import defaultLocale from '../locale/en-US/index'
import subMilliseconds from '../subMilliseconds/index'
import toDate from '../toDate/index'
import assign from '../_lib/assign/index'
import longFormatters from '../_lib/format/longFormatters/index'
import getTimezoneOffsetInMilliseconds from '../_lib/getTimezoneOffsetInMilliseconds/index'
import {
  isProtectedDayOfYearToken,
  isProtectedWeekYearToken,
  throwProtectedError
} from '../_lib/protectedTokens/index'
import toInteger from '../_lib/toInteger/index'
import parsers from './_lib/parsers/index'
import requiredArgs from '../_lib/requiredArgs/index'

var TIMEZONE_UNIT_PRIORITY = 10

// This RegExp consists of three parts separated by `|`:
// - [yYQqMLwIdDecihHKkms]o matches any available ordinal number token
//   (one of the certain letters followed by `o`)
// - (\w)\1* matches any sequences of the same letter
// - '' matches two quote characters in a row
// - '(''|[^'])+('|$) matches anything surrounded by two quote characters ('),
//   except a single quote symbol, which ends the sequence.
//   Two quote characters do not end the sequence.
//   If there is no matching single quote
//   then the sequence will continue until the end of the string.
// - . matches any single character unmatched by previous parts of the RegExps
var formattingTokensRegExp = /[yYQqMLwIdDecihHKkms]o|(\w)\1*|''|'(''|[^'])+('|$)|./g

// This RegExp catches symbols escaped by quotes, and also
// sequences of symbols P, p, and the combinations like `PPPPPPPppppp`
var longFormattingTokensRegExp = /P+p+|P+|p+|''|'(''|[^'])+('|$)|./g

