + checksum
    size_t n = 1 + addr_len + 1 + 1 + data_len + 1;
    if (n > cap || data_len > 0xFF)
        return 0;
 
    size_t i = 0;
    out[i++] = delimiter;
    memcpy(out + i, addr, addr_len);
    i += addr_len;
    out[i++] = command;
    out[i++] = (uint8_t)data_len; // byte count
    if (data_len)
    {
        memcpy(out + i, data, data_len);
        i += data_len;
    }
    out[i] = detws_hart_checksum(out, i); // XOR over delimiter..last data byte
    i++;
    return i;
}
 
bool detws_hart_parse(const uint8_t *frame, size_t len, HartFrame *out)
{
    if (!frame || !out)
        return false;
    size_t addr_len = (frame[0] & HartDelim::HART_DELIM_LONG_ADDR) ? 5 : 1;
    // delimiter + addr + command + byte-count + checksum = minimum with no data
    size_t min = 1 + addr_len + 1 + 1 + 1;
    if (len < min)
        return false;
 
    size_t bc_idx = 1 + addr_len + 1; // index of the byte-count field
    uint8_t byte_count = frame[bc_idx];
    size_t expect = 1 + addr_len + 1 + 1 + byte_count + 1; // full frame incl checksum
    if (len < expect)
        return false;
 
    uint8_t want = detws_hart_checksum(frame, expect - 1);
    if (want != frame[expect - 1])
        return false;
 
    out->delimiter = frame[0];
    out->addr = frame + 1;
    out->addr_len = addr_len;
    out->command = frame[1 + addr_len];
    out->byte_count = byte_count;
    out->data = byte_count ? (frame + bc_idx + 1) : nullptr;
    out->data_len = byte_count;
    return true;
}
 
size_t detws_hartip_build_header(uint8_t msg_type, uint8_t msg_id, uint8_t status, uint16_t seq, uint16_t total_len,
                                 uint8_t *out, size_t cap)
{
    if (cap < HartIp::HARTIP_HEADER_LEN || !out)
        return 0;
    out[0] = 1; // HART-IP protocol version
    out[1] = msg_type;
    out[2] = msg_id;
    out[3] = status;
    out[4] = (uint8_t)(seq >> 8);
    out[5] = (uint8_t)seq;
    out[6] = (uint8_t)(total_len >> 8);
    out[7] = (uint8_t)total_len;
    return HartIp::HARTIP_HEADER_LEN;
}
 
#endif // DETWS_ENABLE_HART