Added a shape fitter

FastRng/Double/ShapeFitter.cs

@@ -0,0 +1,43 @@
+using System;
+using System.Runtime.CompilerServices;
+using System.Threading.Tasks;
+
+namespace FastRng.Double
+{
+    public sealed class ShapeFitter
+    {
+        private readonly double[] probabilities;
+        private readonly double[] samples;
+        private readonly IRandom rng;
+        private readonly ushort sampleSize;
+        
+        public ShapeFitter(Func<double, double> shapeFunction, IRandom rng, ushort sampleSize = 100)
+        {
+            this.rng = rng;
+            this.sampleSize = sampleSize;
+            this.samples = new double[sampleSize];
+            this.probabilities = new double[sampleSize];
+            
+            var sampleStepSize = 1.0 / sampleSize;
+            var nextStep = 0.0 + sampleStepSize;
+            for (var n = 0; n < sampleSize; n++)
+            {
+                this.probabilities[n] = shapeFunction(nextStep);
+                nextStep += sampleStepSize;
+            }
+        }
+
+        public async ValueTask<double> NextNumber()
+        {
+            while (true)
+            {
+                var nextNumber = await this.rng.GetUniform();
+                var nextBucket = (int)Math.Floor(nextNumber * this.sampleSize);
+                this.samples[nextBucket] += this.probabilities[nextBucket];
+
+                if (this.samples[nextBucket] >= 1.0)
+                    return nextNumber;
+            }
+        }
+    }
+}