var escapedStringRegExp = /^'([^]*?)'?$/
var doubleQuoteRegExp = /''/g

var notWhitespaceRegExp = /\S/
var unescapedLatinCharacterRegExp = /[a-zA-Z]/

/**
 * @name parse
 * @category Common Helpers
 * @summary Parse the date.
 *
 * @description
 * Return the date parsed from string using the given format string.
 *
 * > ⚠️ Please note that the `format` tokens differ from Moment.js and other libraries.
 * > See: https://git.io/fxCyr
 *
 * The characters in the format string wrapped between two single quotes characters (') are escaped.
 * Two single quotes in a row, whether inside or outside a quoted sequence, represent a 'real' single quote.
 *
 * Format of the format string is based on Unicode Technical Standard #35:
 * https://www.unicode.org/reports/tr35/tr35-dates.html#Date_Field_Symbol_Table
 * with a few additions (see note 5 below the table).
 *
 * Not all tokens are compatible. Combinations that don't make sense or could lead to bugs are prohibited
 * and will throw `RangeError`. For example usage of 24-hour format token with AM/PM token will throw an exception:
 *
 * ```javascript
 * parse('23 AM', 'HH a', new Date())
 * //=> RangeError: The format string mustn't contain `HH` and `a` at the same time
 * ```
 *
 * See the compatibility table: https://docs.google.com/spreadsheets/d/e/2PACX-1vQOPU3xUhplll6dyoMmVUXHKl_8CRDs6_ueLmex3SoqwhuolkuN3O05l4rqx5h1dKX8eb46Ul-CCSrq/pubhtml?gid=0&single=true
 *
 * Accepted format string patterns:
 * | Unit                            |Prior| Pattern | Result examples                   | Notes |
 * |---------------------------------|-----|---------|-----------------------------------|-------|
 * | Era                             | 140 | G..GGG  | AD, BC                            |       |
 * |                                 |     | GGGG    | Anno Domini, Before Christ        | 2     |
 * |                                 |     | GGGGG   | A, B                              |       |
 * | Calendar year                   | 130 | y       | 44, 1, 1900, 2017, 9999           | 4     |
 * |                                 |     | yo      | 44th, 1st, 1900th, 9999999th      | 4,5   |
 * |                                 |     | yy      | 44, 01, 00, 17                    | 4     |
 * |                                 |     | yyy     | 044, 001, 123, 999                | 4     |
 * |                                 |     | yyyy    | 0044, 0001, 1900, 2017            | 4     |
 * |                                 |     | yyyyy   | ...                               | 2,4   |
 * | Local week-numbering year       | 130 | Y       | 44, 1, 1900, 2017, 9000           | 4     |
 * |                                 |     | Yo      | 44th, 1st, 1900th, 9999999th      | 4,5   |
 * |                                 |     | YY      | 44, 01, 00, 17                    | 4,6   |
 * |                                 |     | YYY     | 044, 001, 123, 999                | 4     |
 * |                                 |     | YYYY    | 0044, 0001, 1900, 2017            | 4,6   |
 * |                                 |     | YYYYY   | ...                               | 2,4   |
 * | ISO week-numbering year         | 130 | R       | -43, 1, 1900, 2017, 9999, -9999   | 4,5   |
 * |                                 |     | RR      | -43, 01, 00, 17                   | 4,5   |
 * |                                 |     | RRR     | -043, 001, 123, 999, -999         | 4,5   |
 * |                                 |     | RRRR    | -0043, 0001, 2017, 9999, -9999    | 4,5   |
 * |                                 |     | RRRRR   | ...                               | 2,4,5 |
 * | Extended year                   | 130 | u       | -43, 1, 1900, 2017, 9999, -999    | 4     |
 * |                                 |     | uu      | -43, 01, 99, -99                  | 4     |
 * |                                 |     | uuu     | -043, 001, 123, 999, -999         | 4     |
 * |                                 |     | uuuu    | -0043, 0001, 2017, 9999, -9999    | 4     |
 * |                                 |     | uuuuu   | ...                               | 2,4   |
 * | Quarter (formatting)            | 120 | Q       | 1, 2, 3, 4                        |       |
 * |                                 |     | Qo      | 1st, 2nd, 3rd, 4th                | 5     |
 * |                                 |     | QQ      | 01, 02, 03, 04                    |       |
 * |                                 |     | QQQ     | Q1, Q2, Q3, Q4                    |       |
 * |                                 |     | QQQQ    | 1st quarter, 2nd quarter, ...     | 2     |
