(function(){
  "use strict";

  var Timestring = function(settings) {
    // default settings
    var defaults = {
      hoursPerDay: 24,
      daysPerWeek: 7,
      weeksPerMonth: 4,
      monthsPerYear: 12
    };

    // merge default settings with user settings
    settings = settings || {};
    this.settings = defaults;
    for (var s in settings) { this.settings[s] = settings[s]; }

    // time units
    this.units = {
      s: ['s', 'sec', 'secs', 'second', 'seconds'],
      m: ['m', 'min', 'mins', 'minute', 'minutes'],
      h: ['h', 'hr', 'hrs', 'hour', 'hours'],
      d: ['d', 'day', 'days'],
      w: ['w', 'week', 'weeks'],
      mth: ['mth', 'mths','month', 'months'],
      y: ['y', 'yr', 'yrs', 'year', 'years']
    };

    // time unit seconds mappings
    this.unitValues = {
      s: 1,
      m: 60,
      h: 3600
    };

    // dynamic time unit seconds mappings
    // these are dynamic based on the settings
    this.unitValues.d = this.settings.hoursPerDay * this.unitValues.h;
    this.unitValues.w = this.settings.daysPerWeek * this.unitValues.d;
    this.unitValues.mth = this.settings.weeksPerMonth * this.unitValues.w;
    this.unitValues.y = this.settings.monthsPerYear * this.unitValues.mth;
  };

  Timestring.prototype.parse = function(string, returnUnit) {
    // reference to this
    var that = this;

    // get unit key helper
    function getUnitKey(unit) {
      for (var k in that.units) {
        for (var u in that.units[k]) {
          if (unit === that.units[k][u]) {
            return k;
          }
        }
      }

      // throw error if invalid unit was passed
      throw new Error('The unit [' + unit + '] is not supported by timestring');
    }

    // convert a value to a specific unit
    function convert(value, unit) {
      var baseValue = that.unitValues[getUnitKey(unit)];

      return value / baseValue;
    }

    // get a value in seconds based on a specific unit
    function getSeconds(value, unit) {
      var baseValue = that.unitValues[getUnitKey(unit)];

      return value * baseValue;
    }

    // seconds counter
    var totalSeconds = 0;

    // split string into groups and get total seconds for each group
    var groups = string
                  .toLowerCase() // convert words to lower case
                  .replace(/[^\.\w+-]+/g, '') // remove white space
                  .match(/[-+]?[0-9]+[a-z]+/g); // match time groups (digit followed by time unit - i.e 5d 15m = 2 time groups)

    if (groups !== null) {
      for(var i = 0; i < groups.length; i++) {
        var g = groups[i];
        var value = g.match(/[0-9]+/g)[0];
        var unit = g.match(/[a-z]+/g)[0];

        totalSeconds += getSeconds(value, unit);
      }
    }

    // return total, convert if needed
    return (returnUnit) ? convert(totalSeconds, returnUnit) : totalSeconds;
  };

  // add convenience method to string prototype
  String.prototype.parseTime = function (unit, settings) {
    return (new Timestring(settings)).parse(this, unit);
  };

  // export Timestring object
  if (typeof module !== 'undefined' && module.exports) {
    module.exports = Timestring;
  }
  else {
    this.Timestring = Timestring;
  }

}).call(this);