MinBy & MaxBy

Author: dragon7307

.editorconfig

@@ -50,6 +50,7 @@ csharp_style_var_when_type_is_apparent = true:suggestion
 csharp_style_var_elsewhere = false:suggestion
 dotnet_diagnostic.CA1001.severity = warning
 dotnet_diagnostic.CA1309.severity = warning
+dotnet_diagnostic.CA1510.severity = none
 
 [*.{cs,vb}]
 #### Naming styles ####

src/Extensions/ReadOnlySpan/ReadOnlySpanExtensions.Linq.cs

@@ -1,4 +1,6 @@
 using System;
+using System.Collections;
+using System.Collections.Generic;
 using System.Numerics;
 
 namespace SpanExtensions
@@ -1920,5 +1922,162 @@ public static BigInteger Max<T>(this ReadOnlySpan<T> source, Func<T, BigInteger>
             return max;
         }
 #endif
+
+        /// <summary>
+        /// Returns the minimum value in a generic sequence according to a specified key selector function.
+        /// </summary>
+        /// <typeparam name="TSource">The type of elements in <paramref name="source"/>.</typeparam>
+        /// <typeparam name="TKey">The type of key to compare elements by.</typeparam>
+        /// <param name="source">A <see cref="ReadOnlySpan{TSource}"/> to determine the minimum value of.</param>
+        /// <param name="keySelector">A function to extract the key for each element.</param>
+        /// <returns>The value with the minimum key in <paramref name="source"/>.</returns>
+        /// <exception cref="ArgumentNullException"><paramref name="keySelector"/> is null.</exception>
+        /// <exception cref="InvalidOperationException"><typeparamref name="TSource"/> is a primitive type and <paramref name="source"/> is empty.</exception>
+        /// <remarks>If <paramref name="source"/> is empty and <typeparamref name="TSource"/> is a non-nullable struct, such as a primitive type, an <see cref="InvalidOperationException"/> is thrown.</remarks>
+        public static TSource MinBy<TSource, TKey>(this ReadOnlySpan<TSource> source, Func<TSource, TKey> keySelector) where TKey : IComparable<TKey>
+        {
+            if(keySelector is null)
+            {
+                throw new ArgumentNullException(nameof(keySelector));
+            }
+            if(source.IsEmpty)
+            {
+                throw new InvalidOperationException($"{nameof(source)} is empty");
+            }
+
+            TSource min = source[0];
+            TKey minKey = keySelector(min);
+            for(int i = 1; i < source.Length; i++)
+            {
+                TSource value = source[i];
+                TKey current = keySelector(value);
+                if(current.CompareTo(minKey) < 0)
+                {
+                    min = value;
+                    minKey = current;
+                }
+            }
+            return min;
+        }
+
+        /// <summary>
+        /// Returns the minimum value in a generic sequence according to a specified key selector function.
+        /// </summary>
+        /// <typeparam name="TSource">The type of elements in <paramref name="source"/>.</typeparam>
+        /// <typeparam name="TKey">The type of key to compare elements by.</typeparam>
+        /// <param name="source">A <see cref="ReadOnlySpan{TSource}"/> to determine the minimum value of.</param>
+        /// <param name="keySelector">A function to extract the key for each element.</param>
+        /// <param name="comparer">The <see cref="IComparer{TSource}"/> to compare keys.</param>
+        /// <returns>The value with the minimum key in <paramref name="source"/>.</returns>
+        /// <exception cref="ArgumentNullException"><paramref name="keySelector"/> is null.</exception>
+        /// <exception cref="InvalidOperationException"><typeparamref name="TSource"/> is a primitive type and <paramref name="source"/> is empty.</exception>
+        /// <remarks>If <paramref name="source"/> is empty and <typeparamref name="TSource"/> is a non-nullable struct, such as a primitive type, an <see cref="InvalidOperationException"/> is thrown.</remarks>
+        public static TSource MinBy<TSource, TKey>(this ReadOnlySpan<TSource> source, Func<TSource, TKey> keySelector, IComparer<TKey> comparer)
+        {
+            if(keySelector is null)
+            {
+                throw new ArgumentNullException(nameof(keySelector));
+            }
+            if(comparer is null)
+            {
+                throw new ArgumentNullException(nameof(comparer));
+            }
+            if(source.IsEmpty)
+            {
+                throw new InvalidOperationException($"{nameof(source)} is empty");
+            }
+
+            TSource min = source[0];
+            TKey minKey = keySelector(min);
+            for(int i = 1; i < source.Length; i++)
+            {
+                TSource value = source[i];
+                TKey current = keySelector(value);
+                if(comparer.Compare(current, minKey) < 0)
