// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

// copied from dart:io

/// Utility functions for working with dates with HTTP specific date
/// formats.
class HttpDate {
  // From RFC-2616 section "3.3.1 Full Date",
  // http://tools.ietf.org/html/rfc2616#section-3.3.1
  //
  // HTTP-date    = rfc1123-date | rfc850-date | asctime-date
  // rfc1123-date = wkday "," SP date1 SP time SP "GMT"
  // rfc850-date  = weekday "," SP date2 SP time SP "GMT"
  // asctime-date = wkday SP date3 SP time SP 4DIGIT
  // date1        = 2DIGIT SP month SP 4DIGIT
  //                ; day month year (e.g., 02 Jun 1982)
  // date2        = 2DIGIT "-" month "-" 2DIGIT
  //                ; day-month-year (e.g., 02-Jun-82)
  // date3        = month SP ( 2DIGIT | ( SP 1DIGIT ))
  //                ; month day (e.g., Jun  2)
  // time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT
  //                ; 00:00:00 - 23:59:59
  // wkday        = "Mon" | "Tue" | "Wed"
  //              | "Thu" | "Fri" | "Sat" | "Sun"
  // weekday      = "Monday" | "Tuesday" | "Wednesday"
  //              | "Thursday" | "Friday" | "Saturday" | "Sunday"
  // month        = "Jan" | "Feb" | "Mar" | "Apr"
  //              | "May" | "Jun" | "Jul" | "Aug"
  //              | "Sep" | "Oct" | "Nov" | "Dec"

  /// Format a date according to
  /// [RFC-1123](http://tools.ietf.org/html/rfc1123 "RFC-1123"),
  /// e.g. `Thu, 1 Jan 1970 00:00:00 GMT`.
  static String format(DateTime date) {
    const List wkday = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"];
    const List month = [
      "Jan",
      "Feb",
      "Mar",
      "Apr",
      "May",
      "Jun",
      "Jul",
      "Aug",
      "Sep",
      "Oct",
      "Nov",
      "Dec",
    ];

    DateTime d = date.toUtc();
    StringBuffer sb = StringBuffer()
      ..write(wkday[d.weekday - 1])
      ..write(", ")
      ..write(d.day <= 9 ? "0" : "")
      ..write(d.day.toString())
      ..write(" ")
      ..write(month[d.month - 1])
      ..write(" ")
      ..write(d.year.toString())
      ..write(d.hour <= 9 ? " 0" : " ")
      ..write(d.hour.toString())
      ..write(d.minute <= 9 ? ":0" : ":")
      ..write(d.minute.toString())
      ..write(d.second <= 9 ? ":0" : ":")
      ..write(d.second.toString())
      ..write(" GMT");
    return sb.toString();
  }

  /// Parse a date string in either of the formats
  /// [RFC-1123](http://tools.ietf.org/html/rfc1123 "RFC-1123"),
  /// [RFC-850](http://tools.ietf.org/html/rfc850 "RFC-850") or
  /// ANSI C's asctime() format. These formats are listed here.
  ///
  ///     Thu, 1 Jan 1970 00:00:00 GMT
  ///     Thursday, 1-Jan-1970 00:00:00 GMT
  ///     Thu Jan  1 00:00:00 1970
  ///
  /// For more information see [RFC-2616 section
  /// 3.1.1](http://tools.ietf.org/html/rfc2616#section-3.3.1
  /// "RFC-2616 section 3.1.1").
  static DateTime parse(String date) {
    final int sp = 32;
    const List wkdays = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"];
    const List weekdays = [
      "Monday",
      "Tuesday",
      "Wednesday",
      "Thursday",
      "Friday",
      "Saturday",
      "Sunday",
    ];
    const List months = [
      "Jan",
      "Feb",
      "Mar",
      "Apr",
      "May",
      "Jun",
      "Jul",
      "Aug",
      "Sep",
      "Oct",
      "Nov",
      "Dec",
    ];

    final int formatRfc1123 = 0;
    final int formatRfc850 = 1;
    final int formatAsctime = 2;

    int index = 0;
    String tmp;

    void expect(String s) {
      if (date.length - index < s.length) {
        throw Exception("Invalid HTTP date $date");
      }
      String tmp = date.substring(index, index + s.length);
      if (tmp != s) {
        throw Exception("Invalid HTTP date $date");
      }
      index += s.length;
    }

    int expectWeekday() {
      int weekday;
      // The formatting of the weekday signals the format of the date string.
      int pos = date.indexOf(",", index);
      if (pos == -1) {
        int pos = date.indexOf(" ", index);
        if (pos == -1) throw Exception("Invalid HTTP date $date");
        tmp = date.substring(index, pos);
        index = pos + 1;
        weekday = wkdays.indexOf(tmp);
        if (weekday != -1) {
          return formatAsctime;
        }
      } else {
        tmp = date.substring(index, pos);
        index = pos + 1;
        weekday = wkdays.indexOf(tmp);
        if (weekday != -1) {
          return formatRfc1123;
        }
        weekday = weekdays.indexOf(tmp);
        if (weekday != -1) {
          return formatRfc850;
        }
      }
      throw Exception("Invalid HTTP date $date");
    }

    int expectMonth(String separator) {
      int pos = date.indexOf(separator, index);
      if (pos - index != 3) throw Exception("Invalid HTTP date $date");
      tmp = date.substring(index, pos);
      index = pos + 1;
      int month = months.indexOf(tmp);
      if (month != -1) return month;
      throw Exception("Invalid HTTP date $date");
    }

    int expectNum(String separator) {
      int pos;
      if (separator.isNotEmpty) {
        pos = date.indexOf(separator, index);
      } else {
        pos = date.length;
      }
      String tmp = date.substring(index, pos);
      index = pos + separator.length;
      try {
        int value = int.parse(tmp);
        return value;
      } on FormatException {
        throw Exception("Invalid HTTP date $date");
      }
    }

    void expectEnd() {
      if (index != date.length) {
        throw Exception("Invalid HTTP date $date");
      }
    }

    int format = expectWeekday();
    int year;
    int month;
    int day;
    int hours;
    int minutes;
    int seconds;
    if (format == formatAsctime) {
      month = expectMonth(" ");
      if (date.codeUnitAt(index) == sp) index++;
      day = expectNum(" ");
      hours = expectNum(":");
      minutes = expectNum(":");
      seconds = expectNum(" ");
      year = expectNum("");
    } else {
      expect(" ");
      day = expectNum(format == formatRfc1123 ? " " : "-");
      month = expectMonth(format == formatRfc1123 ? " " : "-");
      year = expectNum(" ");
      hours = expectNum(":");
      minutes = expectNum(":");
      seconds = expectNum(" ");
      expect("GMT");
    }
    expectEnd();
    return DateTime.utc(year, month + 1, day, hours, minutes, seconds, 0);
  }
}