/**
 * DateTimeOffset functions.
 *
 * Note: Date instances are always DateObjects in local
 * timezone (because JS dates are all kinds of messed up).
 * A local date returns UTC epoch when `.getTime()` is called.
 *
 * Basically; invariant: date.getTime() always return UTC time.
 */

import { int64, toInt64, toFloat64 } from "./BigInt.ts";
import { FSharpRef } from "./Types.ts";
import { Exception, compareDates, DateTimeKind, dateOffset, IDateTime, IDateTimeOffset, padWithZeros } from "./Util.ts";

export type OffsetInMinutes = number;
export type Offset = "Z" | OffsetInMinutes | null;

const shortDays =
  [
    "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"
  ]

const longDays =
  [
    "Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"
  ]

const shortMonths =
  [
    "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"
  ]

const longMonths =
  [
    "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"
  ]


function parseRepeatToken(format: string, pos: number, patternChar: string) {
  let tokenLength = 0;
  let internalPos = pos;
  while (internalPos < format.length && format[internalPos] === patternChar) {
    internalPos++;
    tokenLength++;
  }

  return tokenLength;
}

function parseNextChar(format: string, pos: number) {
  if (pos >= format.length - 1) {
    return -1
  }

  return format.charCodeAt(pos + 1);
}

function parseQuotedString(format: string, pos: number): [string, number] {
  let beginPos = pos;
  // Get the character used to quote the string
  const quoteChar = format[pos];

  let result = "";
  let foundQuote = false;

  while (pos < format.length) {
    pos++;
    const currentChar = format[pos];
    if (currentChar === quoteChar) {
      foundQuote = true;
      break;
    } else if (currentChar === "\\") {
      if (pos < format.length) {
        pos++;
        result += format[pos];
      } else {
        // This means that '\' is the last character in the string.
        throw new Exception("Invalid string format");
      }
    } else {
      result += currentChar;
    }
  }

  if (!foundQuote) {
    // We could not find the matching quote
    throw new Exception(`Invalid string format could not find matching quote for ${quoteChar}`);
  }

  return [result, pos - beginPos + 1];
}

function dateToStringWithCustomFormat(date: Date, format: string, utc: boolean) {
  let cursorPos = 0;
  let tokenLength = 0;
  let result = "";
  const localizedDate = utc ? DateTime(date.getTime(), DateTimeKind.Utc) : date;

  while (cursorPos < format.length) {
    const token = format[cursorPos];

