🍰 Layered Cakewalk [Dart] - Parses/splits a command-line String into a List<String> of the individual argument elements processing quotes.

CommandlineSplittingConverter.dart

//
// Original implementation: https://stackoverflow.com/a/39177861/638848
//
// Re-made by: Leslie Godwin (leslie.godwin@gmail.com)

import 'dart:convert';

/// Parses/splits a command-line String into a List<String> of the individual argument elements.
/// Handles whitespace, quotes and back\slash escape.
/// 
/// ### Example: 
/// ```
/// command-line    $> command -x value -y "value with 'spaces'"  -z value\ with\'\ spaces
/// result-arguments : ["command", "-x", "value", "y", "value with 'spaces'", "-z", "value with' spaces"]
/// ```
class CommandlineSplittingConverter extends Converter<String, List<String>>
{
  @override
  List<String> convert(String input) 
  {
    if (input == null || input.isEmpty) { return []; } // ⤴️

    final result = <String>[];

    var    current    = "";
    String inEscape   = null;
    int    inPosition = null;

    for (int index = 0; index < input.length; index++)
    {
        final token = input[index];

        if (inEscape != null)
        {
          switch (inEscape)
          {
            case "\\":

              inEscape   = null;
              inPosition = null;

              current += token;
              continue; // ⤴️

            default:

              if (token == inEscape) 
              {
                  result.add(current); // allow empty
                  current = "";

                  inEscape   = null;
                  inPosition = null;
              } 
              else 
              {
                  current += token;
              }
          }
        }
        else
        {
          switch (token) 
          {
            case "'":  // '  (Single quote)
            case '"':  // "" (Double quote)
            case "\\": // \  (Escape)

              inEscape   = token;
              inPosition = index;
              continue; // ⤴️

            case " ": // space

              if (current.isNotEmpty)
              {
                result.add(current);
                current = "";
              }
              continue; // ⤴️

            default:

              current += token;
          }
        }
    }

    if (inEscape != null)
    {
        throw new FormatException("Unbalanced quote $inEscape in input.", input, inPosition);
    }

    if (current.isNotEmpty) 
    {
        result.add(current);
    }

    return result;
  }
}

// //Tests
//
// Set<E> diff <E> (Iterable<E> lhs, Iterable<E> rhs)
// {
//   final a = Set.of(lhs);
//   final b = Set.of(rhs);
//   return a.difference(b)..addAll(b.difference(a));
// }

// Set<String> commandlineDiff (String lhs, List<String> rhs)
// {
//   return diff(CommandlineSplittingConverter().convert(lhs), rhs);
// }

// test("$CommandlineSplittingConverter", ()
// {
//   expect
//   (
//     commandlineDiff("", []), 
//     isEmpty
//   );

//   expect
//   (
//     commandlineDiff("command", ["command"]), 
//     isEmpty
//   );

//   expect
//   (
//     commandlineDiff("command   -arg 1  -arg2 'f r\"ed'", ["command", "-arg", "1", "-arg2", "f r\"ed"]), 
//     isEmpty
//   );

//   expect
//   (
//     commandlineDiff('command -arg   1 -arg2  "f r\'ed"', ["command", "-arg", "1", "-arg2", "f r'ed"]), 
//     isEmpty
//   );
  
//   expect
//   (
//     commandlineDiff("command -x value -y \"value with 'spaces'\" -z value\\ with\\'\\ spaces", ["command", "-x", "value", "-y", "value with 'spaces'", "-z", "value with' spaces"]), 
//     isEmpty
//   );

//   expect
//   (
//     () => CommandlineSplittingConverter().convert('command -arg 1 -arg2 "f r\'ed'), 
//     throwsFormatException
//   );
// });