Add A006577 sequence: number of '3n+1' steps to reach 1 (#302)

Author: vbe5-ua

Algorithms.Tests/Sequences/ThreeNPlusOneStepsSequenceTests.cs

@@ -0,0 +1,22 @@
+using System.Linq;
+using System.Numerics;
+using Algorithms.Sequences;
+using FluentAssertions;
+using NUnit.Framework;
+
+namespace Algorithms.Tests.Sequences {
+    public class ThreeNPlusOneStepsSequenceTests {
+        [Test]
+        public void First50ElementsCorrect() {
+            var sequence = new ThreeNPlusOneStepsSequence().Sequence.Take(50);
+            var first50 = new BigInteger[] {
+                0, 1, 7, 2, 5, 8, 16, 3, 19, 6,
+                14, 9, 9, 17, 17, 4, 12, 20, 20, 7,
+                7, 15, 15, 10, 23, 10, 111, 18, 18, 18,
+                106, 5, 26, 13, 13, 21, 21, 21, 34, 8,
+                109, 8, 29, 16, 16, 16, 104, 11, 24, 24
+            };
+            sequence.SequenceEqual(first50).Should().BeTrue();
+        }
+    }
+}

Algorithms/Sequences/ThreeNPlusOneStepsSequence.cs

@@ -0,0 +1,53 @@
+using System.Collections.Generic;
+using System.Numerics;
+
+namespace Algorithms.Sequences
+{
+    /// <summary>
+    ///     <para>
+    ///         Number of halving and tripling steps to reach 1 in the '3n+1' problem.
+    ///     </para>
+    ///     <para>
+    ///         Wikipedia: https://en.wikipedia.org/wiki/Collatz_conjecture.
+    ///     </para>
+    ///     <para>
+    ///         OEIS: https://oeis.org/A006577.
+    ///     </para>
+    /// </summary>
+    public class ThreeNPlusOneStepsSequence : ISequence
+    {
+        /// <summary>
+        /// Gets sequence of number of halving and tripling steps to reach 1 in the '3n+1' problem.
+        /// </summary>
+        public IEnumerable<BigInteger> Sequence
+        {
+            get
+            {
+                BigInteger startingValue = 1;
+
+                while (true)
+                {
+                    BigInteger counter = 0;
+                    BigInteger currentValue = startingValue;
+
+                    while (currentValue != 1)
+                    {
+                        if (currentValue.IsEven)
+                        {
+                            currentValue /= 2;
+                        }
+                        else
+                        {
+                            currentValue = 3 * currentValue + 1;
+                        }
+
+                        counter++;
+                    }
+
+                    yield return counter;
+                    startingValue++;
+                }
+            }
+        }
+    }
+}

README.md

@@ -115,6 +115,7 @@ This repository contains algorithms and data structures implemented in C# for ed
     * [A001478 Negative Integers](./Algorithms/Sequences/NegativeIntegersSequence.cs)
     * [A002110 Primorial Numbers](./Algorithms/Sequences/PrimorialNumbersSequence.cs)
     * [A005132 Recaman's](./Algorithms/Sequences/RecamansSequence.cs)
+    * [A006577 Number of '3n+1' steps to reach 1](./Algorithms/Sequences/ThreeNPlusOneStepsSequence.cs)
     * [A006862 Euclid Numbers](./Algorithms/Sequences/EuclidNumbersSequence.cs)
     * [A006879 Number of Primes by Number of Digits](./Algorithms/Sequences/NumberOfPrimesByNumberOfDigitsSequence.cs)
     * [A006880 Number of Primes by Powers of 10](./Algorithms/Sequences/NumberOfPrimesByPowersOf10Sequence.cs)