using System;

namespace FrostFS.SDK.ClientV2;

internal sealed class Sampler
{
    private readonly object _lock = new();

    private Random random = new();

    internal double[] Probabilities { get; set; }
    internal int[] Alias { get; set; }

    internal Sampler(double[] probabilities)
    {
        var small = new WorkList();
        var large = new WorkList();

        var n = probabilities.Length;

        //  sampler.randomGenerator = rand.New(source)
        Probabilities = new double[n];
        Alias = new int[n];

        // Compute scaled probabilities.
        var p = new double[n];

        for (int i = 0; i < n; i++)
        {
            p[i] = probabilities[i] * n;
            if (p[i] < 1)
                small.Add(i);
            else
                large.Add(i);
        }

        while (small.Length > 0 && large.Length > 0)
        {
            var l = small.Remove();
            var g = large.Remove();

            Probabilities[l] = p[l];
            Alias[l] = g;

            p[g] = p[g] + p[l] - 1;

            if (p[g] < 1)
                small.Add(g);
            else
                large.Add(g);
        }

        while (large.Length > 0)
        {
            var g = large.Remove();
            Probabilities[g] = 1;
        }

        while (small.Length > 0)
        {
            var l = small.Remove();
            probabilities[l] = 1;
        }
    }

    internal int Next()
    {
        var n = Alias.Length;

        int i;
        double f;
        lock (_lock)
        {
            i = random.Next(0, n - 1);
            f = random.NextDouble();
        }

        if (f < Probabilities[i])
        {
            return i;
        }

        return Alias[i];
    }
}