From 7003422e8f120c69de31319b13f81988588bda17 Mon Sep 17 00:00:00 2001
From: Gerard Smit <gsmit1996@gmail.com>
Date: Thu, 30 May 2024 17:41:57 +0200
Subject: [PATCH] Reduce allocations in UPath.Combine

---
 src/Zio/UPath.cs | 107 +++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 98 insertions(+), 9 deletions(-)

diff --git a/src/Zio/UPath.cs b/src/Zio/UPath.cs
index f80d9a5..2d30b75 100644
--- a/src/Zio/UPath.cs
+++ b/src/Zio/UPath.cs
@@ -129,6 +129,33 @@ public static explicit operator string(UPath path)
     /// </exception>
     /// <exception cref="System.ArgumentException">If an error occurs while trying to combine paths.</exception>
     public static UPath Combine(UPath path1, UPath path2)
+    {
+        if (TryGetAbsolute(path1, path2, out var result))
+        {
+            return result;
+        }
+
+        try
+        {
+#if NET7_0_OR_GREATER
+            return string.Create(path1.FullName.Length + path2.FullName.Length + 1, new KeyValuePair<UPath, UPath>(path1, path2), (span, state) =>
+            {
+                var (left, right) = state;
+                left.FullName.AsSpan().CopyTo(span);
+                span[left.FullName.Length] = '/';
+                right.FullName.AsSpan().CopyTo(span.Slice(left.FullName.Length + 1));
+            });
+#else
+            return new UPath($"{path1.FullName}/{path2.FullName}");
+#endif
+        }
+        catch (ArgumentException ex)
+        {
+            throw new ArgumentException($"Unable to combine path `{path1}` with `{path2}`", ex);
+        }
+    }
+
+    private static bool TryGetAbsolute(UPath path1, UPath path2, out UPath result)
     {
         if (path1.FullName is null)
             throw new ArgumentNullException(nameof(path1));
@@ -138,26 +165,88 @@ public static UPath Combine(UPath path1, UPath path2)
 
         // If the right path is absolute, it takes priority over path1
         if (path1.IsEmpty || path2.IsAbsolute)
-            return path2;
-
-        try
         {
-            return new UPath($"{path1.FullName}/{path2.FullName}");
-        }
-        catch (ArgumentException ex)
-        {
-            throw new ArgumentException($"Unable to combine path `{path1}` with `{path2}`", ex);
+            result = path2;
+            return true;
         }
+
+        result = default;
+        return false;
     }
 
     public static UPath Combine(UPath path1, UPath path2, UPath path3)
     {
+        if (TryGetAbsolute(path1, path2, out var result))
+        {
+            return Combine(result, path3);
+        }
+
+        if (TryGetAbsolute(path2, path3, out result))
+        {
+            return Combine(path1, result);
+        }
+
+#if NET7_0_OR_GREATER
+        return string.Create(path1.FullName.Length + path2.FullName.Length + path3.FullName.Length + 2, (path1, path2, path3), (span, state) =>
+        {
+            var (p1, p2, p3) = state;
+            var remaining = span;
+
+            p1.FullName.AsSpan().CopyTo(remaining);
+            remaining[p1.FullName.Length] = '/';
+            remaining = remaining.Slice(p1.FullName.Length + 1);
+
+            p2.FullName.AsSpan().CopyTo(remaining);
+            remaining[p2.FullName.Length] = '/';
+
+            remaining = remaining.Slice(p2.FullName.Length + 1);
+            p3.FullName.AsSpan().CopyTo(remaining);
+        });
+#else
         return UPath.Combine(UPath.Combine(path1, path2), path3);
+#endif
     }
 
     public static UPath Combine(UPath path1, UPath path2, UPath path3, UPath path4)
     {
-        return UPath.Combine(Combine(path1, path2), Combine(path3, path4));
+        if (TryGetAbsolute(path1, path2, out var result))
+        {
+            return Combine(result, path3, path4);
+        }
+
+        if (TryGetAbsolute(path2, path3, out result))
+        {
+            return Combine(path1, result, path4);
+        }
+
+        if (TryGetAbsolute(path3, path4, out result))
+        {
+            return Combine(path1, path2, result);
+        }
+
+#if NET7_0_OR_GREATER
+        return string.Create(path1.FullName.Length + path2.FullName.Length + path3.FullName.Length + path4.FullName.Length + 3, (path1, path2, path3, path4), (span, state) =>
+        {
+            var (p1, p2, p3, p4) = state;
+            var remaining = span;
+
+            p1.FullName.AsSpan().CopyTo(remaining);
+            remaining[p1.FullName.Length] = '/';
+            remaining = remaining.Slice(p1.FullName.Length + 1);
+
+            p2.FullName.AsSpan().CopyTo(remaining);
+            remaining[p2.FullName.Length] = '/';
+            remaining = remaining.Slice(p2.FullName.Length + 1);
+
+            p3.FullName.AsSpan().CopyTo(remaining);
+            remaining[p3.FullName.Length] = '/';
+            remaining = remaining.Slice(p3.FullName.Length + 1);
+
+            p4.FullName.AsSpan().CopyTo(remaining);
+        });
+#else
+        return UPath.Combine(UPath.Combine(path1, path2), UPath.Combine(path3, path4));
+#endif
     }
 
     public static UPath Combine(params UPath[] paths)