const DateFormatCache = new Map<string, DateFormat>();

declare type DateFormatPart = {
  type: "quoted" | "literal" | "pattern";
  text: string;
};

export class DateFormat {
  static get(format: string, locale = "en") {
    const cached = DateFormatCache.get(`${locale} ${format}`);
    if (cached) {
      return cached;
    }

    const re =
      /('(?:''|[^'])*')|(P+p+|(?:G+|y+|Y+|R+|u+|Q+|q+|M+|L+|w+|I+|d+|E+|i+|e+|c+|a+|b+|B+|h+|H+|K+|k+|m+|s+|S+|X+|x+|O+|z+|t+|T+|P+|p+)o?)|(.+?)/y;
    const parts = new Array<DateFormatPart>();
    let m: RegExpMatchArray | null;
    while ((m = re.exec(format)) != null) {
      if (m[1]) {
        parts.push({ type: "quoted", text: m[1] });
      } else if (m[2]) {
        parts.push({ type: "pattern", text: m[2] });
      } else {
        parts.push({ type: "literal", text: m[3] });
      }
    }

    if (DateFormatCache.size >= 32) {
      for (const entry of DateFormatCache) {
        DateFormatCache.delete(entry[0]);
        break;
      }
    }

    const dformat = new DateFormat(parts);
    DateFormatCache.set(`${locale} ${format}`, dformat);
    return dformat;
  }

  public parts: Array<DateFormatPart>;

  constructor(parts: Array<DateFormatPart>) {
    this.parts = parts;
  }

  toString() {
    let str = "";
    let quoted = false;
    for (let i = 0; i < this.parts.length; i++) {
      const part = this.parts[i];
      if (part.type === "quoted") {
        if (!quoted) {
          str += "'";
          quoted = true;
        }
        str += part.text.substring(1, part.text.length - 1);
      } else {
        if (quoted) {
          str += "'";
          quoted = false;
        }
        str += part.text;
      }
    }
    if (quoted) {
      str += "'";
      quoted = false;
    }
    return str;
  }
}