 * |                                 |     | QQQQQ   | 1, 2, 3, 4                        | 4     |
 * | Quarter (stand-alone)           | 120 | q       | 1, 2, 3, 4                        |       |
 * |                                 |     | qo      | 1st, 2nd, 3rd, 4th                | 5     |
 * |                                 |     | qq      | 01, 02, 03, 04                    |       |
 * |                                 |     | qqq     | Q1, Q2, Q3, Q4                    |       |
 * |                                 |     | qqqq    | 1st quarter, 2nd quarter, ...     | 2     |
 * |                                 |     | qqqqq   | 1, 2, 3, 4                        | 3     |
 * | Month (formatting)              | 110 | M       | 1, 2, ..., 12                     |       |
 * |                                 |     | Mo      | 1st, 2nd, ..., 12th               | 5     |
 * |                                 |     | MM      | 01, 02, ..., 12                   |       |
 * |                                 |     | MMM     | Jan, Feb, ..., Dec                |       |
 * |                                 |     | MMMM    | January, February, ..., December  | 2     |
 * |                                 |     | MMMMM   | J, F, ..., D                      |       |
 * | Month (stand-alone)             | 110 | L       | 1, 2, ..., 12                     |       |
 * |                                 |     | Lo      | 1st, 2nd, ..., 12th               | 5     |
 * |                                 |     | LL      | 01, 02, ..., 12                   |       |
 * |                                 |     | LLL     | Jan, Feb, ..., Dec                |       |
 * |                                 |     | LLLL    | January, February, ..., December  | 2     |
 * |                                 |     | LLLLL   | J, F, ..., D                      |       |
 * | Local week of year              | 100 | w       | 1, 2, ..., 53                     |       |
 * |                                 |     | wo      | 1st, 2nd, ..., 53th               | 5     |
 * |                                 |     | ww      | 01, 02, ..., 53                   |       |
 * | ISO week of year                | 100 | I       | 1, 2, ..., 53                     | 5     |
 * |                                 |     | Io      | 1st, 2nd, ..., 53th               | 5     |
 * |                                 |     | II      | 01, 02, ..., 53                   | 5     |
 * | Day of month                    |  90 | d       | 1, 2, ..., 31                     |       |
 * |                                 |     | do      | 1st, 2nd, ..., 31st               | 5     |
 * |                                 |     | dd      | 01, 02, ..., 31                   |       |
 * | Day of year                     |  90 | D       | 1, 2, ..., 365, 366               | 7     |
 * |                                 |     | Do      | 1st, 2nd, ..., 365th, 366th       | 5     |
 * |                                 |     | DD      | 01, 02, ..., 365, 366             | 7     |
 * |                                 |     | DDD     | 001, 002, ..., 365, 366           |       |
 * |                                 |     | DDDD    | ...                               | 2     |
 * | Day of week (formatting)        |  90 | E..EEE  | Mon, Tue, Wed, ..., Sun           |       |
 * |                                 |     | EEEE    | Monday, Tuesday, ..., Sunday      | 2     |
 * |                                 |     | EEEEE   | M, T, W, T, F, S, S               |       |
 * |                                 |     | EEEEEE  | Mo, Tu, We, Th, Fr, Su, Sa        |       |
 * | ISO day of week (formatting)    |  90 | i       | 1, 2, 3, ..., 7                   | 5     |
 * |                                 |     | io      | 1st, 2nd, ..., 7th                | 5     |
 * |                                 |     | ii      | 01, 02, ..., 07                   | 5     |
 * |                                 |     | iii     | Mon, Tue, Wed, ..., Sun           | 5     |
 * |                                 |     | iiii    | Monday, Tuesday, ..., Sunday      | 2,5   |
 * |                                 |     | iiiii   | M, T, W, T, F, S, S               | 5     |
 * |                                 |     | iiiiii  | Mo, Tu, We, Th, Fr, Su, Sa        | 5     |
 * | Local day of week (formatting)  |  90 | e       | 2, 3, 4, ..., 1                   |       |
 * |                                 |     | eo      | 2nd, 3rd, ..., 1st                | 5     |
 * |                                 |     | ee      | 02, 03, ..., 01                   |       |
 * |                                 |     | eee     | Mon, Tue, Wed, ..., Sun           |       |
