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Lag: IList<> implementation #404

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May 10, 2023
Merged

Lag: IList<> implementation #404

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edbf519
`Lag`: `IList<>` implementation
viceroypenguin May 10, 2023
0172e98
Test `IList<>` behavior
viceroypenguin May 10, 2023
dee4505
Improve coverage
viceroypenguin May 10, 2023
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46 changes: 37 additions & 9 deletions Source/SuperLinq/Lag.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -17,10 +17,7 @@ public static partial class SuperEnumerable
/// </remarks>
public static IEnumerable<(TSource current, TSource? lag)> Lag<TSource>(this IEnumerable<TSource> source, int offset)
{
Guard.IsNotNull(source);

return source.Select(Some)
.Lag(offset, default, (curr, lag) => (curr.Value, lag is (true, var some) ? some : default));
return source.Lag(offset, ValueTuple.Create);
}

/// <summary>
Expand All @@ -41,11 +38,7 @@ public static partial class SuperEnumerable
/// </remarks>
public static IEnumerable<TResult> Lag<TSource, TResult>(this IEnumerable<TSource> source, int offset, Func<TSource, TSource?, TResult> resultSelector)
{
Guard.IsNotNull(source);
Guard.IsNotNull(resultSelector);

return source.Select(Some)
.Lag(offset, default, (curr, lag) => resultSelector(curr.Value, lag is (true, var some) ? some : default));
return source.Lag(offset, default!, resultSelector);
}

/// <summary>
Expand All @@ -70,6 +63,9 @@ public static IEnumerable<TResult> Lag<TSource, TResult>(this IEnumerable<TSourc
Guard.IsNotNull(resultSelector);
Guard.IsGreaterThanOrEqualTo(offset, 1);

if (source is IList<TSource> list)
return new LagIterator<TSource, TResult>(list, offset, defaultLagValue, resultSelector);

return Core(source, offset, defaultLagValue, resultSelector);

static IEnumerable<TResult> Core(IEnumerable<TSource> source, int offset, TSource defaultLagValue, Func<TSource, TSource, TResult> resultSelector)
Expand All @@ -84,4 +80,36 @@ static IEnumerable<TResult> Core(IEnumerable<TSource> source, int offset, TSourc
}
}
}

private sealed class LagIterator<TSource, TResult> : ListIterator<TResult>
{
private readonly IList<TSource> _source;
private readonly int _offset;
private readonly TSource _defaultLagValue;
private readonly Func<TSource, TSource, TResult> _resultSelector;

public LagIterator(IList<TSource> source, int offset, TSource defaultLagValue, Func<TSource, TSource, TResult> resultSelector)
{
_source = source;
_offset = offset;
_defaultLagValue = defaultLagValue;
_resultSelector = resultSelector;
}

public override int Count => _source.Count;

protected override IEnumerable<TResult> GetEnumerable()
{
var cnt = (uint)_source.Count;
for (var i = 0; i < cnt; i++)
yield return _resultSelector(
_source[i],
i < _offset ? _defaultLagValue : _source[i - _offset]);
}

protected override TResult ElementAt(int index) =>
_resultSelector(
_source[index],
index < _offset ? _defaultLagValue : _source[index - _offset]);
}
}
180 changes: 101 additions & 79 deletions Tests/SuperLinq.Test/LagTest.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -35,108 +35,130 @@ public void TestLagZeroOffset()
new BreakingSequence<int>().Lag(0, (val, lagVal) => val + lagVal));
}

/// <summary>
/// Verify that lag can accept an propagate a default value passed to it.
/// </summary>
[Fact]
public void TestLagExplicitDefaultValue()
public static IEnumerable<object[]> GetIntSequences() =>
Enumerable.Range(1, 100)
.GetListSequences()
.Select(x => new object[] { x });

[Theory]
[MemberData(nameof(GetIntSequences))]
public void TestLagExplicitDefaultValue(IDisposableEnumerable<int> seq)
{
using var sequence = Enumerable.Range(1, 100).AsTestingSequence();

var result = sequence.Lag(10, -1, (val, lagVal) => lagVal).ToList();
Assert.Equal(100, result.Count);
Assert.Equal(Enumerable.Repeat(-1, 10), result.Take(10));
using (seq)
{
var result = seq.Lag(10, -1, (val, lagVal) => lagVal);
result.AssertSequenceEqual(
Enumerable.Repeat(-1, 10).Concat(Enumerable.Range(1, 90)));
}
}

[Fact]
public void TestLagTuple()
[Theory]
[MemberData(nameof(GetIntSequences))]
public void TestLagTuple(IDisposableEnumerable<int> seq)
{
using var sequence = Enumerable.Range(1, 100).AsTestingSequence();

var result = sequence.Lag(10).ToList();
Assert.Equal(100, result.Count);
result.AssertSequenceEqual(
Enumerable.Range(1, 100).Select(x => (x, x <= 10 ? default : x - 10)));
using (seq)
{
var result = seq.Lag(10);
result.AssertSequenceEqual(
Enumerable.Range(1, 100).Select(x => (x, x <= 10 ? default : x - 10)));
}
}