+                {
+                    min = value;
+                    minKey = current;
+                }
+            }
+            return min;
+        }
+
+        /// <summary>
+        /// Returns the maximum value in a generic sequence according to a specified key selector function.
+        /// </summary>
+        /// <typeparam name="TSource">The type of elements in <paramref name="source"/>.</typeparam>
+        /// <typeparam name="TKey">The type of key to compare elements by.</typeparam>
+        /// <param name="source">A <see cref="ReadOnlySpan{TSource}"/> to determine the maximum value of.</param>
+        /// <param name="keySelector">A function to extract the key for each element.</param>
+        /// <returns>The value with the maximum key in <paramref name="source"/>.</returns>
+        /// <exception cref="ArgumentNullException"><paramref name="keySelector"/> is null.</exception>
+        /// <exception cref="InvalidOperationException"><typeparamref name="TSource"/> is a primitive type and <paramref name="source"/> is empty.</exception>
+        /// <remarks>If <paramref name="source"/> is empty and <typeparamref name="TSource"/> is a non-nullable struct, such as a primitive type, an <see cref="InvalidOperationException"/> is thrown.</remarks>
+        public static TSource MaxBy<TSource, TKey>(this ReadOnlySpan<TSource> source, Func<TSource, TKey> keySelector) where TKey : IComparable<TKey>
+        {
+            if(keySelector is null)
+            {
+                throw new ArgumentNullException(nameof(keySelector));
+            }
+            if(source.IsEmpty)
+            {
+                throw new InvalidOperationException($"{nameof(source)} is empty");
+            }
+            TSource max = source[0];
+            TKey maxKey = keySelector(max);
+            for(int i = 1; i < source.Length; i++)
+            {
+                TSource value = source[i];
+                TKey current = keySelector(value);
+                if(current.CompareTo(maxKey) > 0)
+                {
+                    max = value;
+                    maxKey = current;
+                }
+            }
+            return max;
+        }
+
+        /// <summary>
+        /// Returns the maximum value in a generic sequence according to a specified key selector function.
+        /// </summary>
+        /// <typeparam name="TSource">The type of elements in <paramref name="source"/>.</typeparam>
+        /// <typeparam name="TKey">The type of key to compare elements by.</typeparam>
+        /// <param name="source">A <see cref="ReadOnlySpan{TSource}"/> to determine the maximum value of.</param>
+        /// <param name="keySelector">A function to extract the key for each element.</param>
+        /// <param name="comparer">The <see cref="IComparer{TSource}"/> to compare keys.</param>
+        /// <returns>The value with the maximum key in <paramref name="source"/>.</returns>
+        /// <exception cref="ArgumentNullException"><paramref name="keySelector"/> is null.</exception>
+        /// <exception cref="InvalidOperationException"><typeparamref name="TSource"/> is a primitive type and <paramref name="source"/> is empty.</exception>
+        /// <remarks>If <paramref name="source"/> is empty and <typeparamref name="TSource"/> is a non-nullable struct, such as a primitive type, an <see cref="InvalidOperationException"/> is thrown.</remarks>
+        public static TSource MaxBy<TSource, TKey>(this ReadOnlySpan<TSource> source, Func<TSource, TKey> keySelector, IComparer<TKey> comparer)
+        {
+            if(keySelector is null)
+            {
+                throw new ArgumentNullException(nameof(keySelector));
+            }
+            if(comparer is null)
+            {
+                throw new ArgumentNullException(nameof(comparer));
+            }
+            if(source.IsEmpty)
+            {
+                throw new InvalidOperationException($"{nameof(source)} is empty");
+            }
+
+            TSource max = source[0];
+            TKey maxKey = keySelector(max);
+            for(int i = 1; i < source.Length; i++)
+            {
+                TSource value = source[i];
+                TKey current = keySelector(value);
+                if(comparer.Compare(current, maxKey) > 0)
+                {
+                    max = value;
+                    maxKey = current;
+                }
+            }
+            return max;
+        }
     }
 }

src/Extensions/Span/SpanExtensions.Linq.cs

@@ -820,7 +820,7 @@ public static Half Max(this Span<Half> source)
         {
             return ReadOnlySpanExtensions.Max(source);
         }
-  
+
         /// <summary>
         /// Invokes a transform function on each element in <paramref name="source"/> and returns the maximum resulting value.
         /// </summary>