 * |                                 |     | eeee    | Monday, Tuesday, ..., Sunday      | 2     |
 * |                                 |     | eeeee   | M, T, W, T, F, S, S               |       |
 * |                                 |     | eeeeee  | Mo, Tu, We, Th, Fr, Su, Sa        |       |
 * | Local day of week (stand-alone) |  90 | c       | 2, 3, 4, ..., 1                   |       |
 * |                                 |     | co      | 2nd, 3rd, ..., 1st                | 5     |
 * |                                 |     | cc      | 02, 03, ..., 01                   |       |
 * |                                 |     | ccc     | Mon, Tue, Wed, ..., Sun           |       |
 * |                                 |     | cccc    | Monday, Tuesday, ..., Sunday      | 2     |
 * |                                 |     | ccccc   | M, T, W, T, F, S, S               |       |
 * |                                 |     | cccccc  | Mo, Tu, We, Th, Fr, Su, Sa        |       |
 * | AM, PM                          |  80 | a..aaa  | AM, PM                            |       |
 * |                                 |     | aaaa    | a.m., p.m.                        | 2     |
 * |                                 |     | aaaaa   | a, p                              |       |
 * | AM, PM, noon, midnight          |  80 | b..bbb  | AM, PM, noon, midnight            |       |
 * |                                 |     | bbbb    | a.m., p.m., noon, midnight        | 2     |
 * |                                 |     | bbbbb   | a, p, n, mi                       |       |
 * | Flexible day period             |  80 | B..BBB  | at night, in the morning, ...     |       |
 * |                                 |     | BBBB    | at night, in the morning, ...     | 2     |
 * |                                 |     | BBBBB   | at night, in the morning, ...     |       |
 * | Hour [1-12]                     |  70 | h       | 1, 2, ..., 11, 12                 |       |
 * |                                 |     | ho      | 1st, 2nd, ..., 11th, 12th         | 5     |
 * |                                 |     | hh      | 01, 02, ..., 11, 12               |       |
 * | Hour [0-23]                     |  70 | H       | 0, 1, 2, ..., 23                  |       |
 * |                                 |     | Ho      | 0th, 1st, 2nd, ..., 23rd          | 5     |
 * |                                 |     | HH      | 00, 01, 02, ..., 23               |       |
 * | Hour [0-11]                     |  70 | K       | 1, 2, ..., 11, 0                  |       |
 * |                                 |     | Ko      | 1st, 2nd, ..., 11th, 0th          | 5     |
 * |                                 |     | KK      | 01, 02, ..., 11, 00               |       |
 * | Hour [1-24]                     |  70 | k       | 24, 1, 2, ..., 23                 |       |
 * |                                 |     | ko      | 24th, 1st, 2nd, ..., 23rd         | 5     |
 * |                                 |     | kk      | 24, 01, 02, ..., 23               |       |
 * | Minute                          |  60 | m       | 0, 1, ..., 59                     |       |
 * |                                 |     | mo      | 0th, 1st, ..., 59th               | 5     |
 * |                                 |     | mm      | 00, 01, ..., 59                   |       |
 * | Second                          |  50 | s       | 0, 1, ..., 59                     |       |
 * |                                 |     | so      | 0th, 1st, ..., 59th               | 5     |
 * |                                 |     | ss      | 00, 01, ..., 59                   |       |
 * | Seconds timestamp               |  40 | t       | 512969520                         |       |
 * |                                 |     | tt      | ...                               | 2     |
 * | Fraction of second              |  30 | S       | 0, 1, ..., 9                      |       |
 * |                                 |     | SS      | 00, 01, ..., 99                   |       |
 * |                                 |     | SSS     | 000, 0001, ..., 999               |       |
 * |                                 |     | SSSS    | ...                               | 2     |
 * | Milliseconds timestamp          |  20 | T       | 512969520900                      |       |
 * |                                 |     | TT      | ...                               | 2     |
 * | Timezone (ISO-8601 w/ Z)        |  10 | X       | -08, +0530, Z                     |       |
 * |                                 |     | XX      | -0800, +0530, Z                   |       |
