// from https://introcs.cs.princeton.edu/java/31datatype/Soundex.java.html

static S soundex(S s) { 
  if (empty(s)) ret "";
  char[] x = s.toUpperCase().toCharArray();
  char firstLetter = x[0];

  // convert letters to numeric code
  for (int i = 0; i < x.length; i++) {
      switch (x[i]) {

          case 'B':
          case 'F':
          case 'P':
          case 'V':
              x[i] = '1';
              break;

          case 'C':
          case 'G':
          case 'J':
          case 'K':
          case 'Q':
          case 'S':
          case 'X':
          case 'Z':
              x[i] = '2';
              break;

          case 'D':
          case 'T':
              x[i] = '3';
              break;

          case 'L':
              x[i] = '4';
              break;

          case 'M':
          case 'N':
              x[i] = '5';
              break;

          case 'R':
              x[i] = '6';
              break;

          default:
              x[i] = '0';
              break;
      }
  }

  // remove duplicates
  new StringBuilder output;
  output.append(firstLetter);
  for (int i = 1; i < x.length; i++)
      if (x[i] != x[i-1] && x[i] != '0')
          output.append(x[i]);

  // pad with 0's or truncate
  output.append("0000");
  return takeFirst(str(output), 4);
}