Reduce allocations in FindEntry by manually doing binary search

ValvePak/ValvePak.Test/WriteTest.cs

@@ -1,4 +1,3 @@
-using System;
 using System.IO;
 using System.Text;
 using NUnit.Framework;

ValvePak/ValvePak/Package.cs

@@ -195,7 +195,7 @@ public PackageEntry FindEntry(string filePathStr)
  return default;
  }
 
- // Remove the trailing slash
+ // Remove the trailing and leading slash
  directory = directory.Trim(DirectorySeparatorChar);
 
  // If the directory is empty after trimming, set it to a space to match Valve's behaviour
@@ -204,29 +204,65 @@ public PackageEntry FindEntry(string filePathStr)
  directory = Space;
  }
 
- /// Searches for a given file entry in the file list after it has been optimized with <see cref="OptimizeEntriesForBinarySearch"/>.
- /// This also supports case insensitive search by using a different <see cref="StringComparison"/>.
- if (Comparer != null)
+ int hi = entriesForExtension.Count - 1;
+
+ if (Comparer == null)
  {
- var searchEntry = new PackageEntry
+ for (var i = 0; i <= hi; i++) // Don't use foreach
  {
- DirectoryName = directory.ToString(),
- FileName = fileName.ToString(),
- TypeName = extension,
- };
-
- var index = entriesForExtension.BinarySearch(searchEntry, Comparer);
+ var entry = entriesForExtension[i];
+ if (directory.SequenceEqual(entry.DirectoryName) && fileName.SequenceEqual(entry.FileName))
+ {
+  return entry;
+ }
+ }
 
- return index < 0 ? default : entriesForExtension[index];
+ return default;
  }
 
- for(var i = 0; i < entriesForExtension.Count; i++) // Don't use foreach
+ /// Searches for a given file entry in the file list after it has been optimized with <see cref="OptimizeEntriesForBinarySearch"/>.
+ /// This also supports case insensitive search by using a different <see cref="StringComparison"/>.
+ ///
+ /// Manually implement binary search to avoid allocating new strings for file and directory names.
+ /// See <see cref="MemoryExtensions.BinarySearch{T}(ReadOnlySpan{T}, IComparable{T})"/> for reference.
+
+ int lo = 0;
+
+ while (lo <= hi)
  {
+ var i = (int)(((uint)hi + (uint)lo) >> 1);
  var entry = entriesForExtension[i];
- if (directory.SequenceEqual(entry.DirectoryName) && fileName.SequenceEqual(entry.FileName))
+
+ /// This code must match <see cref="CaseInsensitivePackageEntryComparer.Compare(PackageEntry, PackageEntry)"/>
+ var comp = fileName.Length.CompareTo(entry.FileName.Length);
+
+ if (comp == 0)
+ {
+ comp = directory.Length.CompareTo(entry.DirectoryName.Length);
+
+ if (comp == 0)
+ {
+ comp = fileName.CompareTo(entry.FileName, Comparer.Comparison);
+
+ if (comp == 0)
+ {
+ comp = directory.CompareTo(entry.DirectoryName, Comparer.Comparison);
+ }
+ }
+ }
+
+ if (comp == 0)
  {
  return entry;
  }
+ else if (comp > 0)
+ {
+ lo = i + 1;
+ }
+ else
+ {
+ hi = i - 1;
+ }
  }
 
  return default;