bacnet.cpp

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// Copyright (C) 2026 Douglas Quigg (dstroy0) <dquigg123@gmail.com>
// SPDX-License-Identifier: AGPL-3.0-or-later
 
/**
 * @file bacnet.cpp
 * @brief BACnet/IP BVLC + NPDU builder + parser (pure, host-tested).
 */
 
#include "services/bacnet/bacnet.h"
 
#if DETWS_ENABLE_BACNET
 
#include <string.h>
 
size_t bvlc_build(uint8_t *buf, size_t cap, uint8_t function, const uint8_t *npdu, size_t npdu_len)
{
    if (!buf || (npdu_len && !npdu))
        return 0;
    size_t total = BVLC_HEADER_SIZE + npdu_len;
    if (total > 0xFFFF || total > cap)
        return 0;
    buf[0] = BVLC_TYPE_BIP;
    buf[1] = function;
    buf[2] = (uint8_t)(total >> 8); // length, big-endian, the whole BVLL
    buf[3] = (uint8_t)(total & 0xFF);
    if (npdu_len)
        memcpy(buf + BVLC_HEADER_SIZE, npdu, npdu_len);
    return total;
}
 
bool bvlc_parse(const uint8_t *buf, size_t len, uint8_t *function, const uint8_t **npdu, size_t *npdu_len)
{
    if (!buf || len < BVLC_HEADER_SIZE || buf[0] != BVLC_TYPE_BIP)
        return false;
    size_t total = ((size_t)buf[2] << 8) | buf[3];
    if (total < BVLC_HEADER_SIZE || total > len)
        return false;
    if (function)
        *function = buf[1];
    if (npdu)
        *npdu = buf + BVLC_HEADER_SIZE;
    if (npdu_len)
        *npdu_len = total - BVLC_HEADER_SIZE;
    return true;
}
 
size_t npdu_build(uint8_t *buf, size_t cap, bool expecting_reply, uint8_t priority, bool has_dest, uint16_t dnet,
                  const uint8_t *dadr, uint8_t dadr_len, uint8_t hop_count, const uint8_t *apdu, size_t apdu_len)
{
    if (!buf || (apdu_len && !apdu) || (dadr_len && !dadr))
        return 0;
    size_t need = 2 + apdu_len; // version + control + apdu
    if (has_dest)
        need += 2 + 1 + dadr_len + 1; // DNET + DLEN + DADR + hop count
    if (need > cap)
        return 0;
 
    size_t p = 0;
    buf[p++] = NPDU_VERSION;
    uint8_t control = (uint8_t)(priority & NPCI_PRIORITY_MASK);
    if (expecting_reply)
        control |= NPCI_EXPECTING_REPLY;
    if (has_dest)
        control |= NPCI_DEST_PRESENT;
    buf[p++] = control;
    if (has_dest)
    {
        buf[p++] = (uint8_t)(dnet >> 8);
        buf[p++] = (uint8_t)(dnet & 0xFF);
        buf[p++] = dadr_len;
        if (dadr_len)
        {
            memcpy(buf + p, dadr, dadr_len);
            p += dadr_len;
        }
        buf[p++] = hop_count; // follows the (absent) source fields when a destination is present
    }
    if (apdu_len)
    {
        memcpy(buf + p, apdu, apdu_len);
        p += apdu_len;
    }
    return p;
}
 
bool npdu_parse(const uint8_t *buf, size_t len, NpduInfo *out)
{
    if (!buf || !out || len < 2 || buf[0] != NPDU_VERSION)
        return false;
    uint8_t control = buf[1];
    size_t p = 2;
 
    out->control = control;
    out->network_message = (control & NPCI_NETWORK_MSG) != 0;
    out->dest_present = (control & NPCI_DEST_PRESENT) != 0;
    out->src_present = (control & NPCI_SRC_PRESENT) != 0;
    out->dnet = 0;
    out->snet = 0;
    out->hop_count = 0;
 
    if (out->dest_present)
    {
        if (p + 3 > len)
            return false;
        out->dnet = (uint16_t)((buf[p] << 8) | buf[p + 1]);
        uint8_t dlen = buf[p + 2];
        p += 3 + dlen;
        if (p > len)
            return false;
    }
    if (out->src_present)
    {
        if (p + 3 > len)
            return false;
        out->snet = (uint16_t)((buf[p] << 8) | buf[p + 1]);
        uint8_t slen = buf[p + 2];
        p += 3 + slen;
        if (p > len)
            return false;
    }
    if (out->dest_present) // the hop count follows the source fields
    {
        if (p + 1 > len)
            return false;
        out->hop_count = buf[p++];
    }
    out->apdu = buf + p;
    out->apdu_len = len - p;
    return true;
}
 
#endif // DETWS_ENABLE_BACNET