From 800d3ec13e257083a8933db357f35f5b03528521 Mon Sep 17 00:00:00 2001
From: Leszek Bochenek <leszek.bochenek@anixe.pl>
Date: Tue, 8 Jul 2025 23:33:02 +0200
Subject: [PATCH] Add `ReadonlySpan<char>` API

closes #15
---
 src/Fastenshtein/AutoCompleteLevenshtein.cs | 23 ++++++++++++++++-----
 src/Fastenshtein/Fastenshtein.csproj        | 13 ++++++++++++
 src/Fastenshtein/Levenshtein.cs             | 20 ++++++++++++++----
 src/Fastenshtein/StaticLevenshtein.cs       | 23 ++++++++++++++++-----
 4 files changed, 65 insertions(+), 14 deletions(-)

diff --git a/src/Fastenshtein/AutoCompleteLevenshtein.cs b/src/Fastenshtein/AutoCompleteLevenshtein.cs
index b9046d9..00a1820 100644
--- a/src/Fastenshtein/AutoCompleteLevenshtein.cs
+++ b/src/Fastenshtein/AutoCompleteLevenshtein.cs
@@ -1,4 +1,6 @@
-namespace Fastenshtein
+using System;
+
+namespace Fastenshtein
 {
     /// <summary>
     /// Measures the difference between two strings.
@@ -14,15 +16,26 @@ public static class AutoCompleteLevenshtein
         /// <returns>0 exact match less a positive value, lower the value the best match</returns>
         public static int Distance(string value1, string value2)
         {
-            if (value1.Length == 0)
+            return Distance(value1.AsSpan(), value2.AsSpan());
+        }
+
+        /// <summary>
+        /// Compares the two spans of characters and returns a measure of their summarily with 0 being an exact match.
+        /// </summary>
+        /// <param name="value1">the incomplete value entered by the user</param>
+        /// <param name="value2">the value to compare value1 against</param>
+        /// <returns>0 exact match less a positive value, lower the value the best match</returns>
+        public static unsafe int Distance(ReadOnlySpan<char> value1, ReadOnlySpan<char> value2)
+        {
+            if (value1.IsEmpty)
             {
                 return 0;
             }
 
-            int[] costs = new int[value1.Length];
+            int* costs = stackalloc int[value1.Length];
 
             // Add indexing for insertion to first row
-            for (int i = 0; i < costs.Length;)
+            for (int i = 0; i < value1.Length;)
             {
                 costs[i] = ++i;
             }
@@ -69,7 +82,7 @@ public static int Distance(string value1, string value2)
                 }
             }
 
-            return costs[costs.Length - 1];
+            return costs[value1.Length - 1];
         }
     }
 }
diff --git a/src/Fastenshtein/Fastenshtein.csproj b/src/Fastenshtein/Fastenshtein.csproj
index 47d7df5..743f840 100644
--- a/src/Fastenshtein/Fastenshtein.csproj
+++ b/src/Fastenshtein/Fastenshtein.csproj
@@ -34,6 +34,18 @@
     </None>
   </ItemGroup>
 
+  <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard1.0' ">
+    <PackageReference Include="System.Memory" Version="4.4.0-preview1-25305-02" />
+  </ItemGroup>
+
+    <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard2.0' ">
+        <PackageReference Include="System.Memory" Version="4.5.5" />
+    </ItemGroup>
+
+    <ItemGroup Condition=" '$(TargetFramework)' == 'net452' ">
+        <PackageReference Include="System.Memory" Version="4.5.5" />
+    </ItemGroup>
+    
   <ItemGroup>
     <PackageReference Include="Microsoft.SourceLink.GitHub" Version="8.0.0">
       <PrivateAssets>all</PrivateAssets>
@@ -56,6 +68,7 @@
   <PropertyGroup>
     <IsAotCompatible Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net7.0'))">true</IsAotCompatible>
     <Nullable Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net6.0'))">enable</Nullable>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Release|AnyCPU'">
diff --git a/src/Fastenshtein/Levenshtein.cs b/src/Fastenshtein/Levenshtein.cs
index 085b2b1..c889fa6 100644
--- a/src/Fastenshtein/Levenshtein.cs
+++ b/src/Fastenshtein/Levenshtein.cs
@@ -1,4 +1,6 @@
-namespace Fastenshtein
+using System;
+
+namespace Fastenshtein
 {
     /// <summary>
     /// Measures the difference between two strings.
@@ -7,7 +9,7 @@
     public partial class Levenshtein
     {
         /*
-         * WARRING this class is performance critical (Speed).
+         * WARNING this class is performance critical (Speed).
          */
 
         private readonly string _storedValue;
@@ -30,11 +32,21 @@ public Levenshtein(string value)
         public int StoredLength => _storedValue.Length;
 
         /// <summary>
-        /// Compares a value to the stored value. 
+        /// Compares a value to the stored value.
         /// Not thread safe.
         /// </summary>
         /// <returns>Difference. 0 complete match.</returns>
         public int DistanceFrom(string value)
+        {
+            return DistanceFrom(value.AsSpan());
+        }
+
+        /// <summary>
+        /// Compares a value to the stored value.
+        /// Not thread safe.
+        /// </summary>
+        /// <returns>Difference. 0 complete match.</returns>
+        public int DistanceFrom(ReadOnlySpan<char> value)
         {
             // copying to local variables allows JIT to remove bounds checks, as it understands they can not change
             var costs = _costs;
@@ -42,7 +54,7 @@ public int DistanceFrom(string value)
 
             if (costs.Length == 0
                 // this will never be ture, however it allows the JIT to remove a bounds check
-                || costs.Length != storedValue.Length) 
+                || costs.Length != storedValue.Length)
             {
                 return value.Length;
             }
diff --git a/src/Fastenshtein/StaticLevenshtein.cs b/src/Fastenshtein/StaticLevenshtein.cs
index deaf457..904042b 100644
--- a/src/Fastenshtein/StaticLevenshtein.cs
+++ b/src/Fastenshtein/StaticLevenshtein.cs
@@ -1,4 +1,6 @@
-namespace Fastenshtein
+using System;
+
+namespace Fastenshtein
 {
     /// <summary>
     /// Measures the difference between two strings.
@@ -7,21 +9,32 @@
     public partial class Levenshtein
     {
         /// <summary>
-        /// Compares the two values to find the minimum Levenshtein distance. 
+        /// Compares the two values to find the minimum Levenshtein distance.
         /// Thread safe.
         /// </summary>
         /// <returns>Difference. 0 complete match.</returns>
         public static int Distance(string value1, string value2)
+        {
+            return Distance(value1.AsSpan(), value2.AsSpan());
+        }
+
+        /// <summary>
+        /// Compares the two values to find the minimum Levenshtein distance.
+        /// Thread safe.
+        /// </summary>
+        /// <returns>Difference. 0 complete match.</returns>
+        public static unsafe int Distance(ReadOnlySpan<char> value1, ReadOnlySpan<char> value2)
         {
             if (value2.Length == 0)
             {
                 return value1.Length;
             }
 
-            int[] costs = new int[value2.Length];
+            int costsLength = value2.Length;
+            int* costs = stackalloc int[costsLength];
 
             // Add indexing for insertion to first row
-            for (int i = 0; i < costs.Length;)
+            for (int i = 0; i < costsLength;)
             {
                 costs[i] = ++i;
             }
@@ -66,7 +79,7 @@ public static int Distance(string value1, string value2)
                 }
             }
 
-            return costs[costs.Length - 1];
+            return costs[costsLength - 1];
         }
     }
 }