/*
 * Copyright 2016-2021 The NATS Authors
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

"use strict";

const digits = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
const base = 36;
const preLen = 12;
const seqLen = 10;
const maxSeq = 3656158440062976; // base^seqLen == 36^10
const minInc = 33;
const maxInc = 333;
const totalLen = preLen + seqLen;

function _getRandomValues(a: Uint8Array) {
  for (let i = 0; i < a.length; i++) {
    a[i] = Math.floor(Math.random() * 255);
  }
}

function fillRandom(a: Uint8Array) {
  if (globalThis?.crypto?.getRandomValues) {
    globalThis.crypto.getRandomValues(a);
  } else {
    _getRandomValues(a);
  }
}

/**
 * Create and initialize a nuid.
 *
 * @api private
 */
export class Nuid {
  buf: Uint8Array;
  seq!: number;
  inc!: number;
  inited: boolean;

  constructor() {
    this.buf = new Uint8Array(totalLen);
    this.inited = false;
  }

  /**
   * Initializes a nuid with a crypto random prefix,
   * and pseudo-random sequence and increment.
   *
   * @api private
   */
  private init() {
    this.inited = true;
    this.setPre();
    this.initSeqAndInc();
    this.fillSeq();
  }

  /**
   * Initializes the pseudo randmon sequence number and the increment range.
   *
   * @api private
   */
  private initSeqAndInc() {
    this.seq = Math.floor(Math.random() * maxSeq);
    this.inc = Math.floor(Math.random() * (maxInc - minInc) + minInc);
  }

  /**
   * Sets the prefix from crypto random bytes. Converts to base36.
   *
   * @api private
   */
  private setPre() {
    const cbuf = new Uint8Array(preLen);
    fillRandom(cbuf);
    for (let i = 0; i < preLen; i++) {
      const di = cbuf[i] % base;
      this.buf[i] = digits.charCodeAt(di);
    }
  }

  /**
   * Fills the sequence part of the nuid as base36 from this.seq.
   *
   * @api private
   */
  private fillSeq() {
    let n = this.seq;
    for (let i = totalLen - 1; i >= preLen; i--) {
      this.buf[i] = digits.charCodeAt(n % base);
      n = Math.floor(n / base);
    }
  }

  /**
   * Returns the next nuid.
   *
   * @api private
   */
  next(): string {
    if (!this.inited) {
      this.init();
    }
    this.seq += this.inc;
    if (this.seq > maxSeq) {
      this.setPre();
      this.initSeqAndInc();
    }
    this.fillSeq();
    // @ts-ignore - Uint8Arrays can be an argument
    return String.fromCharCode.apply(String, this.buf);
  }

  reset() {
    this.init();
  }
}

export const nuid = new Nuid();