 * |                                 |     | XXX     | -08:00, +05:30, Z                 |       |
 * |                                 |     | XXXX    | -0800, +0530, Z, +123456          | 2     |
 * |                                 |     | XXXXX   | -08:00, +05:30, Z, +12:34:56      |       |
 * | Timezone (ISO-8601 w/o Z)       |  10 | x       | -08, +0530, +00                   |       |
 * |                                 |     | xx      | -0800, +0530, +0000               |       |
 * |                                 |     | xxx     | -08:00, +05:30, +00:00            | 2     |
 * |                                 |     | xxxx    | -0800, +0530, +0000, +123456      |       |
 * |                                 |     | xxxxx   | -08:00, +05:30, +00:00, +12:34:56 |       |
 * | Long localized date             |  NA | P       | 05/29/1453                        | 5,8   |
 * |                                 |     | PP      | May 29, 1453                      |       |
 * |                                 |     | PPP     | May 29th, 1453                    |       |
 * |                                 |     | PPPP    | Sunday, May 29th, 1453            | 2,5,8 |
 * | Long localized time             |  NA | p       | 12:00 AM                          | 5,8   |
 * |                                 |     | pp      | 12:00:00 AM                       |       |
 * | Combination of date and time    |  NA | Pp      | 05/29/1453, 12:00 AM              |       |
 * |                                 |     | PPpp    | May 29, 1453, 12:00:00 AM         |       |
 * |                                 |     | PPPpp   | May 29th, 1453 at ...             |       |
 * |                                 |     | PPPPpp  | Sunday, May 29th, 1453 at ...     | 2,5,8 |
 * Notes:
 * 1. "Formatting" units (e.g. formatting quarter) in the default en-US locale
 *    are the same as "stand-alone" units, but are different in some languages.
 *    "Formatting" units are declined according to the rules of the language
 *    in the context of a date. "Stand-alone" units are always nominative singular.
 *    In `format` function, they will produce different result:
 *
 *    `format(new Date(2017, 10, 6), 'do LLLL', {locale: cs}) //=> '6. listopad'`
 *
 *    `format(new Date(2017, 10, 6), 'do MMMM', {locale: cs}) //=> '6. listopadu'`
 *
 *    `parse` will try to match both formatting and stand-alone units interchangably.
 *
 * 2. Any sequence of the identical letters is a pattern, unless it is escaped by
 *    the single quote characters (see below).
 *    If the sequence is longer than listed in table:
 *    - for numerical units (`yyyyyyyy`) `parse` will try to match a number
 *      as wide as the sequence
 *    - for text units (`MMMMMMMM`) `parse` will try to match the widest variation of the unit.
 *      These variations are marked with "2" in the last column of the table.
 *
 * 3. `QQQQQ` and `qqqqq` could be not strictly numerical in some locales.
 *    These tokens represent the shortest form of the quarter.
 *
 * 4. The main difference between `y` and `u` patterns are B.C. years:
 *
 *    | Year | `y` | `u` |
 *    |------|-----|-----|
 *    | AC 1 |   1 |   1 |
 *    | BC 1 |   1 |   0 |
 *    | BC 2 |   2 |  -1 |
 *
 *    Also `yy` will try to guess the century of two digit year by proximity with `referenceDate`:
 *
 *    `parse('50', 'yy', new Date(2018, 0, 1)) //=> Sat Jan 01 2050 00:00:00`
 *
 *    `parse('75', 'yy', new Date(2018, 0, 1)) //=> Wed Jan 01 1975 00:00:00`
 *
 *    while `uu` will just assign the year as is:
 *
 *    `parse('50', 'uu', new Date(2018, 0, 1)) //=> Sat Jan 01 0050 00:00:00`
 *
 *    `parse('75', 'uu', new Date(2018, 0, 1)) //=> Tue Jan 01 0075 00:00:00`
 *
 *    The same difference is true for local and ISO week-numbering years (`Y` and `R`),
 *    except local week-numbering years are dependent on `options.weekStartsOn`
 *    and `options.firstWeekContainsDate` (compare [setISOWeekYear]{@link https://date-fns.org/docs/setISOWeekYear}
 *    and [setWeekYear]{@link https://date-fns.org/docs/setWeekYear}).
 *
 * 5. These patterns are not in the Unicode Technical Standard #35:
 *    - `i`: ISO day of week
 *    - `I`: ISO week of year
 *    - `R`: ISO week-numbering year
 *    - `o`: ordinal number modifier
 *    - `P`: long localized date
 *    - `p`: long localized time
 *
 * 6. `YY` and `YYYY` tokens represent week-numbering years but they are often confused with years.
 *    You should enable `options.useAdditionalWeekYearTokens` to use them. See: https://git.io/fxCyr
 *
 * 7. `D` and `DD` tokens represent days of the year but they are ofthen confused with days of the month.