    switch (token) {
      case "d":
        tokenLength = parseRepeatToken(format, cursorPos, "d");
        cursorPos += tokenLength;
        switch (tokenLength) {
          case 1:
            result += day(localizedDate);
            break;
          case 2:
            result += padWithZeros(day(localizedDate), 2);
            break;
          case 3:
            result += shortDays[dayOfWeek(localizedDate)];
            break;
          case 4:
          default:
            result += longDays[dayOfWeek(localizedDate)];
            break;
        }
        break;
      case "f":
        tokenLength = parseRepeatToken(format, cursorPos, "f");
        cursorPos += tokenLength;
        if (tokenLength <= 3) {
          const precision = 10 ** (3 - tokenLength);
          result += padWithZeros(Math.floor(millisecond(localizedDate) / precision), tokenLength);
        } else if (tokenLength <= 7) {
          // JavaScript Date only support precision to the millisecond
          // so we fill the rest of the precision with 0 as if the date didn't have
          // milliseconds provided to it.
          // This is to have the same behavior as .NET when doing:
          // DateTime(1, 2, 3, 4, 5, 6, DateTimeKind.Utc).ToString("fffff") => 00000
          result += ("" + millisecond(localizedDate)).padEnd(tokenLength, "0");
        } else {
          throw "Input string was not in a correct format.";
        }
        break;
      case "F":
        tokenLength = parseRepeatToken(format, cursorPos, "F");
        cursorPos += tokenLength;
        if (tokenLength <= 3) {
          const precision = 10 ** (3 - tokenLength);
          const value = Math.floor(millisecond(localizedDate) / precision)
          if (value != 0) {
            result += padWithZeros(value, tokenLength);
          }
        } else if (tokenLength <= 7) {
          // JavaScript Date only support precision to the millisecond
          // so we can't go beyond that.
          // We also need to pad start with 0 if the value is not 0
          const value = millisecond(localizedDate);
          if (value != 0) {
            result += padWithZeros(value, 3);
          }
        } else {
          throw "Input string was not in a correct format.";
        }
        break;
      case "g":
        tokenLength = parseRepeatToken(format, cursorPos, "g");
        cursorPos += tokenLength;
        result += "A.D.";
        break;
      case "h":
        tokenLength = parseRepeatToken(format, cursorPos, "h");
        cursorPos += tokenLength;
        switch (tokenLength) {
          case 1:
            const h1Value = hour(localizedDate) % 12;
            result += h1Value ? h1Value : 12;
            break;
          case 2:
          default:
            const h2Value = hour(localizedDate) % 12;
            result += padWithZeros(h2Value ? h2Value : 12, 2);
            break;
        }
        break;
      case "H":
        tokenLength = parseRepeatToken(format, cursorPos, "H");
        cursorPos += tokenLength;
        switch (tokenLength) {
          case 1:
            result += hour(localizedDate);
            break;
          case 2:
          default:
            result += padWithZeros(hour(localizedDate), 2);
            break;
        }
        break;
      case "K":
        tokenLength = parseRepeatToken(format, cursorPos, "K");
        cursorPos += tokenLength;
        switch (tokenLength) {
          case 1:
            switch (getKind(localizedDate)) {
              case DateTimeKind.Utc:
                result += "Z";
                break;
              case DateTimeKind.Local:
                result += dateOffsetToString(localizedDate.getTimezoneOffset() * -60_000);
                break;
              case DateTimeKind.Unspecified:
                break;
            }
            break;
          default:
            break;
        }
        break;
      case "m":
        tokenLength = parseRepeatToken(format, cursorPos, "m");
        cursorPos += tokenLength;
        switch (tokenLength) {
          case 1:
            result += minute(localizedDate);
            break;
          case 2:
          default:
            result += padWithZeros(minute(localizedDate), 2);
            break;
        }
        break;
      case "M":
        tokenLength = parseRepeatToken(format, cursorPos, "M");
        cursorPos += tokenLength;
        switch (tokenLength) {
          case 1:
            result += month(localizedDate);
            break;
          case 2:
            result += padWithZeros(month(localizedDate), 2);
            break;
          case 3:
            result += shortMonths[month(localizedDate) - 1];
            break;
          case 4:
          default:
            result += longMonths[month(localizedDate) - 1];
            break;
        }
        break;
      case "s":
        tokenLength = parseRepeatToken(format, cursorPos, "s");
        cursorPos += tokenLength;
        switch (tokenLength) {
          case 1:
            result += second(localizedDate);
            break;
          case 2:
          default:
            result += padWithZeros(second(localizedDate), 2);
            break;
        }
        break;
      case "t":
        tokenLength = parseRepeatToken(format, cursorPos, "t");
        cursorPos += tokenLength;
        switch (tokenLength) {
          case 1:
            result += localizedDate.getHours() < 12 ? "A" : "P";
            break;
          case 2:
          default:
            result += localizedDate.getHours() < 12 ? "AM" : "PM";
            break;
        }
        break;
      case "y":
        tokenLength = parseRepeatToken(format, cursorPos, "y");
        cursorPos += tokenLength;
        switch (tokenLength) {
          case 1:
            result += year(localizedDate) % 100;
            break;
          case 2:
            result += padWithZeros(year(localizedDate) % 100, 2);
            break;
          default:
            result += padWithZeros(year(localizedDate), tokenLength);
            break;
        }
        break;
      case "z":
        tokenLength = parseRepeatToken(format, cursorPos, "z");
        cursorPos += tokenLength;
        let utcOffsetText = "";

