MakeRelative Task (#3750)

* MakeRelative Task

* PR Feedback

* Fix naming conflict

build_projects/dotnet-cli-build/MakeRelative.cs

@@ -0,0 +1,119 @@
+using System;
+using System.IO;
+using Microsoft.Build.Utilities;
+using Microsoft.Build.Framework;
+using Microsoft.DotNet.Cli.Build.Framework;
+using Newtonsoft.Json;
+
+namespace Microsoft.DotNet.Cli.Build
+{
+    public class MakeRelative : Task
+    {
+        [Required]
+        public string Path1 { get; set; }
+
+        [Required]
+        public string Path2 { get; set; }
+
+        public char SeparatorChar { get; set; }
+
+        [Output]
+        public ITaskItem RelativePath { get; set; }
+
+        public override bool Execute()
+        {
+            if (SeparatorChar == default(char))
+            {
+                SeparatorChar = Path.DirectorySeparatorChar;
+            }
+
+            var relativePath = GetRelativePath(Path1, Path2, SeparatorChar);
+
+            RelativePath = ToTaskItem(Path1, Path2, relativePath);
+
+            return true;
+        }
+
+        private static TaskItem ToTaskItem(string path1, string path2, string relativePath)
+        {
+            var framework = new TaskItem();
+            framework.ItemSpec = relativePath;
+
+            framework.SetMetadata("Path1", path1);
+            framework.SetMetadata("Path2", path2);
+            framework.SetMetadata("RelativePath", relativePath);
+
+            return framework;
+        }
+
+        private static string GetRelativePath(string path1, string path2, char separator = default(char))
+        {
+
+            StringComparison compare;
+            if (CurrentPlatform.IsWindows)
+            {
+                compare = StringComparison.OrdinalIgnoreCase;
+                // check if paths are on the same volume
+                if (!string.Equals(Path.GetPathRoot(path1), Path.GetPathRoot(path2)))
+                {
+                    // on different volumes, "relative" path is just Path2
+                    return path2;
+                }
+            }
+            else
+            {
+                compare = StringComparison.Ordinal;
+            }
+
+            var index = 0;
+            var path1Segments = path1.Split(Path.DirectorySeparatorChar, Path.AltDirectorySeparatorChar);
+            var path2Segments = path2.Split(Path.DirectorySeparatorChar, Path.AltDirectorySeparatorChar);
+            // if path1 does not end with / it is assumed the end is not a directory
+            // we will assume that is isn't a directory by ignoring the last split
+            var len1 = path1Segments.Length - 1;
+            var len2 = path2Segments.Length;
+
+            // find largest common absolute path between both paths
+            var min = Math.Min(len1, len2);
+            while (min > index)
+            {
+                if (!string.Equals(path1Segments[index], path2Segments[index], compare))
+                {
+                    break;
+                }
+                // Handle scenarios where folder and file have same name (only if os supports same name for file and directory)
+                // e.g. /file/name /file/name/app
+                else if ((len1 == index && len2 > index + 1) || (len1 > index && len2 == index + 1))
+                {
+                    break;
+                }
+                ++index;
+            }
+
+            var path = "";
+
+            // check if path2 ends with a non-directory separator and if path1 has the same non-directory at the end
+            if (len1 + 1 == len2 && !string.IsNullOrEmpty(path1Segments[index]) &&
+                string.Equals(path1Segments[index], path2Segments[index], compare))
+            {
+                return path;
+            }
+
+            for (var i = index; len1 > i; ++i)
+            {
+                path += ".." + separator;
+            }
+            for (var i = index; len2 - 1 > i; ++i)
+            {
+                path += path2Segments[i] + separator;
+            }
+            // if path2 doesn't end with an empty string it means it ended with a non-directory name, so we add it back
+            if (!string.IsNullOrEmpty(path2Segments[len2 - 1]))
+            {
+                path += path2Segments[len2 - 1];
+            }
+
+            return path;
+        }
+    }
+}