Sequential Alphanumeric IDs

AlphanumericIdUtil.java

public class AlphanumericIdUtil {

  private static final String VALID_CHARACTERS = "23456789abcdefghjkmnpqrstuvwxyz";

  public static void main(String[] args) {
    System.out.println(getNumberFromId("a8e2fz"));
    System.out.println(getIdFromNumber(24));
  }

  /**
   * Go from number (like 12345) to alphanumeric ID (like a8e7z4)
   *
   * @param number The number you wish to convert to ID
   * @return the alphanumeric ID as a string
   */

  public static String getIdFromNumber(Integer number) {
    // The magic number we're trying to get down to 0
    long magicNumber = number;

    // Some placeholder values
    long wholeNum = 0;
    int remainder = 0;
    String id = "";

    // The value we'll use as our modulo, just stored in a variable because it's simpler
    int modVal = VALID_CHARACTERS.length();

    // Loop until the magic number (AKA the whole number) is not zero (>0 also works)
    while (magicNumber != 0) {

      // First, take the magic number, divide it by the modulo value, and store that as our whole
      // number
      wholeNum = (long) Math.floor(magicNumber / modVal);

      // Next, find the remainder of when we take the magic number and divide it by the modulo value
      remainder = (int) magicNumber % modVal;

      // Then, take the remainder that we just got, find what position it is in our valid
      // characters, and store it in our ID (yes, this means the ID is backwards, but we fix that
      // later)
      id += VALID_CHARACTERS.charAt(remainder);

      // Lastly, make the magic number the whole number from above (since we don't need to deal with
      // the remainder)
      magicNumber = wholeNum;

      // Loop again until the whole num is zero!

    }

    // One last thing: reverse the string. StringBuilder has a nice reverse method, so that's what
    // I'm using to reverse the string.
    return new StringBuilder(id).reverse().toString();

  }

  /**
   * Go from alphanumeric ID (like a8e7z4) to number (like 12345)
   *
   * @param id The string with the alphanumeric ID
   * @return The number as an int
   */

  public static Integer getNumberFromId(String id) {

    // A placeholder for the base 10 number
    Integer base10Number = 0;

    // Loop over each character in the input string
    for (int i = 0; i < id.length(); i++) {

      // Get the "current" character
      char c = id.charAt(i);

      // Find where that character is in the list of valid characters
      // TODO: What do we do if it's not in that list?
      int index = VALID_CHARACTERS.indexOf(c);

      // Check if the character was able to be found
      if (index == -1) {
        throw new IllegalArgumentException("Character \"" + c
            + "\" is not a valid character. The only valid characters are: " + VALID_CHARACTERS);
      }

      // Take the base (31) to the power of the total number of characters, minus i, minus 1, and
      // mutiply it by the index
      base10Number += (int) Math.pow(VALID_CHARACTERS.length(), id.length() - i - 1) * index;

    }

    return base10Number;

  }

}