/**
 * Exports
 */

module.exports = parseTimestring

/**
 * Default options to use when parsing a timestring
 *
 * @type {Object}
 */

const DEFAULT_OPTS = {
  hoursPerDay: 24,
  daysPerWeek: 7,
  weeksPerMonth: 4,
  monthsPerYear: 12,
  daysPerYear: 365.25
}

/**
 * Map of accepted strings to unit
 *
 * @type {Object}
 */

const UNIT_MAP = {
  ms: ['ms', 'milli', 'millisecond', 'milliseconds'],
  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: ['mon', 'mth', 'mths', 'month', 'months'],
  y: ['y', 'yr', 'yrs', 'year', 'years']
}

/**
 * Parse a timestring
 *
 * @param   {string} string
 * @param   {string} returnUnit
 * @param   {Object} opts
 * @returns {number}
 */

function parseTimestring (string, returnUnit, opts) {
  opts = Object.assign({}, DEFAULT_OPTS, opts || {})

  let totalSeconds = 0
  const unitValues = getUnitValues(opts)
  const groups = string
    .toLowerCase()
    .replace(/[^.\w+-]+/g, '')
    .match(/[-+]?[0-9.]+[a-z]+/g)

  if (groups === null) {
    throw new Error(`The string [${string}] could not be parsed by timestring`)
  }

  groups.forEach(group => {
    const value = group.match(/[0-9.]+/g)[0]
    const unit = group.match(/[a-z]+/g)[0]

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

  if (returnUnit) {
    return convert(totalSeconds, returnUnit, unitValues)
  }

  return totalSeconds
}

/**
 * Get unit values based on the passed options
 *
 * @param   {Object} opts
 * @returns {Object}
 */

function getUnitValues (opts) {
  const unitValues = {
    ms: 0.001,
    s: 1,
    m: 60,
    h: 3600
  }

  unitValues.d = opts.hoursPerDay * unitValues.h
  unitValues.w = opts.daysPerWeek * unitValues.d
  unitValues.mth = (opts.daysPerYear / opts.monthsPerYear) * unitValues.d
  unitValues.y = opts.daysPerYear * unitValues.d

  return unitValues
}

/**
 * Get the key for a unit
 *
 * @param   {string} unit
 * @returns {string}
 */

function getUnitKey (unit) {
  for (const key of Object.keys(UNIT_MAP)) {
    if (UNIT_MAP[key].indexOf(unit) > -1) {
      return key
    }
  }

  throw new Error(`The unit [${unit}] is not supported by timestring`)
}

/**
 *  Get the number of seconds for a value, based on the unit
 *
 * @param   {number} value
 * @param   {string} unit
 * @param   {Object} unitValues
 * @returns {number}
 */

function getSeconds (value, unit, unitValues) {
  return value * unitValues[getUnitKey(unit)]
}

/**
 * Convert a value from its existing unit to a new unit
 *
 * @param   {number} value
 * @param   {string} unit
 * @param   {Object} unitValues
 * @returns {number}
 */

function convert (value, unit, unitValues) {
  return value / unitValues[getUnitKey(unit)]
}