lwm2m_tlv.cpp

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// Copyright (C) 2026 Douglas Quigg (dstroy0) <dquigg123@gmail.com>
// SPDX-License-Identifier: AGPL-3.0-or-later
 
/**
 * @file lwm2m_tlv.cpp
 * @brief OMA LwM2M TLV writer + reader (pure, host-tested).
 */
 
#include "services/lwm2m/lwm2m_tlv.h"
 
#if DETWS_ENABLE_LWM2M
 
#include <string.h>
 
void lwm2m_tlv_init(Lwm2mTlvWriter *w, uint8_t *buf, size_t cap)
{
    w->buf = buf;
    w->cap = cap;
    w->pos = 0;
    w->error = false;
}
 
bool lwm2m_tlv_write(Lwm2mTlvWriter *w, uint8_t id_type, uint16_t id, const uint8_t *value, size_t value_len)
{
    if (!w || (value_len && !value))
        return false;
    uint8_t type = (uint8_t)(id_type & LWM2M_TLV_IDKIND_MASK);
    bool id16 = id > 0xFF; // a 16-bit identifier is needed past 255
    if (id16)
        type |= LWM2M_TLV_ID16_FLAG;
 
    size_t lenbytes;
    if (value_len <= LWM2M_TLV_INLINE_LEN_MASK) // 0..7 fits inline (length-type 0)
    {
        lenbytes = 0;
        type |= (uint8_t)value_len;
    }
    else if (value_len <= 0xFF) // 8-bit length field (length-type 1)
    {
        lenbytes = 1;
        type |= (uint8_t)(1 << LWM2M_TLV_LENTYPE_SHIFT);
    }
    else if (value_len <= 0xFFFF) // 16-bit length field (length-type 2)
    {
        lenbytes = 2;
        type |= (uint8_t)(2 << LWM2M_TLV_LENTYPE_SHIFT);
    }
    else if (value_len <= 0xFFFFFF) // 24-bit length field (length-type 3)
    {
        lenbytes = 3;
        type |= (uint8_t)(3 << LWM2M_TLV_LENTYPE_SHIFT);
    }
    else
        return false;
 
    size_t need = 1 + (id16 ? 2 : 1) + lenbytes + value_len;
    if (w->error || w->pos + need > w->cap)
    {
        w->error = true;
        return false;
    }
 
    w->buf[w->pos++] = type;
    if (id16)
        w->buf[w->pos++] = (uint8_t)(id >> 8);
    w->buf[w->pos++] = (uint8_t)id;
    for (size_t i = 0; i < lenbytes; i++)
        w->buf[w->pos++] = (uint8_t)(value_len >> (8 * (lenbytes - 1 - i)));
    if (value_len)
    {
        memcpy(w->buf + w->pos, value, value_len);
        w->pos += value_len;
    }
    return true;
}
 
bool lwm2m_tlv_write_int(Lwm2mTlvWriter *w, uint16_t id, int64_t v)
{
    size_t n;
    if (v >= -128 && v <= 127)
        n = 1;
    else if (v >= -32768 && v <= 32767)
        n = 2;
    else if (v >= -2147483648LL && v <= 2147483647LL)
        n = 4;
    else
        n = 8;
    uint8_t b[8];
    for (size_t i = 0; i < n; i++)
        b[i] = (uint8_t)((uint64_t)v >> (8 * (n - 1 - i)));
    return lwm2m_tlv_write(w, LWM2M_TLV_RESOURCE, id, b, n);
}
 
bool lwm2m_tlv_write_bool(Lwm2mTlvWriter *w, uint16_t id, bool v)
{
    uint8_t b = v ? 1 : 0;
    return lwm2m_tlv_write(w, LWM2M_TLV_RESOURCE, id, &b, 1);
}
 
bool lwm2m_tlv_write_string(Lwm2mTlvWriter *w, uint16_t id, const char *s)
{
    if (!s)
        return false;
    return lwm2m_tlv_write(w, LWM2M_TLV_RESOURCE, id, (const uint8_t *)s, strnlen(s, w->cap + 1));
}
 
bool lwm2m_tlv_write_float(Lwm2mTlvWriter *w, uint16_t id, double v)
{
    uint64_t bits;
    memcpy(&bits, &v, 8);
    uint8_t b[8];
    for (size_t i = 0; i < 8; i++)
        b[i] = (uint8_t)(bits >> (8 * (7 - i))); // big-endian
    return lwm2m_tlv_write(w, LWM2M_TLV_RESOURCE, id, b, 8);
}
 
size_t lwm2m_tlv_finish(Lwm2mTlvWriter *w)
{
    return w->error ? 0 : w->pos;
}
 
bool lwm2m_tlv_read(const uint8_t *buf, size_t len, size_t *pos, Lwm2mTlv *out)
{
    if (!buf || !pos || !out || *pos >= len)
        return false;
    size_t p = *pos;
    uint8_t type = buf[p++];
    bool id16 = (type & LWM2M_TLV_ID16_FLAG) != 0;
    if (p + (id16 ? 2u : 1u) > len)
        return false;
    uint16_t id = buf[p++];
    if (id16)
        id = (uint16_t)((id << 8) | buf[p++]);
 
    uint8_t lentype = (uint8_t)((type >> LWM2M_TLV_LENTYPE_SHIFT) & LWM2M_TLV_LENTYPE_MASK);
    size_t vlen;
    if (lentype == 0)
        vlen = type & LWM2M_TLV_INLINE_LEN_MASK;
    else
    {
        if (p + lentype > len)
            return false;
        vlen = 0;
        for (uint8_t i = 0; i < lentype; i++)
            vlen = (vlen << 8) | buf[p++];
    }
    if (p + vlen > len)
        return false;
 
    out->id_type = (uint8_t)(type & LWM2M_TLV_IDKIND_MASK);
    out->id = id;
    out->value = buf + p;
    out->value_len = vlen;
    *pos = p + vlen;
    return true;
}
 
bool lwm2m_tlv_value_int(const uint8_t *value, size_t len, int64_t *out)
{
    if (!value || (len != 1 && len != 2 && len != 4 && len != 8))
        return false;
    int64_t r = (value[0] & 0x80) ? -1 : 0; // sign-extend from the MSB
    for (size_t i = 0; i < len; i++)
        r = (int64_t)(((uint64_t)r << 8) | value[i]);
    if (out)
        *out = r;
    return true;
}
 
#endif // DETWS_ENABLE_LWM2M