Combine lists

index.html

<h1 id="header">Combinations</h1>

<p>This tiny tool will combine any number of lists, combining every option with every other option. Separate lists by a blank line.</p>

<textarea id="input" rows=20 cols=40>
In college, I had 

good grades, 
bad grades, 

I slept well, 
I got no sleep, 

and 

I went to parties.
I had no social life.
</textarea>

<pre id="output">(nothing yet)</pre>

main.dart

import 'dart:html';

void main() {
  final textArea = querySelector('#input')! as TextAreaElement;
  final output = querySelector('#output')!;
  textArea.onInput.listen((_) => output.text = _update(textArea.value!));

  // Run algorithm on the example input on load.
  output.text = _update(textArea.value!);
}

/// Takes [text], combines the lists inside, and returns all combinations
/// as lines in a single string.
String _update(String text) {
  var listStrings = text.split(RegExp('\n{2,}'));
  var lists = listStrings
      .map((s) =>
          s.split('\n').where((s) => s.isNotEmpty).toList(growable: false))
      .where((l) => l.isNotEmpty)
      .toList(growable: false);
  print(lists.join('x'));

  var permutations = _combine(lists.first, lists.sublist(1));
  return permutations.join('\n');
}

/// Recursively combines the strings in [head] with the strings in [tail].
List<String> _combine(List<String> head, List<List<String>> tail) {
  if (head.isEmpty) return const [];

  if (tail.isEmpty) return head;

  return _combineTwo(
    head,
    _combine(
      tail.first,
      tail.sublist(1),
    ),
  ).toList(growable: false);
}

/// Combine just two string.
Iterable<String> _combineTwo(List<String> a, List<String> b) sync* {
  for (final first in a) {
    for (final second in b) {
      yield '$first$second';
    }
  }
}

style.css

body {
  color: white;
  font-family: Arial, Helvetica, sans-serif;
  
  padding: 4em;
}

#output {
  margin: 4em 0;
  max-width: 80vw;
  
  overflow-x: scroll;
  padding-bottom: 1em;
}