How compute shannon entropy of a string in Dart.

entropy.dart

// Copyright 2021 Google LLC.
// SPDX-License-Identifier: Apache-2.0

import 'dart:math';

double entropy(String s) {
  final length = s.length.toDouble();
  
  // TODO: Explore if this could be done faster without any allocations, since strings are often short.
  final frequencies = <int, int>{};
  for (final rune in s.runes) {
    frequencies[rune] = (frequencies[rune] ?? 0) + 1;
  }
  var sum = 0.0;
  for (final frequency in frequencies.values) {
    sum -= frequency / length * (log(frequency / length) / log(2));
  }
  return sum;
}