Ranges.js

var _ = require('lodash');

function getRangesForNumbers(numbers) {
  return _.reduce(numbers, function(list, item) {
    if (_.isEmpty(list)) {
      return [item];
    }

    var last = _.last(list);
    var initial = _.initial(list);

    if (_.isNumber(last) && last + 1 === item) {
      return initial.concat([[last, item]]);
    } else if (_.isArray(last) && last[1] + 1 === item) {
      return initial.concat([[last[0], item]]);
    }
    
    return list.concat(item);
  }, []);
}

function getNumbersForRanges(ranges) {
  return _.reduce(ranges, function(list, numberOrRange) {
    if (_.isNumber(numberOrRange)) {
      return list.concat([numberOrRange]);
    }

    if (_.isArray(numberOrRange)) {
      return list.concat(_.range(
        numberOrRange[0], 
        numberOrRange[1] + 1
      ));
    } 
  }, []);
}

function formatRanges(ranges) {
  return ranges.map(function(numberOrRange) {
    if (_.isArray(numberOrRange)) {
      return numberOrRange.join('-');
    }
    return numberOrRange;
  }).join(',');
}

function parseRanges(formatted) {
  return formatted.split(',').reduce(function(ranges, formattedSection) {
    var section = formattedSection.split('-');

    if (section.length === 1) {
      var possibleNumber = parseInt(section[0], 10);
      return isNaN(possibleNumber) ? ranges : ranges.concat([possibleNumber]);
    }

    var range = section.map((number) => parseInt(number, 10));
    return ranges.concat([range]);
  }, []);
}

var Ranges = {
  getRangesForNumbers,
  getNumbersForRanges,
  formatRanges,
  parseRanges
};

module.exports = Ranges;