        switch (getKind(localizedDate)) {
          case DateTimeKind.Utc:
            utcOffsetText = "+00:00";
            break;
          case DateTimeKind.Local:
            utcOffsetText = dateOffsetToString(localizedDate.getTimezoneOffset() * -60_000);
            break;
          case DateTimeKind.Unspecified:
            utcOffsetText = dateOffsetToString(toLocalTime(localizedDate).getTimezoneOffset() * -60_000);
            break;
        }

        const sign = utcOffsetText[0] === "-" ? "-" : "+";
        const hours = parseInt(utcOffsetText.substring(1, 3), 10);
        const minutes = parseInt(utcOffsetText.substring(4, 6), 10);

        switch (tokenLength) {
          case 1:
            result += `${sign}${hours}`;
            break;
          case 2:
            result += `${sign}${padWithZeros(hours, 2)}`;
            break;
          default:
            result += `${sign}${padWithZeros(hours, 2)}:${padWithZeros(minutes, 2)}`;
            break;
        }
        break;
      case ":":
        result += ":";
        cursorPos++;
        break;
      case "/":
        result += "/";
        cursorPos++;
        break;
      case "'":
      case '"':
        const [quotedString, quotedStringLenght] = parseQuotedString(format, cursorPos);
        result += quotedString;
        cursorPos += quotedStringLenght;
        break;
      case "%":
        const nextChar = parseNextChar(format, cursorPos);
        if (nextChar >= 0 && nextChar !== "%".charCodeAt(0)) {
          cursorPos += 2;
          result += dateToStringWithCustomFormat(localizedDate, String.fromCharCode(nextChar), utc);
        } else {
          throw new Exception("Invalid format string");
        }
        break;
      case "\\":
        const nextChar2 = parseNextChar(format, cursorPos);
        if (nextChar2 >= 0) {
          cursorPos += 2;
          result += String.fromCharCode(nextChar2);
        } else {
          throw new Exception("Invalid format string");
        }
        break;
      default:
        cursorPos++;
        result += token;
        break;
    }
  }

  return result;
}

export function getKind(value: IDateTime): DateTimeKind {
  return value.kind ?? DateTimeKind.Unspecified;
}

export function unixEpochMillisecondsToTicks(ms: number, offset: number): int64 {
  return toInt64(((BigInt(ms) + 62135596800000n) + BigInt(offset)) * 10000n);
}

export function ticksToUnixEpochMilliseconds(ticks: number | bigint): number {
  return Number(((BigInt(ticks) / 10000n) - 62135596800000n));
}

export function dateOffsetToString(offset: number) {
  const isMinus = offset < 0;
  offset = Math.abs(offset);
  const hours = ~~(offset / 3600000);
  const minutes = (offset % 3600000) / 60_000;
  return (isMinus ? "-" : "+") +
    padWithZeros(hours, 2) + ":" +
    padWithZeros(minutes, 2);
}

function dateToISOString(d: IDateTime, utc: boolean) {
  if (utc) {
    return d.toISOString();
  } else {
    // JS Date is always local
    const printOffset = d.kind == null ? true : d.kind === DateTimeKind.Local;
    return padWithZeros(d.getFullYear(), 4) + "-" +
      padWithZeros(d.getMonth() + 1, 2) + "-" +
      padWithZeros(d.getDate(), 2) + "T" +
      padWithZeros(d.getHours(), 2) + ":" +
      padWithZeros(d.getMinutes(), 2) + ":" +
      padWithZeros(d.getSeconds(), 2) + "." +
      padWithZeros(d.getMilliseconds(), 3) +
      (printOffset ? dateOffsetToString(d.getTimezoneOffset() * -60_000) : "");
  }
}

function dateToISOStringWithOffset(dateWithOffset: Date, offset: number) {
  const str = dateWithOffset.toISOString();
  return str.substring(0, str.length - 1) + dateOffsetToString(offset);
}

