// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Microsoft.Extensions.CommandLineUtils
{
    /// <summary>
    /// A utility for escaping arguments for new processes.
    /// </summary>
    internal static class ArgumentEscaper
    {
        /// <summary>
        /// Undo the processing which took place to create string[] args in Main, so that the next process will
        /// receive the same string[] args.
        /// </summary>
        /// <remarks>
        /// See https://blogs.msdn.microsoft.com/twistylittlepassagesallalike/2011/04/23/everyone-quotes-command-line-arguments-the-wrong-way/
        /// </remarks>
        /// <param name="args"></param>
        /// <returns></returns>
        public static string EscapeAndConcatenate(IEnumerable<string> args)
            => string.Join(" ", args.Select(EscapeSingleArg));

        private static string EscapeSingleArg(string arg)
        {
            var sb = new StringBuilder();

            var needsQuotes = ShouldSurroundWithQuotes(arg);
            var isQuoted = needsQuotes || IsSurroundedWithQuotes(arg);

            if (needsQuotes)
            {
                sb.Append('"');
            }

            for (int i = 0; i < arg.Length; ++i)
            {
                var backslashes = 0;

                // Consume all backslashes
                while (i < arg.Length && arg[i] == '\\')
                {
                    backslashes++;
                    i++;
                }

                if (i == arg.Length && isQuoted)
                {
                    // Escape any backslashes at the end of the arg when the argument is also quoted.
                    // This ensures the outside quote is interpreted as an argument delimiter
                    sb.Append('\\', 2 * backslashes);
                }
                else if (i == arg.Length)
                {
                    // At then end of the arg, which isn't quoted,
                    // just add the backslashes, no need to escape
                    sb.Append('\\', backslashes);
                }
                else if (arg[i] == '"')
                {
                    // Escape any preceding backslashes and the quote
                    sb.Append('\\', (2 * backslashes) + 1);
                    sb.Append('"');
                }
                else
                {
                    // Output any consumed backslashes and the character
                    sb.Append('\\', backslashes);
                    sb.Append(arg[i]);
                }
            }

            if (needsQuotes)
            {
                sb.Append('"');
            }

            return sb.ToString();
        }

        private static bool ShouldSurroundWithQuotes(string argument)
        {
            // Don't quote already quoted strings
            if (IsSurroundedWithQuotes(argument))
            {
                return false;
            }

            // Only quote if whitespace exists in the string
            return ContainsWhitespace(argument);
        }

        private static bool IsSurroundedWithQuotes(string argument)
        {
            if (argument.Length <= 1)
            {
                return false;
            }

            return argument[0] == '"' && argument[argument.Length - 1] == '"';
        }

        private static bool ContainsWhitespace(string argument)
            => argument.IndexOfAny(new [] { ' ', '\t', '\n' }) >= 0;
    }
}