 *    You should enable `options.useAdditionalDayOfYearTokens` to use them. See: https://git.io/fxCyr
 *
 * 8. `P+` tokens do not have a defined priority since they are merely aliases to other tokens based
 *    on the given locale.
 *
 *    using `en-US` locale: `P` => `MM/dd/yyyy`
 *    using `en-US` locale: `p` => `hh:mm a`
 *    using `pt-BR` locale: `P` => `dd/MM/yyyy`
 *    using `pt-BR` locale: `p` => `HH:mm`
 *
 * Values will be assigned to the date in the descending order of its unit's priority.
 * Units of an equal priority overwrite each other in the order of appearance.
 *
 * If no values of higher priority are parsed (e.g. when parsing string 'January 1st' without a year),
 * the values will be taken from 3rd argument `referenceDate` which works as a context of parsing.
 *
 * `referenceDate` must be passed for correct work of the function.
 * If you're not sure which `referenceDate` to supply, create a new instance of Date:
 * `parse('02/11/2014', 'MM/dd/yyyy', new Date())`
 * In this case parsing will be done in the context of the current date.
 * If `referenceDate` is `Invalid Date` or a value not convertible to valid `Date`,
 * then `Invalid Date` will be returned.
 *
 * The result may vary by locale.
 *
 * If `formatString` matches with `dateString` but does not provides tokens, `referenceDate` will be returned.
 *
 * If parsing failed, `Invalid Date` will be returned.
 * Invalid Date is a Date, whose time value is NaN.
 * Time value of Date: http://es5.github.io/#x15.9.1.1
 *
 * ### v2.0.0 breaking changes:
 *
 * - [Changes that are common for the whole library](https://github.com/date-fns/date-fns/blob/master/docs/upgradeGuide.md#Common-Changes).
 *
 * - Old `parse` was renamed to `toDate`.
 *   Now `parse` is a new function which parses a string using a provided format.
 *
 *   ```javascript
 *   // Before v2.0.0
 *   parse('2016-01-01')
 *
 *   // v2.0.0 onward (toDate no longer accepts a string)
 *   toDate(1392098430000) // Unix to timestamp
 *   toDate(new Date(2014, 1, 11, 11, 30, 30)) // Cloning the date
 *   parse('2016-01-01', 'yyyy-MM-dd', new Date())
 *   ```
 *
 * @param {String} dateString - the string to parse
 * @param {String} formatString - the string of tokens
 * @param {Date|Number} referenceDate - defines values missing from the parsed dateString
 * @param {Object} [options] - an object with options.
 * @param {Locale} [options.locale=defaultLocale] - the locale object. See [Locale]{@link https://date-fns.org/docs/Locale}
 * @param {0|1|2|3|4|5|6} [options.weekStartsOn=0] - the index of the first day of the week (0 - Sunday)
 * @param {1|2|3|4|5|6|7} [options.firstWeekContainsDate=1] - the day of January, which is always in the first week of the year
 * @param {Boolean} [options.useAdditionalWeekYearTokens=false] - if true, allows usage of the week-numbering year tokens `YY` and `YYYY`;
 *   see: https://git.io/fxCyr
 * @param {Boolean} [options.useAdditionalDayOfYearTokens=false] - if true, allows usage of the day of year tokens `D` and `DD`;
 *   see: https://git.io/fxCyr
 * @returns {Date} the parsed date
 * @throws {TypeError} 3 arguments required
 * @throws {RangeError} `options.weekStartsOn` must be between 0 and 6
 * @throws {RangeError} `options.firstWeekContainsDate` must be between 1 and 7
 * @throws {RangeError} `options.locale` must contain `match` property
 * @throws {RangeError} use `yyyy` instead of `YYYY` for formatting years using [format provided] to the input [input provided]; see: https://git.io/fxCyr
 * @throws {RangeError} use `yy` instead of `YY` for formatting years using [format provided] to the input [input provided]; see: https://git.io/fxCyr
 * @throws {RangeError} use `d` instead of `D` for formatting days of the month using [format provided] to the input [input provided]; see: https://git.io/fxCyr
 * @throws {RangeError} use `dd` instead of `DD` for formatting days of the month using [format provided] to the input [input provided]; see: https://git.io/fxCyr
 * @throws {RangeError} format string contains an unescaped latin alphabet character
 *
 * @example
 * // Parse 11 February 2014 from middle-endian format:
 * var result = parse('02/11/2014', 'MM/dd/yyyy', new Date())
 * //=> Tue Feb 11 2014 00:00:00
 *
 * @example
 * // Parse 28th of February in Esperanto locale in the context of 2010 year:
 * import eo from 'date-fns/locale/eo'
 * var result = parse('28-a de februaro', "do 'de' MMMM", new Date(2010, 0, 1), {
 *   locale: eo
 * })
 * //=> Sun Feb 28 2010 00:00:00
 */
export default function parse(
  dirtyDateString,
  dirtyFormatString,
  dirtyReferenceDate,
  dirtyOptions
) {
  requiredArgs(3, arguments)

