using System;
using System.Security.Cryptography;
using vendorApproverBackend.Interfaces;

public class OtpGenerate : IOtpGenerate
{
    private const int OTP_LENGTH = 4;
    private const string OTP_NUMBERS_NON_ZERO = "123456789";
    private static readonly string OTP_NUMBERS = "0" + OTP_NUMBERS_NON_ZERO;

    public string GetOtpCode(string key)
    {
        if (string.IsNullOrWhiteSpace(key))
            throw new ArgumentException("key must not be empty", nameof(key));

        return BuildOtp();
    }

    private string BuildOtp()
    {
        Span<char> buffer = stackalloc char[OTP_LENGTH];
        var numbers = OTP_NUMBERS;
        using (var rng = RandomNumberGenerator.Create())
        {
            for (int i = 0; i < OTP_LENGTH; i++)
            {
                var r = GetInt(rng, numbers.Length);
                buffer[i] = numbers[r];
            }
        }

        var otp = new string(buffer);

        if (otp.StartsWith("0"))
        {
            char nonZero;
            using (var rng = RandomNumberGenerator.Create())
            {
                nonZero = OTP_NUMBERS_NON_ZERO[GetInt(rng, OTP_NUMBERS_NON_ZERO.Length)];
            }
            otp = nonZero + otp.Substring(1);
        }

        return otp;
    }

    private static int GetInt(RandomNumberGenerator rng, int maxExclusive)
    {
        if (maxExclusive <= 0) throw new ArgumentOutOfRangeException(nameof(maxExclusive));
        var bytes = new byte[4];
        int value;
        do
        {
            rng.GetBytes(bytes);
            value = BitConverter.ToInt32(bytes, 0) & int.MaxValue;
        } while (value >= (int.MaxValue - ((int.MaxValue % maxExclusive) + 1)));

        return value % maxExclusive;
    }
}