expo-ble-app-demo/ble/protocol/ProtocolManager.ts

import {ProtocolFrame} from './types';
import {FRAME_CONSTANTS} from './Constants';

export class ProtocolManager {

    static calculateChecksum(frameData: Uint8Array): number {
        let sum = 0;
        console.debug(`[ProtocolManager] Calculating checksum for frame count: ${frameData.length}`);
        // Checksum is calculated on all bytes except the last one (which is the checksum itself)
        // Example: 0xA0 03 00 01 01 5B
        // 0xA0 + 0x03 + 0x00 + 0x01 + 0x01 = 0xA5
        // 0 - 0xA5 = 0x5B
        for (let i = 0; i < frameData.length; i++) {
            sum += frameData[i];
        }
        const checksumV1 = (0 - sum) & 0xff
        const checksum = (~sum + 1) & 0xff
        console.debug(`[ProtocolManager] Checksum V1 calculated:  0 - ${sum} = ${checksumV1}`);
        console.debug(`[ProtocolManager] Checksum calculated:  0 - ${sum} = ${checksum}`);
        return checksum;
    }

    static verifyChecksum(frameData: Uint8Array, expectedChecksum: number): boolean {
        const calculated = this.calculateChecksum(frameData);
        return calculated === expectedChecksum;
    }

    static createFrame(
        type: number,
        data: Uint8Array,
        head: number = FRAME_CONSTANTS.HEAD_APP_TO_DEVICE,
        requireFragmentation: boolean = true
    ): Uint8Array[] {
        if (data.length <= FRAME_CONSTANTS.MAX_DATA_SIZE || !requireFragmentation) {
            console.debug(`[ProtocolManager] Frame size ${data.length} is less than max size ${FRAME_CONSTANTS.MAX_DATA_SIZE}, not fragmented`);
            return [this.createSingleFrame(type, data, head)];
        } else {
            console.debug(`[ProtocolManager] Frame size ${data.length} is greater than max size ${FRAME_CONSTANTS.MAX_DATA_SIZE}, fragmented`);
            return this.createFragmentedFrames(type, data, head);
        }
    }

    private static createSingleFrame(type: number, data: Uint8Array, head: number): Uint8Array {
        const buffer = new Uint8Array(FRAME_CONSTANTS.HEADER_SIZE + data.length + FRAME_CONSTANTS.FOOTER_SIZE);
        let offset = 0;

        buffer[offset++] = head;
        buffer[offset++] = type;

        // subpageTotal = 0
        buffer[offset++] = 0;
        buffer[offset++] = 0;

        // curPage = 0
        buffer[offset++] = 0;
        buffer[offset++] = 0;

        // dataLen
        buffer[offset++] = (data.length >> 8) & 0xff;
        buffer[offset++] = data.length & 0xff;

        // data
        buffer.set(data, offset);
        offset += data.length;

        // checksum
        // Logic from ProtocolUtilsV2: calculate sum of everything before checksum byte
        const checksum = this.calculateChecksum(buffer.slice(0, offset));
        buffer[offset] = checksum;

        return buffer;
    }

    private static createFragmentedFrames(type: number, data: Uint8Array, head: number): Uint8Array[] {
        const frames: Uint8Array[] = [];
        const totalSize = data.length;
        const maxDataSize = FRAME_CONSTANTS.MAX_DATA_SIZE;
        const totalPages = Math.ceil(totalSize / maxDataSize);

        for (let i = 0; i < totalPages; i++) {
            const start = i * maxDataSize;
            const end = Math.min(start + maxDataSize, totalSize);
            const chunk = data.slice(start, end);

            const buffer = new Uint8Array(FRAME_CONSTANTS.HEADER_SIZE + chunk.length + FRAME_CONSTANTS.FOOTER_SIZE);
            let offset = 0;

            buffer[offset++] = head;
            buffer[offset++] = type;

            // subpageTotal
            buffer[offset++] = (totalPages >> 8) & 0xff;
            buffer[offset++] = totalPages & 0xff;

            // curPage (descending order usually? No, BleManagerV2 comments said: "Protocol specifies: page numbers count down from highest to 0")
            // Wait, ProtocolUtilsV2 code:
            // const curPageVal = totalPages - 1 - i;
            const curPageVal = totalPages - 1 - i;

            buffer[offset++] = (curPageVal >> 8) & 0xff;
            buffer[offset++] = curPageVal & 0xff;

            // dataLen
            buffer[offset++] = (chunk.length >> 8) & 0xff;
            buffer[offset++] = chunk.length & 0xff;
            console.debug(`chunk length = ${chunk.length}, buffer 8 header = ${buffer.slice(0, 8).toString()}`)
            // data
            buffer.set(chunk, offset);
            offset += chunk.length;

            buffer[offset] = this.calculateChecksum(buffer.slice(0, offset));

            frames.push(buffer);
        }

        return frames;
    }

    static parseFrame(data: ArrayBufferLike): ProtocolFrame | null {
        const bytes = new Uint8Array(data);
        if (bytes.length < FRAME_CONSTANTS.HEADER_SIZE + FRAME_CONSTANTS.FOOTER_SIZE) {
            return null;
        }

        const head = bytes[0];
        if (head !== FRAME_CONSTANTS.HEAD_DEVICE_TO_APP && head !== FRAME_CONSTANTS.HEAD_APP_TO_DEVICE) {
            console.warn(`[ProtocolManager] Invalid frame header: 0x${head.toString(16)}`);
            return null;
        }

        const type = bytes[1];
        const subpageTotal = (bytes[2] << 8) | bytes[3];
        const curPage = (bytes[4] << 8) | bytes[5];
        const dataLen = (bytes[6] << 8) | bytes[7];

        if (bytes.length < FRAME_CONSTANTS.HEADER_SIZE + dataLen + FRAME_CONSTANTS.FOOTER_SIZE) {
            // Incomplete
            return null;
        }

        const frameData = bytes.slice(FRAME_CONSTANTS.HEADER_SIZE, FRAME_CONSTANTS.HEADER_SIZE + dataLen);
        const checksum = bytes[FRAME_CONSTANTS.HEADER_SIZE + dataLen];

        // Verify checksum
        const dataToCheck = bytes.slice(0, FRAME_CONSTANTS.HEADER_SIZE + dataLen);
        if (!this.verifyChecksum(dataToCheck, checksum)) {
            console.warn("Checksum mismatch");
            return null;
        }

        return {
            head,
            type,
            subpageTotal,
            curPage,
            dataLen,
            data: frameData.buffer as ArrayBuffer,
            checksum
        };
    }
}