[#25] Client: Implement Patch and Range methods

Signed-off-by: Pavel Gross <p.gross@yadro.com>

src/FrostFS.SDK.ClientV2/Pool/Sampler.cs

@@ -0,0 +1,85 @@
+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];
+    }
+}