/// <summary>
/// Verify that lag will use default(T) if a specific default value is not supplied for the lag value.
/// </summary>
[Fact]
public void TestLagImplicitDefaultValue()
[Theory]
[MemberData(nameof(GetIntSequences))]
public void TestLagImplicitDefaultValue(IDisposableEnumerable<int> seq)
{
using var sequence = Enumerable.Range(1, 100).AsTestingSequence();

var result = sequence.Lag(10, (val, lagVal) => lagVal).ToList();
Assert.Equal(100, result.Count);
Assert.Equal(Enumerable.Repeat(default(int), 10), result.Take(10));
using (seq)
{
var result = seq.Lag(10, (val, lagVal) => lagVal);
result.AssertSequenceEqual(
Enumerable.Repeat(default(int), 10)
.Concat(Enumerable.Range(1, 90)));
}
}

/// <summary>
/// Verify that if the lag offset is greater than the sequence length lag
/// still yields all of the elements of the source sequence.
/// </summary>
[Fact]
public void TestLagOffsetGreaterThanSequenceLength()
[Theory]
[MemberData(nameof(GetIntSequences))]
public void TestLagOffsetGreaterThanSequenceLength(IDisposableEnumerable<int> seq)
{
using var sequence = Enumerable.Range(1, 100).AsTestingSequence();

var result = sequence.Lag(100 + 1, (a, b) => a).ToList();
Assert.Equal(100, result.Count);
Assert.Equal(Enumerable.Range(1, 100), result);
using (seq)
{
var result = seq.Lag(100 + 1, (a, b) => a);
result.AssertSequenceEqual(
Enumerable.Range(1, 100));
}
}

/// <summary>
/// Verify that lag actually yields the correct pair of values from the sequence
/// when offsetting by a single item.
/// </summary>
[Fact]
public void TestLagPassesCorrectLagValueOffsetBy1()
[Theory]
[MemberData(nameof(GetIntSequences))]
public void TestLagPassesCorrectLagValueOffsetBy1(IDisposableEnumerable<int> seq)
{
using var sequence = Enumerable.Range(1, 100).AsTestingSequence();

var result = sequence.Lag(1, (a, b) => new { A = a, B = b }).ToList();
Assert.Equal(100, result.Count);
Assert.True(result.All(x => x.B == (x.A - 1)));
using (seq)
{
var result = seq.Lag(1);
result.AssertSequenceEqual(
Enumerable.Range(1, 100)
.Select(x => (x, x - 1)));
}
}

/// <summary>
/// Verify that lag yields the correct pair of values from the sequence when
/// offsetting by more than a single item.
/// </summary>
[Fact]
public void TestLagPassesCorrectLagValuesOffsetBy2()
[Theory]
[MemberData(nameof(GetIntSequences))]
public void TestLagPassesCorrectLagValuesOffsetBy2(IDisposableEnumerable<int> seq)
{
using var sequence = Enumerable.Range(1, 100).AsTestingSequence();
using (seq)
{
var result = seq.Lag(2);
result.AssertSequenceEqual(
Enumerable.Range(1, 100)
.Select(x => (x, x <= 2 ? 0 : x - 2)));
}
}

public static IEnumerable<object[]> GetStringSequences() =>
Seq("foo", "bar", "baz", "qux")
.GetListSequences()
.Select(x => new object[] { x, });

var result = sequence.Lag(2, (a, b) => new { A = a, B = b }).ToList();
Assert.Equal(100, result.Count);
Assert.True(result.Skip(2).All(x => x.B == (x.A - 2)));
Assert.True(result.Take(2).All(x => (x.A - x.B) == x.A));
[Theory]
[MemberData(nameof(GetStringSequences))]
public void TestLagWithNullableReferences(IDisposableEnumerable<string> seq)
{
using (seq)
{
var result = seq.Lag(2, (a, b) => new { A = a, B = b });
result.AssertSequenceEqual(
new { A = "foo", B = (string?)null },
new { A = "bar", B = (string?)null },
new { A = "baz", B = (string?)"foo" },
new { A = "qux", B = (string?)"bar" });
}
}

[Fact]
public void TestLagWithNullableReferences()
[Theory]
[MemberData(nameof(GetStringSequences))]
public void TestLagWithNonNullableReferences(IDisposableEnumerable<string> seq)
{
using var words = TestingSequence.Of("foo", "bar", "baz", "qux");
var result = words.Lag(2, (a, b) => new { A = a, B = b });
result.AssertSequenceEqual(
new { A = "foo", B = (string?)null },
new { A = "bar", B = (string?)null },
new { A = "baz", B = (string?)"foo" },
new { A = "qux", B = (string?)"bar" });
using (seq)
{
var result = seq.Lag(2, string.Empty, (a, b) => new { A = a, B = b });
result.AssertSequenceEqual(
new { A = "foo", B = string.Empty, },
new { A = "bar", B = string.Empty, },
new { A = "baz", B = "foo" },
new { A = "qux", B = "bar" });
}
}

[Fact]
public void TestLagWithNonNullableReferences()
public void ZipMapListBehavior()
{
using var words = TestingSequence.Of("foo", "bar", "baz", "qux");
var empty = string.Empty;
var result = words.Lag(2, empty, (a, b) => new { A = a, B = b });
result.AssertSequenceEqual(
new { A = "foo", B = empty },
new { A = "bar", B = empty },
new { A = "baz", B = "foo" },
new { A = "qux", B = "bar" });
using var seq = Enumerable.Range(0, 10_000).AsBreakingList();

var result = seq.Lag(20);
Assert.Equal(10_000, result.Count());
Assert.Equal((10, 0), result.ElementAt(10));
Assert.Equal((50, 30), result.ElementAt(50));
Assert.Equal((9_950, 9_930), result.ElementAt(^50));
}
}