zwave.cpp

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// Copyright (C) 2026 Douglas Quigg (dstroy0) <dquigg123@gmail.com>
// SPDX-License-Identifier: AGPL-3.0-or-later
 
/**
 * @file zwave.cpp
 * @brief Z-Wave Serial API frame codec - implementation.
 *
 * Data frame: SOF | LEN | Type | Command | Data | Checksum, where LEN counts Type..Checksum
 * and Checksum = 0xFF XOR-folded over LEN..last-data (Silicon Labs Serial API spec).
 */
 
#include "services/zwave/zwave.h"
 
#if DETWS_ENABLE_ZWAVE
 
namespace
{
// Checksum: 0xFF XORed with every byte from LEN through the last data byte.
uint8_t checksum(const uint8_t *from_len, uint16_t n)
{
    uint8_t c = 0xFF;
    for (uint16_t i = 0; i < n; i++)
        c = (uint8_t)(c ^ from_len[i]);
    return c;
}
} // namespace
 
uint16_t zwave_build_frame(zwave_type type, uint8_t cmd, const uint8_t *data, uint8_t data_len, uint8_t *out,
                           uint16_t cap)
{
    if (!out || data_len > DETWS_ZWAVE_MAX_DATA || (data == nullptr && data_len > 0))
        return 0;
    uint8_t frame_len = (uint8_t)(data_len + 3); // Type + Command + Data + Checksum
    uint16_t total = (uint16_t)(2 + frame_len);  // SOF + LEN + frame_len bytes
    if (total > cap)
        return 0;
    out[0] = Zwave::ZWAVE_SOF;
    out[1] = frame_len;
    out[2] = (uint8_t)type;
    out[3] = cmd;
    for (uint8_t i = 0; i < data_len; i++)
        out[4 + i] = data[i];
    // Checksum folds LEN..last-data = out[1 .. 1+frame_len-1] = out[1..frame_len].
    out[1 + frame_len] = checksum(&out[1], frame_len);
    return total;
}
 
int zwave_parse_frame(const uint8_t *raw, uint16_t len, uint8_t *type, uint8_t *cmd, const uint8_t **pdata,
                      uint8_t *pdata_len)
{
    if (!raw || len < 1)
        return 0;
    if (raw[0] != Zwave::ZWAVE_SOF)
        return -1; // not a data frame (could be a control byte - test those first)
    if (len < 2)
        return 0;
    uint8_t frame_len = raw[1];
    if (frame_len < 3 || frame_len > DETWS_ZWAVE_MAX_DATA + 3)
        return -1; // too short for Type+Cmd+Checksum, or implausibly long
    uint16_t total = (uint16_t)(2 + frame_len);
    if (len < total)
        return 0; // wait for the rest
    if (checksum(&raw[1], frame_len) != raw[1 + frame_len])
        return -1; // checksum mismatch
    if (type)
        *type = raw[2];
    if (cmd)
        *cmd = raw[3];
    if (pdata)
        *pdata = &raw[4];
    if (pdata_len)
        *pdata_len = (uint8_t)(frame_len - 3);
    return (int)total;
}
 
bool zwave_is_ack(uint8_t b)
{
    return b == Zwave::ZWAVE_ACK;
}
bool zwave_is_nak(uint8_t b)
{
    return b == Zwave::ZWAVE_NAK;
}
bool zwave_is_can(uint8_t b)
{
    return b == Zwave::ZWAVE_CAN;
}
 
uint16_t zwave_build_ack(uint8_t *out, uint16_t cap)
{
    if (!out || cap < 1)
        return 0;
    out[0] = Zwave::ZWAVE_ACK;
    return 1;
}
 
#endif // DETWS_ENABLE_ZWAVE