'expand' and 'shrink' year ranges, made in about an hour after an initial version in js only with the expand function

year-range.ts

export class YearRange {
  static testPattern = /[\d -,]+/g;
  static splitPattern = / *[-,] */g;

  private static getContext(input: string): YearRangeContext {
    console.assert(this.testPattern.test(input), 'Invalid pattern match');

    const fragments = input.split(this.splitPattern);
    const symbols = input.match(this.splitPattern) ?? [];

    console.assert(fragments[0].length === 4, 'Invalid initial year range');

    return { fragments, symbols };
  }

  static expand(input: string): string {
    const { fragments, symbols } = this.getContext(input);

    if (symbols.length === 0) return fragments[0];

    const output: string[] = [];
    let previousYear = '';

    for (const index in fragments) {
      const fragment = fragments[index];
      let result: string = fragment;

      if (index !== '0') {
        result = previousYear.substring(0, previousYear.length - fragment.length) + fragment;
      }

      output.push(result);
      previousYear = result;
    }

    return String.raw({ raw: output }, ...symbols);
  }

  static shrink(input: string): string {
    const { fragments, symbols } = this.getContext(input);

    if (symbols.length === 0) return fragments[0];

    const output: string[] = [];
    let previousYear = '';

    for (const index in fragments) {
      const fragment = fragments[index];
      let result: string = fragment;

      if (index !== '0') {
        const index = result
          .slice()
          .split('')
          .findIndex((char, index) => char === previousYear[index]);

        result = result.slice(result.length - index - 1);
      }

      output.push(result);
      previousYear = fragment;
    }

    return String.raw({ raw: output }, ...symbols);
  }
}

interface Context {
  fragments: string[];
  symbols: string[];
}

export type YearRangeContext = Context;

console.log(YearRange.shrink('2002 - 2003,2005'));

console.log(YearRange.expand('2002 - 3, 5') // 2002 - 2003, 2005
console.log(YearRange.shrink('2002-2003,2005') // 2002-3,5

Known issues with providing year strings in the incorrect order (shrink: 2012 - 2003), it's not something I'm willing to fix given the time in which it was smashed together. While expand doesn't validate any of this, it continues to work as intended regardless of the flaws that its sibling has.