@seriousme/opifex/dist/mqttPacket/length.js

MQTT client & server for Deno & NodeJS

/**
 * Encodes a number into a variable-length byte array using a modified base-128 encoding
 * @param n The number to encode
 * @returns Array of bytes representing the encoded number
 * @throws Error if the encoded length is greater than 4 bytes
 */
export function encodeLength(n) {
    const output = [];
    let x = n;
    do {
        let encodedByte = x % 0x80;
        x = Math.floor(x / 0x80);
        if (x > 0) {
            encodedByte = encodedByte | 0x80;
        }
        output.push(encodedByte);
    } while (x > 0);
    if (output.length > 4) {
        throw Error("length encoding failed");
    }
    return output;
}
/**
 * Decodes a variable-length encoded number from a byte array
 * @param buf The byte array containing the encoded number
 * @param start The starting position in the buffer to begin decoding
 * @returns Object containing the decoded length and number of bytes used for encoding
 * @throws Error if decoding fails
 */
export function decodeLength(buf, start) {
    const decode = getLengthDecoder();
    const lenBuf = buf.subarray(start);
    for (const byte of lenBuf) {
        const { done, length, numLengthBytes } = decode(byte);
        if (done) {
            return { length, numLengthBytes };
        }
    }
    throw Error("length decoding failed");
}
/**
 * Creates a stateful decoder function for processing variable-length encoded numbers
 * @returns Function that processes one byte at a time and returns decoding status
 * @throws Error if the encoded length is greater than 4 bytes
 */
export function getLengthDecoder() {
    let numLengthBytes = 1;
    let length = 0;
    let multiplier = 1;
    return function addLengthByte(encodedByte) {
        length += (encodedByte & 0x7f) * multiplier;
        multiplier *= 0x80;
        if ((encodedByte & 0x80) === 0) {
            return { done: true, length, numLengthBytes };
        }
        if (numLengthBytes++ >= 4) {
            throw Error("length decoding failed");
        }
        return { done: false, length, numLengthBytes };
    };
}
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