function dateToStringWithOffset(date: IDateTimeOffset, format?: string) {
  const d = new Date(date.getTime() + (date.offset ?? 0));
  if (typeof format !== "string") {
    return d.toISOString().replace(/\.\d+/, "").replace(/[A-Z]|\.\d+/g, " ") + dateOffsetToString((date.offset ?? 0));
  } else if (format.length === 1) {
    switch (format) {
      case "D": return dateToString_D(d);
      case "d": return dateToString_d(d);
      case "F": return dateToString_D(d) + " " + dateToString_T(d);
      case "f": return dateToString_D(d) + " " + dateToString_t(d);
      case "G": return dateToString_d(d) + " " + dateToString_T(d);
      case "g": return dateToString_d(d) + " " + dateToString_t(d);
      case "M": case "m": return dateToString_M(d);
      case "O": case "o": return dateToISOStringWithOffset(d, (date.offset ?? 0));
      case "R": case "r": {
        const utcDate = DateTime(date.getTime(), DateTimeKind.Utc);
        return dateToString_R(utcDate);
      }
      case "s": return dateToString_s(toUniversalTime(d));
      case "T": return dateToString_T(toUniversalTime(d));
      case "t": return dateToString_t(toUniversalTime(d));
      case "u": {
        const utcDate = DateTime(date.getTime(), DateTimeKind.Utc);
        return dateToString_u(utcDate);
      }
      case "U": {
        const utcDate = DateTime(date.getTime(), DateTimeKind.Utc);
        return dateToString_D(utcDate) + " " + dateToString_T(utcDate);
      }
      case "Y": case "y": return dateToString_Y(d);
      default: throw new Exception("Unrecognized Date print format");
    }
  } else {
    return dateToStringWithCustomFormat(d, format, true);
  }
}

function dateToString_D(date: IDateTime) {
  return longDays[dayOfWeek(date)]
    + ", " + padWithZeros(day(date), 2)
    + " " + longMonths[month(date) - 1]
    + " " + year(date);
}

function dateToString_d(date: IDateTime) {
  return padWithZeros(month(date), 2)
    + "/" + padWithZeros(day(date), 2)
    + "/" + year(date);
}

function dateToString_T(date: IDateTime) {
  return padWithZeros(hour(date), 2)
    + ":" + padWithZeros(minute(date), 2)
    + ":" + padWithZeros(second(date), 2);
}

function dateToString_t(date: IDateTime) {
  return padWithZeros(hour(date), 2)
    + ":" + padWithZeros(minute(date), 2);
}

// RFC 1123: "Thu, 01 Jan 2009 00:00:00 GMT" — always UTC
function dateToString_R(date: IDateTime) {
  const utcDate = toUniversalTime(date);
  return shortDays[dayOfWeek(utcDate)] + ", "
    + padWithZeros(day(utcDate), 2) + " "
    + shortMonths[month(utcDate) - 1] + " "
    + year(utcDate) + " "
    + padWithZeros(hour(utcDate), 2) + ":"
    + padWithZeros(minute(utcDate), 2) + ":"
    + padWithZeros(second(utcDate), 2) + " GMT";
}

// Sortable ISO 8601, no timezone: "2009-06-15T13:45:30"
function dateToString_s(date: IDateTime) {
  return padWithZeros(year(date), 4) + "-"
    + padWithZeros(month(date), 2) + "-"
    + padWithZeros(day(date), 2) + "T"
    + padWithZeros(hour(date), 2) + ":"
    + padWithZeros(minute(date), 2) + ":"
    + padWithZeros(second(date), 2);
}

// Universal sortable: "2009-06-15 13:45:30Z" — always UTC
function dateToString_u(date: IDateTime) {
  const utcDate = toUniversalTime(date);
  return padWithZeros(year(utcDate), 4) + "-"
    + padWithZeros(month(utcDate), 2) + "-"
    + padWithZeros(day(utcDate), 2) + " "
    + padWithZeros(hour(utcDate), 2) + ":"
    + padWithZeros(minute(utcDate), 2) + ":"
    + padWithZeros(second(utcDate), 2) + "Z";
}

