/**
 * @module node-opcua-secure-channel
 */

/**
 * SequenceNumberGenerator manages a monotonically increasing sequence number.
 * @class SequenceNumberGenerator
 *
 * @see OPC Unified Architecture, Part 6 -  $6.4.2 page 36 -
 *
 * The SequenceNumber shall also monotonically increase for all messages and shall not wrap
 * around until it is greater than 4294966271 (UInt32.MaxValue – 1024). 
 * 
 * The first number after the wrap around shall be less than 1024.
 * 
 * Note that this requirement means that SequenceNumbers do not reset when a new TokenId is issued.
 * 
 * The SequenceNumber shall be incremented by exactly one for each MessageChunk sent unless
 * the communication channel was interrupted and re-established. 
 * 
 * Gaps are permitted between the SequenceNumber for the last MessageChunk received before the
 * interruption and the SequenceNumber for first MessageChunk received after communication
 * was re-established.
 */
export class SequenceNumberGenerator {

    /** 
     * **spec Part 3 says**:
     * 
     * The same sequence number shall not be reused on a Subscription until over
     * four billion NotificationMessages have been sent.
     * At a continuous rate of one thousand NotificationMessages per second on a given 
     * Subscription, it would take roughly fifty days for the same sequence number to be reused. 
     * This allows Clients to safely treat sequence numbers as unique.
     */
    public static MAXVALUE = 4294966271;

    #_counter: number;

    constructor() {
        this.#_counter = 0;
        this._set(1);
    }

    public next(): number {
        const current = this.#_counter;
        this.#_counter += 1;
        if (this.#_counter > SequenceNumberGenerator.MAXVALUE) {
            this._set(1);
        }
        return current;
    }

    public future(): number {
        return this.#_counter;
    }

    /**
     * 
     * @param value forced the sequence number to a new value
     * @private
     */
    public _set(value: number) {
        this.#_counter = value;
    }

}