Remove string allocation/copy from Path.Combine

Path.Combine(string, string, string) is essentially implemented as Path.Combine(Path.Combine(string, string), string), resulting in the temporary string from combining the first two strings and then another string from combining that with the third string.

This commit adds a new overload of CommitNoCheck that does combining for three paths without the intermediate allocation/copy.

Author: stephentoub

src/System.Runtime.Extensions/src/System/IO/Path.cs

@@ -311,7 +311,7 @@ public static string Combine(string path1, string path2, string path3)
             CheckInvalidPathChars(path2);
             CheckInvalidPathChars(path3);
 
-            return CombineNoChecks(CombineNoChecks(path1, path2), path3);
+            return CombineNoChecks(path1, path2, path3);
         }
 
         public static string Combine(params string[] paths)
@@ -402,6 +402,49 @@ private static string CombineNoChecks(string path1, string path2)
                 path1 + DirectorySeparatorCharAsString + path2;
         }
 
+        private static string CombineNoChecks(string path1, string path2, string path3)
+        {
+            if (path1.Length == 0)
+                return CombineNoChecks(path2, path3);
+            if (path2.Length == 0)
+                return CombineNoChecks(path1, path3);
+            if (path3.Length == 0)
+                return CombineNoChecks(path1, path2);
+
+            if (IsPathRooted(path3))
+                return path3;
+            if (IsPathRooted(path2))
+                return CombineNoChecks(path2, path3);
+
+            bool hasSep1 = IsDirectoryOrVolumeSeparator(path1[path1.Length - 1]);
+            bool hasSep2 = IsDirectoryOrVolumeSeparator(path2[path2.Length - 1]);
+
+            if (hasSep1 && hasSep2)
+            {
+                return path1 + path2 + path3;
+            }
+            else if (hasSep1)
+            {
+                return path1 + path2 + DirectorySeparatorCharAsString + path3;
+            }
+            else if (hasSep2)
+            {
+                return path1 + DirectorySeparatorCharAsString + path2 + path3;
+            }
+            else
+            {
+                // string.Concat only has string-based overloads up to four arguments; after that requires allocating
+                // a params string[].  Instead, try to use a cached StringBuilder.
+                StringBuilder sb = StringBuilderCache.Acquire(path1.Length + path2.Length + path3.Length + 2);
+                sb.Append(path1)
+                  .Append(DirectorySeparatorChar)
+                  .Append(path2)
+                  .Append(DirectorySeparatorChar)
+                  .Append(path3);
+                return StringBuilderCache.GetStringAndRelease(sb);
+            }
+        }
+
         private static readonly char[] s_Base32Char = {
                 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h',
                 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p',

src/System.Runtime.Extensions/tests/System/IO/Path.Combine.cs

@@ -51,8 +51,15 @@ public static void PathIsNullWihtoutRootedAfterArgumentNull()
     }
 
     [Fact]
-    [PlatformSpecific(PlatformID.Windows)]
     public static void ContainsInvalidCharWithoutRootedAfterArgumentNull()
+    {
+        //any path contains invalid character without rooted after (AE)
+        CommonCasesException<ArgumentException>("ab\0cd");
+    }
+
+    [Fact]
+    [PlatformSpecific(PlatformID.Windows)]
+    public static void ContainsInvalidCharWithoutRootedAfterArgumentNull_Windows()
     {
         //any path contains invalid character without rooted after (AE)
         CommonCasesException<ArgumentException>("ab\"cd");
@@ -66,16 +73,23 @@ public static void ContainsInvalidCharWithoutRootedAfterArgumentNull()
     }
 
     [Fact]
-    [PlatformSpecific(PlatformID.Windows)]
     public static void ContainsInvalidCharWithRootedAfterArgumentNull()
+    {
+        //any path contains invalid character with rooted after (AE)
+        CommonCasesException<ArgumentException>("ab\0cd", s_separator + "abc");
+    }
+
+
+    [Fact]
+    [PlatformSpecific(PlatformID.Windows)]
+    public static void ContainsInvalidCharWithRootedAfterArgumentNull_Windows()
     {
         //any path contains invalid character with rooted after (AE)
         CommonCasesException<ArgumentException>("ab\"cd", s_separator + "abc");
         CommonCasesException<ArgumentException>("ab<cd", s_separator + "abc");
         CommonCasesException<ArgumentException>("ab>cd", s_separator + "abc");
         CommonCasesException<ArgumentException>("ab|cd", s_separator + "abc");
         CommonCasesException<ArgumentException>("ab\bcd", s_separator + "abc");
-        CommonCasesException<ArgumentException>("ab\0cd", s_separator + "abc");
         CommonCasesException<ArgumentException>("ab\tcd", s_separator + "abc");
     }
 
@@ -110,6 +124,7 @@ public static void PathIsSingleElement()
     {
         //any path is single element
         CommonCases("abc");
+        CommonCases("abc" + s_separator);
     }
 
     [Fact]
@@ -119,10 +134,13 @@ public static void PathIsMultipleElements()
         CommonCases(Path.Combine("abc", Path.Combine("def", "ghi")));
     }
 
-    public static void NoPathIsRooted()
+    [Fact]
+    public static void PathElementsAllSeparated()
     {
-        //no path is rooted
-        CommonCases("abc");
+        Verify(new string[] { "abc" + s_separator, "def" + s_separator });
+        Verify(new string[] { "abc" + s_separator, "def" + s_separator, "ghi" + s_separator });
+        Verify(new string[] { "abc" + s_separator, "def" + s_separator, "ghi" + s_separator, "jkl" + s_separator });
+        Verify(new string[] { "abc" + s_separator, "def" + s_separator, "ghi" + s_separator, "jkl" + s_separator, "mno" + s_separator });
     }
 
     private static void Verify(string[] paths)