// Month/day (InvariantCulture "MMMM dd"): "June 15"
function dateToString_M(date: IDateTime) {
  return longMonths[month(date) - 1] + " " + padWithZeros(day(date), 2);
}

// Year/month (InvariantCulture "yyyy MMMM"): "2009 June"
function dateToString_Y(date: IDateTime) {
  return year(date) + " " + longMonths[month(date) - 1];
}

function dateToStringWithKind(date: IDateTime, format?: string) {
  const utc = date.kind === DateTimeKind.Utc;
  if (typeof format !== "string") {
    return utc ? date.toUTCString() : date.toLocaleString();
  } else if (format.length === 1) {
    switch (format) {
      case "D": return dateToString_D(date);
      case "d": return dateToString_d(date);
      case "F": return dateToString_D(date) + " " + dateToString_T(date);
      case "f": return dateToString_D(date) + " " + dateToString_t(date);
      case "G": return dateToString_d(date) + " " + dateToString_T(date);
      case "g": return dateToString_d(date) + " " + dateToString_t(date);
      case "M": case "m": return dateToString_M(date);
      case "O": case "o": return dateToISOString(date, utc);
      case "R": case "r": return dateToString_R(date);
      case "s": return dateToString_s(date);
      case "T": return dateToString_T(date);
      case "t": return dateToString_t(date);
      case "u": return dateToString_u(date);
      case "U": return dateToString_D(toUniversalTime(date)) + " " + dateToString_T(toUniversalTime(date));
      case "Y": case "y": return dateToString_Y(date);
      default:
        throw new Exception("Unrecognized Date print format");
    }
  } else {
    return dateToStringWithCustomFormat(date, format, utc);
  }
}

export function toString(date: IDateTime | IDateTimeOffset, format?: string, _provider?: any) {
  return (date as IDateTimeOffset).offset != null
    ? dateToStringWithOffset(date, format)
    : dateToStringWithKind(date, format);
}

export function DateTime(value: number, kind?: DateTimeKind) {
  const d = new Date(value) as IDateTime;
  d.kind = (kind == null ? DateTimeKind.Unspecified : kind);
  return d;
}

export function fromTicks(ticks: number | bigint, kind?: DateTimeKind) {
  kind = kind != null ? kind : DateTimeKind.Local; // better default than Unspecified
  let date = DateTime(ticksToUnixEpochMilliseconds(ticks), kind);

  // Ticks are local to offset (in this case, either UTC or Local/Unknown).
  // If kind is anything but UTC, that means that the tick number was not
  // in utc, thus getTime() cannot return UTC, and needs to be shifted.
  if (kind !== DateTimeKind.Utc) {
    date = DateTime(date.getTime() - dateOffset(date), kind);
  }

  return date;
}

export function fromDateTime(dateOnly: IDateTime, timeOnly: number, kind?: DateTimeKind) {
  const d = DateTime(dateOnly.getTime() + timeOnly, kind);
  return DateTime(d.getTime() - dateOffset(d), kind);
}

export function fromDateTimeOffset(date: IDateTimeOffset, kind: DateTimeKind) {
  switch (kind) {
    case DateTimeKind.Utc: return DateTime(date.getTime(), DateTimeKind.Utc);
    case DateTimeKind.Local: return DateTime(date.getTime(), DateTimeKind.Local);
    default:
      const d = DateTime(date.getTime() + (date.offset ?? 0), kind);
      return DateTime(d.getTime() - dateOffset(d), kind);
  }
}

export function getTicks(date: IDateTime | IDateTimeOffset) {
  return unixEpochMillisecondsToTicks(date.getTime(), dateOffset(date));
}

export function minValue() {
  // This is "0001-01-01T00:00:00.000Z", actual JS min value is -8640000000000000
  return DateTime(-62135596800000, DateTimeKind.Utc);
}

export function maxValue() {
  // This is "9999-12-31T23:59:59.999Z", actual JS max value is 8640000000000000
  return DateTime(253402300799999, DateTimeKind.Utc);
}

