Add a class for tracking noise in row counts.

Author: dandanlen

src/explorer/Common/NoisyCount.cs

@@ -0,0 +1,61 @@
+namespace Explorer.Common
+{
+    using System;
+
+    internal struct NoisyCount : IEquatable<NoisyCount>
+    {
+        private NoisyCount(long count, double variance)
+        {
+            Count = count;
+            Variance = variance;
+        }
+
+        public static NoisyCount Zero => default;
+
+        public long Count { get; private set; }
+
+        public double Variance { get; private set; }
+
+        public double Noise => Math.Sqrt(Variance);
+
+        public static bool operator ==(NoisyCount left, NoisyCount right) => left.Equals(right);
+
+        public static bool operator !=(NoisyCount left, NoisyCount right) => !(left == right);
+
+        public static NoisyCount operator +(NoisyCount left, NoisyCount right) => left.Add(right);
+
+        public static NoisyCount Noiseless(long count) => new NoisyCount(count, 0);
+
+        public static NoisyCount WithNoise(long count, double noise) => new NoisyCount(count, noise * noise);
+
+        public static NoisyCount WithVariance(long count, double variance) => new NoisyCount(count, variance);
+
+        public static NoisyCount FromCountableRow(CountableRow row) => WithNoise(row.Count, row.CountNoise ?? 0.0);
+
+        public NoisyCount Add(NoisyCount other) => new NoisyCount(Count + other.Count, Variance + other.Variance);
+
+        public void Add(long count, double noise)
+        {
+            Count += count;
+            Variance += noise * noise;
+        }
+
+        public void Add(CountableRow row)
+        {
+            Add(row.Count, row.CountNoise ?? 0.0);
+        }
+
+        public override bool Equals(object? obj)
+        {
+            return obj is NoisyCount other && Equals(other);
+        }
+
+        public override int GetHashCode()
+        {
+            return HashCode.Combine(Count, Variance);
+        }
+
+        public bool Equals(NoisyCount other) =>
+            Count == other.Count && Variance == other.Variance;
+    }
+}

tests/explorer.api.tests/NoisyCountTests.cs

@@ -0,0 +1,39 @@
+namespace Explorer.Common.Tests
+{
+    using Explorer.Common;
+    using Xunit;
+
+    public sealed class NoisyCountTests
+    {
+        [Fact]
+        public void ZeroIsZero()
+        {
+            var zero = NoisyCount.Zero;
+            Assert.True(zero.Count == 0);
+            Assert.True(zero.Variance == 0);
+            Assert.True(zero.Noise == 0);
+            Assert.True(zero == NoisyCount.Zero);
+        }
+
+        [Theory]
+        [InlineData(100, 1.4)]
+        [InlineData(40, 4.3)]
+        public void AddingZeroDoesNothing(long count, double noise)
+        {
+            var nc = NoisyCount.WithNoise(count, noise);
+            var result = NoisyCount.Zero + nc;
+            Assert.True(result == nc);
+        }
+
+        [Fact]
+        public void NoiseValuesCombinedCorrectly()
+        {
+            var a = NoisyCount.WithNoise(100, 3);
+            var b = NoisyCount.WithNoise(200, 4);
+            var expected = NoisyCount.WithNoise(300, 5);
+
+            var result = a + b;
+            Assert.True(result == expected);
+        }
+    }
+}