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Add Permutations method to EnumerableExtensions with corresponding tests

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This commit is contained in:
Sebastian Lindemeier
2025-12-15 11:05:51 +01:00
parent 56521d39d7
commit acef1dd7d2
2 changed files with 73 additions and 5 deletions
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AdventOfCode.Extensions/EnumerableExtensions.cs
+44 -5
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@@ -16,8 +16,8 @@ public static class EnumerableExtensions
yield return GetCombination(indices, pool);
while (true)
{
int idx;
for(idx = count - 1; idx >= 0; idx--)
var idx = count - 1;
for(;idx >= 0; --idx)
{
var isIndexBelowMax = indices[idx] != idx + poolLength - count;
if (isIndexBelowMax)
@@ -34,8 +34,47 @@ public static class EnumerableExtensions
yield return GetCombination(indices, pool);
}
static TValue[] GetCombination(int[] innerIndices, TValue[] innerPool) =>
innerIndices.Select(i => innerPool[i]).ToArray();
}
public static IEnumerable<TValue[]> Permutations<TValue>(this IEnumerable<TValue> values, int count)
{
var pool = values.ToArray();
if (count > pool.Length)
yield break;
foreach (var p in InnerPermutations(pool, count))
yield return p;
foreach (var p in InnerPermutations([..pool.Reverse()], count))
yield return p;
yield break;
static IEnumerable<TValue[]> InnerPermutations(TValue[] pool, int count)
{
var indices = Enumerable.Repeat(0, count).ToArray();
yield return GetCombination(indices, pool);
while (true)
{
var idx = count - 1;
for(;idx >= 0; --idx)
{
var isIndexBelowMax = indices[idx] != idx + pool.Length - count;
if (isIndexBelowMax)
break;
}
if(idx < 0)
break;
indices[idx] += 1;
for (var j = idx + 1; j < count; j++)
{
indices[j] = indices[j - 1];
}
yield return GetCombination(indices, pool);
}
}
}
private static TValue[] GetCombination<TValue>(int[] innerIndices, TValue[] innerPool) =>
innerIndices.Select(i => innerPool[i]).ToArray();
}