// The only date words .NET's invariant parser recognises: month names, weekday names,
// meridiem designators and zone markers (plus the ISO "T" separator). Anything else is
// rejected. Used to reject JS-permissive inputs (see `parseRaw`).
const recognizedDateWords = new Set([
  "january", "february", "march", "april", "may", "june",
  "july", "august", "september", "october", "november", "december",
  "jan", "feb", "mar", "apr", "jun", "jul", "aug", "sep", "sept", "oct", "nov", "dec",
  "monday", "tuesday", "wednesday", "thursday", "friday", "saturday", "sunday",
  "mon", "tue", "wed", "thu", "fri", "sat", "sun",
  "am", "pm", "gmt", "utc", "ut", "t", "z",
]);

export function parseRaw(input: string): [Date, Offset] {
  function fail() {
    throw new Exception(`The string is not a valid Date: ${input}`);
  }

  if (input == null || input.trim() === "") {
    fail();
  }

  if ((input.match(/[a-z]+/gi) ?? []).some(word => !recognizedDateWords.has(word.toLowerCase()))) {
    fail();
  }

  // ISO dates without TZ are parsed as UTC. Adding time without TZ keeps them local.
  if (input.length === 10 && input[4] === "-" && input[7] === "-") {
    input += "T00:00:00";
  }
  let date = new Date(input);
  let offset: Offset = null;

  if (isNaN(date.getTime())) {
    // Try to check strings JS Date cannot parse (see #1045, #1422)
    const m = /^\s*(\d+[^\w\s:]\d+[^\w\s:]\d+)?\s*(\d+:\d+(?::\d+(?:\.\d+)?)?)?\s*([AaPp][Mm])?\s*(Z|[+-]([01]?\d):?([0-5]?\d)?)?\s*$/.exec(input);
    if (m != null) {
      let baseDate: Date;
      let timeInSeconds = 0;
      if (m[2] != null) {
        const timeParts = m[2].split(":");
        const hourPart = parseInt(timeParts[0], 10);
        timeInSeconds =
          hourPart * 3600 +
          parseInt(timeParts[1] || "0", 10) * 60 +
          parseFloat(timeParts[2] || "0");
        if (m[3] != null && m[3].toUpperCase() === "PM" && hourPart < 12) {
          timeInSeconds += 720;
        }
      }
      if (m[4] != null) { // There's an offset, parse as UTC
        if (m[1] != null) {
          baseDate = new Date(m[1] + " UTC");
        } else {
          const d = new Date();
          baseDate = new Date(d.getUTCFullYear() + "/" + (d.getUTCMonth() + 1) + "/" + d.getUTCDate());
        }
        if (m[4] === "Z") {
          offset = "Z";
        } else {
          let offsetInMinutes = parseInt(m[5], 10) * 60 + parseInt(m[6] || "0", 10);
          if (m[4][0] === "-") {
            offsetInMinutes *= -1;
          }
          offset = offsetInMinutes;
          timeInSeconds -= offsetInMinutes * 60;
        }
      } else {
        if (m[1] != null) {
          baseDate = new Date(m[1]);
        } else {
          const d = new Date();
          baseDate = new Date(d.getFullYear() + "/" + (d.getMonth() + 1) + "/" + d.getDate());
        }
      }
      date = new Date(baseDate.getTime() + timeInSeconds * 1000);
      // correct for daylight savings time
      date = new Date(date.getTime() + (date.getTimezoneOffset() - baseDate.getTimezoneOffset()) * 60_000);
    } else {
      fail();
    }

