/*
 * MEDiDFU — VesiSeek firmware updater
 *
 * VesiSeek BLE identifiers and the NUS command framing shared with the firmware.
 *
 * Author: Charles KWON OhJun <charleskwon@medithings.co.kr>
 * MEDiThings Inc.
 */
package com.medithings.vesiscan.firmware
import java.util.UUID

/**
 * VesiSeek BLE identifiers.
 *
 * During normal operation the device advertises the Nordic UART Service. After the `mdf?` command
 * it disconnects and re-advertises with the Zephyr SMP service instead, keeping the same
 * advertised name (its serial number).
 */
object VesiSeekUuids {
    val NUS_SERVICE: UUID = UUID.fromString("6E400001-B5A3-F393-E0A9-E50E24DCCA9E")
    /** host -> device (write) */
    val NUS_WRITE: UUID = UUID.fromString("6E400002-B5A3-F393-E0A9-E50E24DCCA9E")
    /** device -> host (notify) */
    val NUS_NOTIFY: UUID = UUID.fromString("6E400003-B5A3-F393-E0A9-E50E24DCCA9E")
    val CCCD: UUID = UUID.fromString("00002902-0000-1000-8000-00805F9B34FB")
    /** Zephyr SMP-over-BLE service, advertised only in DFU mode. */
    val SMP_SERVICE: UUID = UUID.fromString("8D53DC1D-1DB7-4CD3-868B-8A527460AA84")
}

/**
 * CRC-16/CCITT (poly 0x1021, init 0xFFFF) — byte-for-byte identical to the firmware's
 * `cmd_crc16_compute` (Nordic SDK `crc16_compute`).
 */
object Crc16 {
    fun compute(data: ByteArray): Int {
        var crc = 0xFFFF
        for (byte in data) {
            crc = crc xor ((byte.toInt() and 0xFF) shl 8)
            repeat(8) {
                crc = if ((crc and 0x8000) != 0) (crc shl 1) xor 0x1021 else crc shl 1
                crc = crc and 0xFFFF
            }
        }
        return crc
    }

    /** Appends the CRC little-endian, matching the firmware's packet layout. */
    fun appendTo(data: ByteArray): ByteArray {
        val crc = compute(data)
        return data + byteArrayOf((crc and 0xFF).toByte(), ((crc shr 8) and 0xFF).toByte())
    }
}

/** NUS packet framing: `[TAG 4B][DATA N][CRC16 2B little-endian]`. */
object NusPacket {

    fun build(tag: String, data: ByteArray = ByteArray(0)): ByteArray {
        require(tag.length == 4) { "tag must be 4 characters, got '$tag'" }
        return Crc16.appendTo(tag.toByteArray(Charsets.US_ASCII) + data)
    }

    /**
     * Enter-DFU command. The firmware ignores the payload but the command dispatcher needs at
     * least one data byte, so a single 0x00 is sent — the same packet the Go host tool uses.
     */
    fun enterDfu(): ByteArray = build("mdf?", byteArrayOf(0x00))

    /** Returns the 4-character response tag, or null if the frame is too short or the CRC fails. */
    fun tagOf(packet: ByteArray): String? {
        if (packet.size < 6) return null
        val body = packet.copyOfRange(0, packet.size - 2)
        val received = (packet[packet.size - 2].toInt() and 0xFF) or
                ((packet[packet.size - 1].toInt() and 0xFF) shl 8)
        if (Crc16.compute(body) != received) return null
        return String(packet, 0, 4, Charsets.US_ASCII)
    }
}