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FallbackIfEmpty: preserve source and fallback collections if possible #340

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Merged
merged 10 commits into from
Aug 23, 2017
Merged

FallbackIfEmpty: preserve source and fallback collections if possible #340

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ee938c0
FallbackIfEmpty: preserve source and fallback collections if possible
fsateler May 12, 2017
a521afa
Brace style
fsateler Aug 8, 2017
f5cb2b5
Better names and localizacion of inner methods
fsateler Aug 8, 2017
ccca164
Turn FallbackIfEmptyPreservesSourceCollectionIfPossible into test cases
fsateler Aug 8, 2017
1d9b052
Use null check on count for choosing fallback
fsateler Aug 10, 2017
bef6cfe
Code formatting
atifaziz Aug 10, 2017
0952f8d
Redundant parens
atifaziz Aug 10, 2017
e247774
Put offending value into ArgumentOutOfRangeException
atifaziz Aug 10, 2017
52966cc
Revert "Turn FallbackIfEmptyPreservesSourceCollectionIfPossible into …
fsateler Aug 10, 2017
16a78a9
Simplification of names
atifaziz Aug 10, 2017
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22 changes: 22 additions & 0 deletions MoreLinq.Test/FallbackIfEmptyTest.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -39,6 +39,28 @@ public void FallbackIfEmptyWithEmptySequence()
}

[Test]
public void FallbackIfEmptyPreservesSourceCollectionIfPossible()
{
var source = new int[] { 1 };
// ReSharper disable PossibleMultipleEnumeration
Assert.AreSame(source.FallbackIfEmpty(12), source);
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@atifaziz atifaziz Aug 8, 2017

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Consider turning these into test cases.

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@fsateler fsateler Aug 8, 2017

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I did this on ccca164 , but I'm not sure it ended up being better, as it is a bunch of extra code noise.

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@atifaziz atifaziz Aug 8, 2017

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I agree that ccca164 isn't any better. What I had more in mind was this:

[TestCase(new[] { 12                     })]
[TestCase(new[] { 12, 23                 })]
[TestCase(new[] { 12, 23, 34             })]
[TestCase(new[] { 12, 23, 34, 45         })]
[TestCase(new[] { 12, 23, 34, 45, 56     })]
[TestCase(new[] { 12, 23, 34, 45, 56, 67 })]
public void FallbackIfEmptyPreservesSourceCollectionIfPossible(int[] fallback)
{
    var source = new[] { 1 };
    Assert.AreSame(source.FallbackIfEmpty(fallback), source);
}

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@fsateler fsateler Aug 8, 2017

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But those test cases don't add much value, as they all test the same overload. The idea is to test that all overloads preserve the source collection.

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@atifaziz atifaziz Aug 8, 2017 •
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Duh! My bad. All the overloading made it very opaque at the call site. Let's revert ccca164 then and ignore the remark about breaking it down into test cases.

Assert.AreSame(source.FallbackIfEmpty(12, 23), source);
Assert.AreSame(source.FallbackIfEmpty(12, 23, 34), source);
Assert.AreSame(source.FallbackIfEmpty(12, 23, 34, 45), source);
Assert.AreSame(source.FallbackIfEmpty(12, 23, 34, 45, 56), source);
Assert.AreSame(source.FallbackIfEmpty(12, 23, 34, 45, 56, 67), source);
// ReSharper restore PossibleMultipleEnumeration
}

[Test]
public void FallbackIfEmptyPreservesFallbackCollectionIfPossible()
{
var source = new int[0];
var fallback = new int[] { 1 };
Assert.AreSame(source.FallbackIfEmpty(fallback), fallback);
Assert.AreSame(source.FallbackIfEmpty(fallback.AsEnumerable()), fallback);
}

public void FallbackIfEmptyWithEmptySequenceCollectionOptimized()
{
var source = LinqEnumerable.Empty<int>();
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72 changes: 39 additions & 33 deletions MoreLinq/FallbackIfEmpty.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -19,7 +19,6 @@ namespace MoreLinq
{
using System;
using System.Collections.Generic;
using System.Linq;

static partial class MoreEnumerable
{
Expand Down Expand Up @@ -155,52 +154,59 @@ public static IEnumerable<T> FallbackIfEmpty<T>(this IEnumerable<T> source, IEnu
{
if (source == null) throw new ArgumentNullException(nameof(source));
if (fallback == null) throw new ArgumentNullException(nameof(fallback));
return FallbackIfEmptyImpl(source, 0, default(T), default(T), default(T), default(T), fallback);
return FallbackIfEmptyImpl(source, null, default(T), default(T), default(T), default(T), fallback);
}

static IEnumerable<T> FallbackIfEmptyImpl<T>(IEnumerable<T> source,
int? count, T fallback1, T fallback2, T fallback3, T fallback4,
IEnumerable<T> fallback)
{
var collection = source as ICollection<T>;
if (collection != null && collection.Count == 0)
if (source is ICollection<T> collection)
{
//
// Replace the empty collection with an empty sequence and
// carry on. LINQ's Enumerable.Empty is implemented
// intelligently to return the same enumerator instance and so
// does not incur an allocation. However, the same cannot be
// said for a collection like an empty array or list. This
// permits the rest of the logic while keeping the call to
// source.GetEnumerator() cheap.
//

source = Enumerable.Empty<T>();
}

using (var e = source.GetEnumerator())
{
if (e.MoveNext())
if (collection.Count == 0)
{
do { yield return e.Current; }
while (e.MoveNext());
return Fallback();
}
else
{
e.Dispose(); // eager disposal
if (count > 0 && count <= 4)
{
yield return fallback1;
if (count > 1) yield return fallback2;
if (count > 2) yield return fallback3;
if (count > 3) yield return fallback4;
}
else
return collection;
}
}

return _();

IEnumerable<T> _()
{
using (var e = source.GetEnumerator())
{
if (e.MoveNext())
{
foreach (var item in fallback)
yield return item;
do { yield return e.Current; }
while (e.MoveNext());
yield break;
}
}

foreach (var item in Fallback())
yield return item;
}

IEnumerable<T> Fallback()
{
switch (count)
{
case null: return fallback;
case int n when n >= 1 && n <= 4: return FallbackOnArgs();
default: throw new ArgumentOutOfRangeException(nameof(count), count, null);
}

IEnumerable<T> FallbackOnArgs()
{
yield return fallback1;
if (count > 1) yield return fallback2;
if (count > 2) yield return fallback3;
if (count > 3) yield return fallback4;
}
}
}
}
Expand Down