content: package tim.prune.data;
  
  public abstract class GeocacheCode
  {
  	/** Supported providers of cache codes */
  	private enum Provider
  	{
  		GEOCACHING_COM("GC", "https://coord.info/"),
  		OPENCACHING_DE("OC", "https://www.opencaching.de/");
  
  		private final String _prefix;
  		private final String _url;
  
  		Provider(String inPrefix, String inUrl) {
  			_prefix = inPrefix;
  			_url = inUrl;
  		}
  
  		static Provider getProvider(String inCode)
  		{
  			if (inCode == null || inCode.length() < 2) {
  				return null;
  			}
  			for (Provider provider : values())
  			{
  				// Assume that all prefixes are the first two characters
  				if (provider._prefix.charAt(0) == inCode.charAt(0)
  						&& provider._prefix.charAt(1) == inCode.charAt(1)) {
  					return provider;
  				}
  			}
  			return null;
  		}
  	}
  
  	/** @return true if the given code is recognised and valid */
  	public static boolean isValidCode(String inCode)
  	{
  		if (inCode == null || !isLengthValid(inCode.length())) {
  			return false;
  		}
  		Provider provider = Provider.getProvider(inCode);
  		return provider != null && verifyCharactersAreValid(inCode);
  	}
  
  	/** @return true if the length of the code is acceptable */
  	private static boolean isLengthValid(int inLength) {
  		return inLength > 4 && inLength <= 8;
  	}
  
  	/**
  	 * @return true if the code only contains valid letters and numbers,
  	 * but this doesn't mean that it starts with a recognised prefix
  	 */
  	private static boolean verifyCharactersAreValid(String inCode)
  	{
  		// Note, only upper case ASCII letters and numbers are allowed
  		final String allowedChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
  		for (int i=0; i<inCode.length(); i++)
  		{
  			if (allowedChars.indexOf(inCode.charAt(i)) < 0) {
  				return false;
  			}
  		}
  		return true;
  	}
  
  	/** @return a suitable URL for the given code, or null if not available */
  	public static String getUrl(String inCode)
  	{
  		Provider provider = Provider.getProvider(inCode);
  		if (provider != null
  			&& inCode != null
  			&& isLengthValid(inCode.length())
  			&& verifyCharactersAreValid(inCode))
  		{
  			return provider._url + inCode;
  		}
  		return null;
  	}
  }
  