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Refactor Subsets core into an iterator method #1052

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Nov 22, 2023
Merged

Refactor Subsets core into an iterator method #1052

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24 changes: 24 additions & 0 deletions MoreLinq.Test/SubsetTest.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -193,5 +193,29 @@ public void TestKSubsetExpectedResult()
foreach (var subset in result)
Assert.That(subset, Is.EqualTo(expectedSubsets[index++]));
}

[Test(Description = "https://github.com/morelinq/MoreLINQ/issues/1047")]
public void TestEnumeratorCurrentReturnsSameReferenceOnEachAccess()
{
var source = Seq(1, 2, 3, 4, 5);

using var e = source.Subsets(3).GetEnumerator();
var moved = e.MoveNext();
var first = e.Current;
var second = e.Current;

Assert.That(moved, Is.True);
Assert.That(first, Is.SameAs(second));
}

[Test]
public void TestEachSubsetInstanceIsUnique()
{
var source = Seq(1, 2, 3, 4, 5);

var subsets = source.Subsets(2).ToArray();

Assert.That(subsets[0], Is.Not.SameAs(subsets[1]));
}
}
}
172 changes: 51 additions & 121 deletions MoreLinq/Subsets.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -18,7 +18,6 @@
namespace MoreLinq
{
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

Expand Down Expand Up @@ -57,23 +56,22 @@ public static IEnumerable<IList<T>> Subsets<T>(this IEnumerable<T> sequence)
var sequenceAsList = sequence.ToList();
var sequenceLength = sequenceAsList.Count;

// the first subset is the empty set
yield return new List<T>();
yield return []; // the first subset is the empty set

// all other subsets are computed using the subset generator
// this check also resolves the case of permuting empty sets
if (sequenceLength > 0)
// next check also resolves the case of permuting empty sets
if (sequenceLength is 0)
yield break;

// all other subsets are computed using the subset generator...

for (var k = 1; k < sequenceLength; k++)
{
for (var i = 1; i < sequenceLength; i++)
{
// each intermediate subset is a lexographically ordered K-subset
var subsetGenerator = new SubsetGenerator<T>(sequenceAsList, i);
foreach (var subset in subsetGenerator)
yield return subset;
}

yield return sequenceAsList; // the last subset is the original set itself
// each intermediate subset is a lexographically ordered K-subset
foreach (var subset in Subsets(sequenceAsList, k))
yield return subset;
}

yield return sequenceAsList; // the last subset is the original set itself
}
}

Expand Down Expand Up @@ -115,128 +113,60 @@ public static IEnumerable<IList<T>> Subsets<T>(this IEnumerable<T> sequence, int
// preconditions. This however, needs to be carefully considered - and perhaps
// may change after further thought and review.

return new SubsetGenerator<T>(sequence, subsetSize);
return _(); IEnumerable<IList<T>> _()
{
foreach (var subset in Subsets(sequence.ToList(), subsetSize))
yield return subset;
}
}

/// <summary>
/// This class is responsible for producing the lexographically ordered k-subsets
/// Produces lexographically ordered k-subsets.
/// </summary>
/// <remarks>
/// It uses a snapshot of the original sequence, and an iterative,
/// reductive swap algorithm to produce all subsets of a predetermined
/// size less than or equal to the original set size.
/// </remarks>

sealed class SubsetGenerator<T> : IEnumerable<IList<T>>
static IEnumerable<IList<T>> Subsets<T>(List<T> set, int subsetSize)
{
/// <summary>
/// SubsetEnumerator uses a snapshot of the original sequence, and an
/// iterative, reductive swap algorithm to produce all subsets of a
/// predetermined size less than or equal to the original set size.
/// </summary>

sealed class SubsetEnumerator : IEnumerator<IList<T>>
{
readonly IList<T> set; // the original set of elements
readonly T[] subset; // the current subset to return
readonly int[] indices; // indices into the original set
// precondition: subsetSize <= set.Count
if (subsetSize > set.Count)
throw new ArgumentOutOfRangeException(nameof(subsetSize), "Subset size must be <= sequence.Count()");

bool @continue; // termination indicator, set when all subsets have been produced

int prevSwapIndex; // previous swap index (upper index)
int currSwapIndex; // current swap index (lower index)
int subsetSize; // size of the subset being produced
int setSize; // size of the original set (sequence)

public SubsetEnumerator(IList<T> set, int subsetSize)
{
// precondition: subsetSize <= set.Count
if (subsetSize > set.Count)
throw new ArgumentOutOfRangeException(nameof(subsetSize), "Subset size must be <= sequence.Count()");