  var dateString = String(dirtyDateString)
  var formatString = String(dirtyFormatString)
  var options = dirtyOptions || {}

  var locale = options.locale || defaultLocale

  if (!locale.match) {
    throw new RangeError('locale must contain match property')
  }

  var localeFirstWeekContainsDate =
    locale.options && locale.options.firstWeekContainsDate
  var defaultFirstWeekContainsDate =
    localeFirstWeekContainsDate == null
      ? 1
      : toInteger(localeFirstWeekContainsDate)
  var firstWeekContainsDate =
    options.firstWeekContainsDate == null
      ? defaultFirstWeekContainsDate
      : toInteger(options.firstWeekContainsDate)

  // Test if weekStartsOn is between 1 and 7 _and_ is not NaN
  if (!(firstWeekContainsDate >= 1 && firstWeekContainsDate <= 7)) {
    throw new RangeError(
      'firstWeekContainsDate must be between 1 and 7 inclusively'
    )
  }

  var localeWeekStartsOn = locale.options && locale.options.weekStartsOn
  var defaultWeekStartsOn =
    localeWeekStartsOn == null ? 0 : toInteger(localeWeekStartsOn)
  var weekStartsOn =
    options.weekStartsOn == null
      ? defaultWeekStartsOn
      : toInteger(options.weekStartsOn)

  // Test if weekStartsOn is between 0 and 6 _and_ is not NaN
  if (!(weekStartsOn >= 0 && weekStartsOn <= 6)) {
    throw new RangeError('weekStartsOn must be between 0 and 6 inclusively')
  }

  if (formatString === '') {
    if (dateString === '') {
      return toDate(dirtyReferenceDate)
    } else {
      return new Date(NaN)
    }
  }

  var subFnOptions = {
    firstWeekContainsDate: firstWeekContainsDate,
    weekStartsOn: weekStartsOn,
    locale: locale
  }

  // If timezone isn't specified, it will be set to the system timezone
  var setters = [
    {
      priority: TIMEZONE_UNIT_PRIORITY,
      subPriority: -1,
      set: dateToSystemTimezone,
      index: 0
    }
  ]

  var i

  var tokens = formatString
    .match(longFormattingTokensRegExp)
    .map(function(substring) {
      var firstCharacter = substring[0]
      if (firstCharacter === 'p' || firstCharacter === 'P') {
        var longFormatter = longFormatters[firstCharacter]
        return longFormatter(substring, locale.formatLong, subFnOptions)
      }
      return substring
    })
    .join('')
    .match(formattingTokensRegExp)

  const usedTokens = []

  for (i = 0; i < tokens.length; i++) {
    var token = tokens[i]

    if (
      !options.useAdditionalWeekYearTokens &&
      isProtectedWeekYearToken(token)
    ) {
      throwProtectedError(token, formatString, dirtyDateString)
    }
    if (
      !options.useAdditionalDayOfYearTokens &&
      isProtectedDayOfYearToken(token)
    ) {
      throwProtectedError(token, formatString, dirtyDateString)
    }