    // Check again the date is valid after transformations, see #2229
    if (isNaN(date.getTime())) {
      fail();
    }
  }

  return [date, offset];
}

export function parse(str: string, detectUTC = false): IDateTime {
  const [date, offset] = parseRaw(str);
  // .NET always parses DateTime as Local if there's offset info (even "Z")
  // Newtonsoft.Json uses UTC if the offset is "Z"
  const kind = offset != null
    ? (detectUTC && offset === "Z" ? DateTimeKind.Utc : DateTimeKind.Local)
    : DateTimeKind.Unspecified;
  return DateTime(date.getTime(), kind);
}

export function tryParse(v: string, defValue: FSharpRef<IDateTime>): boolean {
  try {
    defValue.contents = parse(v);
    return true;
  } catch (_err) {
    return false;
  }
}

export function create(
  year: number, month: number, day: number,
  h: number = 0, m: number = 0, s: number = 0,
  ms: number = 0, kind?: DateTimeKind) {
  const date = kind === DateTimeKind.Utc
    ? new Date(Date.UTC(year, month - 1, day, h, m, s, ms))
    : new Date(year, month - 1, day, h, m, s, ms);
  if (year <= 99) {
    if (kind === DateTimeKind.Utc) {
      date.setUTCFullYear(year, month - 1, day);
    } else {
      date.setFullYear(year, month - 1, day);
    }
  }
  const dateValue = date.getTime();
  if (isNaN(dateValue)) {
    throw new Exception("The parameters describe an unrepresentable Date.");
  }
  return DateTime(dateValue, kind);
}

export function now() {
  return DateTime(Date.now(), DateTimeKind.Local);
}

export function utcNow() {
  return DateTime(Date.now(), DateTimeKind.Utc);
}

export function today() {
  return date(now());
}

export function isLeapYear(year: number) {
  return year % 4 === 0 && year % 100 !== 0 || year % 400 === 0;
}

export function daysInMonth(year: number, month: number) {
  return month === 2
    ? (isLeapYear(year) ? 29 : 28)
    : (month >= 8 ? (month % 2 === 0 ? 31 : 30) : (month % 2 === 0 ? 30 : 31));
}

export function toUniversalTime(date: IDateTime) {
  return date.kind === DateTimeKind.Utc ? date : DateTime(date.getTime(), DateTimeKind.Utc);
}

export function toLocalTime(date: IDateTime) {
  return date.kind === DateTimeKind.Local ? date : DateTime(date.getTime(), DateTimeKind.Local);
}

export function specifyKind(d: IDateTime, kind: DateTimeKind) {
  return create(year(d), month(d), day(d), hour(d), minute(d), second(d), millisecond(d), kind);
}

export function timeOfDay(d: IDateTime) {
  return hour(d) * 3600000
    + minute(d) * 60_000
    + second(d) * 1000
    + millisecond(d);
}

export function date(d: IDateTime) {
  return create(year(d), month(d), day(d), 0, 0, 0, 0, d.kind);
}

export function day(d: IDateTime) {
  return d.kind === DateTimeKind.Utc ? d.getUTCDate() : d.getDate();
}

export function hour(d: IDateTime) {
  return d.kind === DateTimeKind.Utc ? d.getUTCHours() : d.getHours();
}

export function millisecond(d: IDateTime) {
  return d.kind === DateTimeKind.Utc ? d.getUTCMilliseconds() : d.getMilliseconds();
}

export function minute(d: IDateTime) {
  return d.kind === DateTimeKind.Utc ? d.getUTCMinutes() : d.getMinutes();
}

export function month(d: IDateTime) {
  return (d.kind === DateTimeKind.Utc ? d.getUTCMonth() : d.getMonth()) + 1;
}

export function second(d: IDateTime) {
  return d.kind === DateTimeKind.Utc ? d.getUTCSeconds() : d.getSeconds();
}

export function year(d: IDateTime) {
  return d.kind === DateTimeKind.Utc ? d.getUTCFullYear() : d.getFullYear();
}

export function dayOfWeek(d: IDateTime) {
  return d.kind === DateTimeKind.Utc ? d.getUTCDay() : d.getDay();
}

