/**
 *  Copyright 2026 Angus.Fenying <fenying@litert.org>
 *
 *  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
 *
 *    https://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.
 */

import * as eL from './Errors.js';
import { uuidToString } from './Utils.js';
import { ETimeReversedStrategy } from './Constants.js';

const MAX_TIMESTAMP = 0xFFFF_FFFF_FFFF; // Maximum timestamp for UUIDv7 (48 bits)

const RAND_A_CAP = 4096;

export interface IUuid7GeneratorOptions {

    /**
     * The strategy for handling time reversal events.
     *
     * @default ETimeReversedStrategy.THROW_ERROR
     */
    onTimeReversed?: ETimeReversedStrategy;
}

/**
 * The default strategy for the time reversal event,
 * which throws an `E_TIME_REVERSED` error.
 */
export const UUID7_DEFAULT_TIME_REVERSAL_STRATEGY = ETimeReversedStrategy.THROW_ERROR;

/**
 * The class for generating UUID in version 7.
 */
export class Uuid7Generator {

    private _prevTime: number = 0;

    private readonly _onTimeReversedStrategy: ETimeReversedStrategy;

    public constructor(opts: IUuid7GeneratorOptions = {}) {

        this._onTimeReversedStrategy = opts.onTimeReversed ?? UUID7_DEFAULT_TIME_REVERSAL_STRATEGY;
    }

    /**
     * Generate a UUIDv7 string, which is based on the current timestamp and random values.
     *
     * @returns A UUIDv7 string.
     *
     * @example
     * ```ts
     * const generator = new Uuid7Generator();
     * const uuid = generator.generate();
     * console.log(uuid);
     * ```
     */
    public generate(): string {

        return uuidToString(this.generateAsBuffer());
    }

    /**
     * Generate a binary UUIDv7 as a 16-byte buffer, which is based on the current timestamp and random values.
     *
     * @returns A Buffer containing the UUIDv7.
     *
     * @example
     * ```ts
     * const generator = new Uuid7Generator();
     * const uuidBuffer = generator.generateAsBuffer();
     * console.log(uuidBuffer);
     * ```
     */
    public generateAsBuffer(): Buffer {

        let now = Date.now();

        if (this._prevTime > now) {

            switch (this._onTimeReversedStrategy) {
                case ETimeReversedStrategy.THROW_ERROR:
                    throw new eL.E_TIME_REVERSED({
                        previous: this._prevTime,
                        current: now
                    });
                case ETimeReversedStrategy.USE_REVERSED_TIME:
                    // do nothing, because `timeOffset` is the reversed time.
                    break;
                case ETimeReversedStrategy.USE_PREVIOUS_TIME:
                    now = this._prevTime;
                    break;
            }
        }

        return Uuid7Generator.generateBuffer(this._prevTime = now);

    }

    /**
     * Generate a UUIDv7 string based on the provided timestamp and random value.
     *
     * > This method is implemented in pure JavaScript.
     *
     * @param timestamp  The 48-bit timestamp in milliseconds since the Unix epoch. If not provided, the current time will be used.
     * @param randA A 12-bit random value to ensure uniqueness. If not provided, a random value will be generated.
     *
     * @returns A UUIDv7 string based on the provided timestamp and random value.
     *
     * @throws {RangeError} If the timestamp is not a non-negative integer or exceeds the maximum allowed value.
     *
     * @example
     * ```ts
     * const uuid = Uuid7Generator.generate(Date.parse('2024-01-01T00:00:00Z'), 123456);
     * console.log(uuid);
     * ```
     */
    public static generate(
        timestamp: number = Date.now(),
        randA: number = Math.floor(Math.random() * RAND_A_CAP),
    ): string {

        return uuidToString(Uuid7Generator.generateBuffer(timestamp, randA));
    }

    /**
     * Generate a binary UUIDv7 based on the provided timestamp and random value, and return it as a Buffer.
     *
     * > This method is implemented in pure JavaScript.
     *
     * @param timestamp  The 48-bit timestamp in milliseconds since the Unix epoch. If not provided, the current time will be used.
     * @param randA A 12-bit random value to ensure uniqueness. If not provided, a random value will be generated.
     *
     * @returns A Buffer containing the UUIDv7 based on the provided timestamp and random value.
     *
     * @throws {RangeError} If the timestamp is not a non-negative integer or exceeds the maximum allowed value.
     *
     * @example
     * ```ts
     * const uuid = Uuid7Generator.generateBuffer(Date.parse('2024-01-01T00:00:00Z'), 123456);
     * console.log(uuid);
     * ```
     */
    public static generateBuffer(
        timestamp: number = Date.now(),
        randA: number = Math.floor(Math.random() * RAND_A_CAP),
    ): Buffer {

        if (!Number.isSafeInteger(timestamp) || timestamp < 0 || timestamp > MAX_TIMESTAMP) {

            throw new RangeError(`Timestamp must be an integer within the range of 0 to ${MAX_TIMESTAMP}.`);
        }

        if (!Number.isSafeInteger(randA) || randA < 0 || randA >= RAND_A_CAP) {

            throw new RangeError(`randA must be an integer within the range of 0 to ${RAND_A_CAP - 1}.`);
        }

        const buffer = Buffer.allocUnsafe(16);

        // Write the timestamp (48 bits)
        buffer.writeUIntBE(timestamp, 0, 6);

        // Write the version (4 bits) and the randA (12 bits)
        const versionAndRandA = (7 << 12) | (randA & 0x0FFF);
        buffer.writeUInt16BE(versionAndRandA, 6);

        // Write the variant (2 bits, 0b10) and random bits (62 bits)
        buffer.writeUInt32BE(((2 << 30) | Math.floor(Math.random() * 0x4000_0000)) >>> 0, 8);
        buffer.writeUInt32BE(Math.floor(Math.random() * 0x1_0000_0000), 12);

        return buffer;
    }
}