// initialize set arrays...
this.set = set;
this.subset = new T[subsetSize];
this.indices = new int[subsetSize];
// initialize index counters...
Reset();
}
var indices = new int[subsetSize]; // indices into the original set
var prevSwapIndex = subsetSize; // previous swap index (upper index)
var currSwapIndex = -1; // current swap index (lower index)
var setSize = set.Count;

public void Reset()
do
{
if (currSwapIndex == -1)
{
this.prevSwapIndex = this.subset.Length;
this.currSwapIndex = -1;
this.subsetSize = this.subset.Length;
this.setSize = this.set.Count;
this.@continue = true;
currSwapIndex = 0;
prevSwapIndex = subsetSize;
}

public IList<T> Current => (IList<T>)this.subset.Clone();

object IEnumerator.Current => Current;

public bool MoveNext()
else
{
if (!this.@continue)
return false;

if (this.currSwapIndex == -1)
{
this.currSwapIndex = 0;
this.prevSwapIndex = this.subsetSize;
}
else
{
if (this.currSwapIndex < this.setSize - this.prevSwapIndex)
{
this.prevSwapIndex = 0;
}
this.prevSwapIndex++;
this.currSwapIndex = this.indices[this.subsetSize - this.prevSwapIndex];
}

for (var j = 1; j <= this.prevSwapIndex; j++)
this.indices[this.subsetSize + j - this.prevSwapIndex - 1] = this.currSwapIndex + j;

ExtractSubset();

this.@continue = this.indices is [var i, ..]
// .... count of items excluded from the subset
&& i != this.setSize - this.subsetSize + 1;

return true;
}

void IDisposable.Dispose() { }
if (currSwapIndex < setSize - prevSwapIndex)
prevSwapIndex = 0;

void ExtractSubset()
{
for (var i = 0; i < this.subsetSize; i++)
this.subset[i] = this.set[this.indices[i] - 1];
prevSwapIndex++;
currSwapIndex = indices[subsetSize - prevSwapIndex];
}
}

readonly IEnumerable<T> sequence;
readonly int subsetSize;
for (var i = 1; i <= prevSwapIndex; i++)
indices[subsetSize + i - prevSwapIndex - 1] = currSwapIndex + i;

public SubsetGenerator(IEnumerable<T> sequence, int subsetSize)
{
if (sequence is null)
throw new ArgumentNullException(nameof(sequence));
if (subsetSize < 0)
throw new ArgumentOutOfRangeException(nameof(subsetSize), "{subsetSize} must be between 0 and set.Count()");
this.subsetSize = subsetSize;
this.sequence = sequence;
}
var subset = new T[subsetSize]; // the current subset to return
for (var i = 0; i < subsetSize; i++)
subset[i] = set[indices[i] - 1];

/// <summary>
/// Returns an enumerator that produces all of the k-sized
/// subsets of the initial value set. The enumerator returns
/// and <see cref="IList{T}"/> for each subset.
/// </summary>
/// <returns>an <see cref="IEnumerator"/> that enumerates all k-sized subsets</returns>

public IEnumerator<IList<T>> GetEnumerator()
{
return new SubsetEnumerator(this.sequence.ToList(), this.subsetSize);
yield return subset;
}

IEnumerator IEnumerable.GetEnumerator() { return GetEnumerator(); }
while (indices is [var fi, ..]
// .... count of items excluded from the subset
&& fi != setSize - subsetSize + 1);
}
}
}