export function dayOfYear(d: IDateTime) {
  const _year = year(d);
  const _month = month(d);
  let _day = day(d);
  for (let i = 1; i < _month; i++) {
    _day += daysInMonth(_year, i);
  }
  return _day;
}

export function add(d: IDateTime, ts: number) {
  const newDate = DateTime(d.getTime() + ts, d.kind);
  if (d.kind !== DateTimeKind.Utc) {
    const oldTzOffset = d.getTimezoneOffset();
    const newTzOffset = newDate.getTimezoneOffset();
    return oldTzOffset !== newTzOffset
      ? DateTime(newDate.getTime() + (newTzOffset - oldTzOffset) * 60_000, d.kind)
      : newDate;
  } else {
    return newDate;
  }
}

export function addDays(d: IDateTime, v: number) {
  return add(d, v * 86400000);
}

export function addHours(d: IDateTime, v: number) {
  return add(d, v * 3600000);
}

export function addMinutes(d: IDateTime, v: number) {
  return add(d, v * 60_000);
}

export function addSeconds(d: IDateTime, v: number) {
  return add(d, v * 1000);
}

export function addMilliseconds(d: IDateTime, v: number) {
  return add(d, v);
}

export function addTicks(d: IDateTime, v: int64) {
  return add(d, toFloat64(v / 10000n));
}

export function addYears(d: IDateTime, v: number) {
  const newMonth = month(d);
  const newYear = year(d) + v;
  const _daysInMonth = daysInMonth(newYear, newMonth);
  const newDay = Math.min(_daysInMonth, day(d));
  return create(newYear, newMonth, newDay, hour(d), minute(d), second(d),
    millisecond(d), d.kind);
}

export function addMonths(d: IDateTime, v: number) {
  let newMonth = month(d) + v;
  let newMonth_ = 0;
  let yearOffset = 0;
  if (newMonth > 12) {
    newMonth_ = newMonth % 12;
    yearOffset = Math.floor(newMonth / 12);
    newMonth = newMonth_;
  } else if (newMonth < 1) {
    newMonth_ = 12 + newMonth % 12;
    yearOffset = Math.floor(newMonth / 12) + (newMonth_ === 12 ? -1 : 0);
    newMonth = newMonth_;
  }
  const newYear = year(d) + yearOffset;
  const _daysInMonth = daysInMonth(newYear, newMonth);
  const newDay = Math.min(_daysInMonth, day(d));
  return create(newYear, newMonth, newDay, hour(d), minute(d), second(d),
    millisecond(d), d.kind);
}

export function subtract<Input extends number | IDateTime, Output = Input extends number ? IDateTime : number>(d: IDateTime, that: Input): Output {
  return typeof that === "number"
    ? add(d, -that) as Output
    : d.getTime() - that.getTime() as Output;
}

export function toLongDateString(d: IDateTime) {
  return d.toDateString();
}

export function toShortDateString(d: IDateTime) {
  return d.toLocaleDateString();
}

export function toLongTimeString(d: IDateTime) {
  return d.toLocaleTimeString();
}

export function toShortTimeString(d: IDateTime) {
  return d.toLocaleTimeString().replace(/:\d\d(?!:)/, "");
}

export function equals(d1: IDateTime, d2: IDateTime) {
  return d1.getTime() === d2.getTime();
}

export const compare = compareDates;
export const compareTo = compareDates;

export function op_Addition(x: IDateTime, y: number) {
  return add(x, y);
}

export function op_Subtraction<Input extends number | IDateTime, Output = Input extends number ? IDateTime : number>(x: IDateTime, y: Input): Output {
  return subtract(x, y);
}

export function isDaylightSavingTime(x: IDateTime) {
  const jan = new Date(x.getFullYear(), 0, 1);
  const jul = new Date(x.getFullYear(), 6, 1);
  return isDST(jan.getTimezoneOffset(), jul.getTimezoneOffset(), x.getTimezoneOffset());
}

function isDST(janOffset: number, julOffset: number, tOffset: number) {
  return Math.min(janOffset, julOffset) === tOffset;
}

export default DateTime;