    var firstCharacter = token[0]
    var parser = parsers[firstCharacter]
    if (parser) {
      const { incompatibleTokens } = parser
      if (Array.isArray(incompatibleTokens)) {
        let incompatibleToken
        for (let i = 0; i < usedTokens.length; i++) {
          const usedToken = usedTokens[i].token
          if (
            incompatibleTokens.indexOf(usedToken) !== -1 ||
            usedToken === firstCharacter
          ) {
            incompatibleToken = usedTokens[i]
            break
          }
        }
        if (incompatibleToken) {
          throw new RangeError(
            `The format string mustn't contain \`${incompatibleToken.fullToken}\` and \`${token}\` at the same time`
          )
        }
      } else if (parser.incompatibleTokens === '*' && usedTokens.length) {
        throw new RangeError(
          `The format string mustn't contain \`${token}\` and any other token at the same time`
        )
      }

      usedTokens.push({ token: firstCharacter, fullToken: token })

      var parseResult = parser.parse(
        dateString,
        token,
        locale.match,
        subFnOptions
      )

      if (!parseResult) {
        return new Date(NaN)
      }

      setters.push({
        priority: parser.priority,
        subPriority: parser.subPriority || 0,
        set: parser.set,
        validate: parser.validate,
        value: parseResult.value,
        index: setters.length
      })

      dateString = parseResult.rest
    } else {
      if (firstCharacter.match(unescapedLatinCharacterRegExp)) {
        throw new RangeError(
          'Format string contains an unescaped latin alphabet character `' +
            firstCharacter +
            '`'
        )
      }

      // Replace two single quote characters with one single quote character
      if (token === "''") {
        token = "'"
      } else if (firstCharacter === "'") {
        token = cleanEscapedString(token)
      }

      // Cut token from string, or, if string doesn't match the token, return Invalid Date
      if (dateString.indexOf(token) === 0) {
        dateString = dateString.slice(token.length)
      } else {
        return new Date(NaN)
      }
    }
  }

  // Check if the remaining input contains something other than whitespace
  if (dateString.length > 0 && notWhitespaceRegExp.test(dateString)) {
    return new Date(NaN)
  }

  var uniquePrioritySetters = setters
    .map(function(setter) {
      return setter.priority
    })
    .sort(function(a, b) {
      return b - a
    })
    .filter(function(priority, index, array) {
      return array.indexOf(priority) === index
    })
    .map(function(priority) {
      return setters
        .filter(function(setter) {
          return setter.priority === priority
        })
        .sort(function(a, b) {
          return b.subPriority - a.subPriority
        })
    })
    .map(function(setterArray) {
      return setterArray[0]
    })

  var date = toDate(dirtyReferenceDate)

  if (isNaN(date)) {
    return new Date(NaN)
  }

  // Convert the date in system timezone to the same date in UTC+00:00 timezone.
  // This ensures that when UTC functions will be implemented, locales will be compatible with them.
  // See an issue about UTC functions: https://github.com/date-fns/date-fns/issues/37
  var utcDate = subMilliseconds(date, getTimezoneOffsetInMilliseconds(date))

  var flags = {}
  for (i = 0; i < uniquePrioritySetters.length; i++) {
    var setter = uniquePrioritySetters[i]

    if (
      setter.validate &&
      !setter.validate(utcDate, setter.value, subFnOptions)
    ) {
      return new Date(NaN)
    }

    var result = setter.set(utcDate, flags, setter.value, subFnOptions)
    // Result is tuple (date, flags)
    if (result[0]) {
      utcDate = result[0]
      assign(flags, result[1])
      // Result is date
    } else {
      utcDate = result
    }
  }

  return utcDate
}

function dateToSystemTimezone(date, flags) {
  if (flags.timestampIsSet) {
    return date
  }

  var convertedDate = new Date(0)
  convertedDate.setFullYear(
    date.getUTCFullYear(),
    date.getUTCMonth(),
    date.getUTCDate()
  )
  convertedDate.setHours(
    date.getUTCHours(),
    date.getUTCMinutes(),
    date.getUTCSeconds(),
    date.getUTCMilliseconds()
  )
  return convertedDate
}

function cleanEscapedString(input) {
  return input.match(escapedStringRegExp)[1].replace(doubleQuoteRegExp, "'")
}
