JavaScript unindent tagged template literal

unindent.js

const unindent = (strings, ...values) => {
  // Quick and not so dirty unindented template strings.
  // Handles tabs very roughly if they are present.
  // The very first line is always left as is if not entirely whitespace,
  // and removed otherwise.

  const matchLine = line => {
    const {groups} =
      /^(?<line>(?<indent>[\t ]*)(?<nonws>[^\n]*)\n?)(?<tail>.*)$/s.exec(line);
    return {
      line: groups.line,
      indent: groups.indent.length,
      hasNonWS: !!groups.nonws,
      hasNewline: groups.line.endsWith('\n'),
      tail: groups.tail,
    };
  };

  const shortestIndent = (() => {
    const [SKIP, LINE_START] = ['SKIP', 'LINE_START'];

    let state = {tag: SKIP};
    let indent = Infinity;
    for (let str of strings) {
      if (state.tag === LINE_START) {
        indent = Math.min(indent, state.indent);
        state = {tag: SKIP};
      }

      while (str) {
        const match = matchLine(str);
        switch (state.tag) {
          case SKIP:
            if (match.hasNewline) {
              state = {tag: LINE_START, indent: 0};
            }
            break;

          case LINE_START:
            // Known invariant here: state.indent === 0
            state = {tag: LINE_START, indent: match.indent};
            if (match.hasNonWS) {
              indent = Math.min(indent, match.indent);
              state = {tag: SKIP};
            }
            if (match.hasNewline) {
              state = {tag: LINE_START, indent: 0};
            }
            break;
        }
        str = match.tail;
      }
    }
    return indent === Infinity ? 0 : indent;
  })();

  const fixedStrings = (() => {
    const [START, SKIP, LINE_START] = ['START', 'SKIP', 'LINE_START'];
    let state = START;
    const result = [];
    for (let str of strings) {
      const builder = [];

      while (str) {
        const match = matchLine(str);
        switch (state) {
          case START:
            if (!match.hasNewline || match.hasNonWS) {
              builder.push(match.line);
            }
            state = match.hasNewline ? LINE_START : SKIP;
            break;

          case SKIP:
            builder.push(match.line);
            if (match.hasNewline) state = LINE_START;
            break;

          case LINE_START:
            if (match.line.length <= shortestIndent && match.hasNewline) {
              builder.push('\n');
            } else {
              builder.push(match.line.substring(shortestIndent));
            }
            if (!match.hasNewline) state = SKIP;
            break;
        }
        str = match.tail;
      }
      result.push(builder.join(''));
    }
    return result;
  })();

  return String.raw({raw: fixedStrings}, ...values);
};

const example = vars => {
  return unindent`
    # ${vars.title}

    This markdown was generated from a JS tagged template. Here is some
    properly indented code:

    \`\`\`html
    <div id="${vars.divId}">
      <p class="${vars.paragraphClass}">
        ${vars.paragraph}
      </p>
    </div>
    \`\`\`
  `;
};