introduce dotnet complete command

src/dotnet/commands/dotnet-complete/CompleteCommand.cs

@@ -0,0 +1,68 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System;
+using System.IO;
+using System.Linq;
+using System.Text;
+using Microsoft.DotNet.Cli.CommandLine;
+using Microsoft.DotNet.Cli.Utils;
+using Command = Microsoft.DotNet.Cli.CommandLine.Command;
+
+namespace Microsoft.DotNet.Tools.Complete
+{
+    public class CompleteCommand
+    {
+        private static readonly Command dotnetCommand = Create.DotnetCommand();
+
+        public static int Run(string[] args)
+        {
+            DebugHelper.HandleDebugSwitch(ref args);
+
+            var log = new StringBuilder();
+            log.AppendLine($"args: {string.Join(" ", args.Select(a => $"\"{a}\""))}");
+
+            var result = dotnetCommand["complete"].Parse(args);
+
+            log.AppendLine("diagram (1): " + result.Diagram());
+
+            var complete = result["complete"];
+
+            var suggestions = Suggestions(complete, log);
+
+            log.AppendLine($"suggestions: {Environment.NewLine}{string.Join(Environment.NewLine, suggestions)}");
+
+            File.WriteAllText("parse.log", log.ToString());
+
+            foreach (var suggestion in suggestions)
+            {
+                Console.WriteLine(suggestion);
+            }
+
+            return 0;
+        }
+
+        private static string[] Suggestions(AppliedOption complete, StringBuilder log)
+        {
+            var input = complete.Arguments.SingleOrDefault() ?? "";
+
+            var positionOption = complete.AppliedOptions.SingleOrDefault(a => a.Name == "position");
+            if (positionOption != null)
+            {
+                var position = positionOption.Value<int>();
+
+                if (position > input.Length)
+                {
+                    input += " ";
+                }
+            }
+
+            var result = dotnetCommand.Parse(input);
+
+            log.AppendLine("diagram (2): " + result.Diagram());
+
+            return result.Suggestions()
+                         .ToArray();
+        }
+    }
+}