Add A000720 PrimePi sequence (TheAlgorithms#288)

OleksiiHe · web-flow · commit 66578c5f50c6 · 2022-02-17T22:24:16.000+02:00
diff --git a/Algorithms.Tests/Sequences/PrimePiSequenceTests.cs b/Algorithms.Tests/Sequences/PrimePiSequenceTests.cs
@@ -0,0 +1,19 @@
+﻿using System.Linq;
+using System.Numerics;
+using Algorithms.Sequences;
+using FluentAssertions;
+using NUnit.Framework;
+
+namespace Algorithms.Tests.Sequences
+{
+    public class PrimePiSequenceTests
+    {
+        [Test]
+        public void First10ElementsCorrect()
+        {
+            var sequence = new PrimePiSequence().Sequence.Take(10);
+            sequence.SequenceEqual(new BigInteger[] { 0, 1, 2, 2, 3, 3, 4, 4, 4, 4 })
+                .Should().BeTrue();
+        }
+    }
+}
diff --git a/Algorithms/Sequences/PrimePiSequence.cs b/Algorithms/Sequences/PrimePiSequence.cs
@@ -0,0 +1,42 @@
+using System.Collections.Generic;
+using System.Numerics;
+
+namespace Algorithms.Sequences
+{
+    /// <summary>
+    ///     <para>
+    ///         Sequence of number of primes less than or equal to n (PrimePi(n)).
+    ///     </para>
+    ///     <para>
+    ///         Wikipedia: https://wikipedia.org/wiki/Prime-counting_function.
+    ///     </para>
+    ///     <para>
+    ///         OEIS: https://oeis.org/A000720.
+    ///     </para>
+    /// </summary>
+    public class PrimePiSequence : ISequence
+    {
+        /// <summary>
+        /// Gets sequence of number of primes.
+        /// </summary>
+        public IEnumerable<BigInteger> Sequence
+        {
+            get
+            {
+                ISequence primes = new PrimesSequence();
+                var n = new BigInteger(0);
+                var counter = new BigInteger(0);
+
+                foreach (var p in primes.Sequence)
+                {
+                    for (n++; n < p; n++)
+                    {
+                        yield return counter;
+                    }
+
+                    yield return ++counter;
+                }
+            }
+        }
+    }
+}
diff --git a/README.md b/README.md
@@ -105,6 +105,7 @@ This repository contains algorithms and data structures implemented in C# for ed
     * [A000142 Factorial](./Algorithms/Sequences/FactorialSequence.cs)
     * [A000290 Squares](./Algorithms/Sequences/SquaresSequence.cs)
     * [A000578 Cubes](./Algorithms/Sequences/CubesSequence.cs)
+    * [A000720 PrimePi](./Algorithms/Sequences/PrimePiSequence.cs)
     * [A001462 Golomb's](./Algorithms/Sequences/GolombsSequence.cs)
     * [A001478 Negative Integers](./Algorithms/Sequences/NegativeIntegersSequence.cs)
     * [A005132 Recaman's](./Algorithms/Sequences/RecamansSequence.cs)
