format_number.dart

import 'dart:math';

String _roundTo(num value, int decimals,
    {bool floor = true, bool round = false, bool ceil = false}) {
  assert(floor || round || ceil);

  final sign = value >= 0 ? 1 : -1;

  final current = ((value * pow(10, decimals)) + (sign * 0.001));

  final base = (() {
    if (floor) return current.floor();
    if (round) return current.round();
    if (ceil) return current.ceil();
  })()!;

  return (base / pow(10, decimals)).toStringAsFixed(decimals);
}

String _format(
  num value,
  double base, {
  bool ceil = true,
  bool floor = false,
  bool round = false,
  int fractionDigits = 2,
  bool forceFraction = false,
}) {
  if (value < base) return '$value';

  final minPostfix = base / pow(10, fractionDigits);

  final before = value ~/ base;

  final rest = value - before * base;

  final rounded = _roundTo(
    rest / base,
    fractionDigits,
    ceil: ceil,
    floor: floor,
    round: round,
  );

  final decimalValue = '$rounded'.substring(2);

  final after = forceFraction || rest >= minPostfix
      ? '.${decimalValue.padRight(fractionDigits, '0')}'
      : '';

  return '$before$after';
}

String formatIt(
  num value, {
  int fractionDigits = 2,
  bool floor = true,
  bool round = false,
  bool ceil = false,
  bool forceFraction = false,
}) {
  const bases = [1e15, 1e12, 1e9, 1e6, 1e3];
  const postfixes = ['Q', 'T', 'B', 'M', 'K'];

  if (value < bases.last) return '$value';

  for (var i = 0; i < bases.length; i++) {
    final base = bases[i], postfix = postfixes[i];

    if (value >= base) {
      final formatted = _format(
        value,
        base,
        ceil: ceil,
        floor: floor,
        round: round,
        fractionDigits: fractionDigits,
        forceFraction: forceFraction,
      );

      return '$formatted$postfix';
    }
  }

  throw Exception('We can\'t format $value. Max allowed value: ${bases.first}');
}


/// ======================================================
/// format_number_test.dart
/// ======================================================

import 'package:test/test.dart';

void main() {
  group('[formatNumber] method', () {
    test(
        'only int | Fraction Digits = 2 | floor = true | round = false | ceil = false',
        () {
      final useCases = [
        [1000, '1K'],
        [10001, '10K'],
        [1000000, '1M'],
        [1000001, '1M'],
        [34567677, '34.56M'],
        [48957634, '48.95M'],
        [9435643565, '9.43B'],
        [10100, '10.10K'],
        [1100000, '1.10M'],
        [1111111, '1.11M'],
        [10011, '10.01K'],
        [1010000, '1.01M'],
        [123456789, '123.45M'],
        [56897845, '56.89M'],
        [10100, '10.10K'],
        [999, '999'],
        [1, '1'],
        [10, '10'],
        [150, '150'],
        [1234567, '1.23M'],
      ];

      for (var i = 0; i < useCases.length; i++) {
        expect(formatIt(useCases[i][0] as num), useCases[i][1] as String);
      }
    });
    test(
        'only int | Fraction Digits = 1 | floor = true | round = false | ceil = false',
        () {
      final useCases = [
        [1000, '1K'],
        [10001, '10K'],
        [1000000, '1M'],
        [1000001, '1M'],
        [34567677, '34.5M'],
        [48957634, '48.9M'],
        [9435643565, '9.4B'],
        [10100, '10.1K'],
        [1100000, '1.1M'],
        [1111111, '1.1M'],
        [10011, '10K'],
        [1010000, '1M'],
        [123456789, '123.4M'],
        [56897845, '56.8M'],
        [10100, '10.1K'],
        [999, '999'],
        [1, '1'],
        [10, '10'],
        [150, '150'],
        [1234567, '1.2M'],
      ];

      for (var i = 0; i < useCases.length; i++) {
        expect(
          formatIt(
            useCases[i][0] as num,
            fractionDigits: 1,
          ),
          useCases[i][1] as String,
        );
      }
    });
  });
}