Added feature to get vessel flag from MMSI.

[panoet]

No description provided.

[Copilot]

Pull Request Overview

Adds a utility to derive a vessel’s flag from an MMSI by mapping MIDs to country names and ISO codes.

• Introduces VesselFlag with a static MID→flag map and getters
• Adds getFlagFromMmsi(String) to extract the MID from MMSI prefixes and look up the flag

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[Copilot]

The API exposes a mutable internal map and returns a raw type. Return an unmodifiable Map<Integer, VesselFlag> to prevent external mutation and use generics for type safety. Example: change the field to private static final Map<Integer, VesselFlag> and return Collections.unmodifiableMap(FLAGS_MAP).

[Copilot]

The API exposes a mutable internal map and returns a raw type. Return an unmodifiable Map<Integer, VesselFlag> to prevent external mutation and use generics for type safety. Example: change the field to private static final Map<Integer, VesselFlag> and return Collections.unmodifiableMap(FLAGS_MAP).

[Copilot]

[nitpick] Make VesselFlag immutable and non-extensible: declare fields final, make the constructor private, and consider making the class final. This prevents accidental mutation and clarifies usage as a value object.

[Copilot]

Substring operations can throw StringIndexOutOfBoundsException for short inputs and NumberFormatException for non-digit inputs. Add input validation (length and numeric) and fail fast with an IllegalArgumentException and a clear message before substring/parse.

Suggested change

	public static VesselFlag getFlagFromMmsi(String mmsi) throws NumberFormatException {
	int mid;
	if (mmsi.startsWith("111") || mmsi.startsWith("970")) {
	mid = Integer.parseInt(mmsi.substring(3, 6));
	} else if (mmsi.startsWith("00") || mmsi.startsWith("98") || mmsi.startsWith("99")) {
	mid = Integer.parseInt(mmsi.substring(2, 5));
	} else if (mmsi.startsWith("8") || mmsi.startsWith("0")) {
	mid = Integer.parseInt(mmsi.substring(1, 4));
	} else {
	mid = Integer.parseInt(mmsi.substring(0, 3));
	}
	public static VesselFlag getFlagFromMmsi(String mmsi) {
	if (mmsi == null) {
	throw new IllegalArgumentException("MMSI must not be null");
	}
	// The largest substring index is 6 (mmsi.substring(3, 6)), so require at least 6 characters
	if (mmsi.length() < 6) {
	throw new IllegalArgumentException("MMSI must be at least 6 characters long: " + mmsi);
	}
	int mid;
	String midStr;
	if (mmsi.startsWith("111") || mmsi.startsWith("970")) {
	midStr = mmsi.substring(3, 6);
	} else if (mmsi.startsWith("00") || mmsi.startsWith("98") || mmsi.startsWith("99")) {
	midStr = mmsi.substring(2, 5);
	} else if (mmsi.startsWith("8") || mmsi.startsWith("0")) {
	midStr = mmsi.substring(1, 4);
	} else {
	midStr = mmsi.substring(0, 3);
	}
	if (midStr.length() != 3 || !midStr.chars().allMatch(Character::isDigit)) {
	throw new IllegalArgumentException("MMSI MID substring is not a 3-digit number: " + midStr + " (from MMSI: " + mmsi + ")");
	}
	mid = Integer.parseInt(midStr);

[Copilot]

The ISO 3166-1 alpha-2 code 'UK' is invalid (United Kingdom is 'GB'). Replace the short code with 'GB', and for consistency with other entries use 'United Kingdom' as the country name.

Suggested change

	FLAGS_MAP.put(740, new VesselFlag("UK", "UK"));
	FLAGS_MAP.put(740, new VesselFlag("United Kingdom", "GB"));

[Copilot]

Add Javadoc describing the MMSI parsing rules (special prefixes like 111/970, 00/98/99, 8/0) and the expected MMSI format/length, plus what is returned when no MID match is found (null). This will make the intent and edge-case handling clearer to callers.

Suggested change

	/**
	* Returns the {@link VesselFlag} corresponding to the given MMSI (Maritime Mobile Service Identity).
	* <p>
	* MMSI is expected to be a 9-digit numeric string. The method extracts the Maritime Identification Digits (MID)
	* using the following rules:
	* <ul>
	* <li>If the MMSI starts with "111" or "970", the MID is digits 4-6 (positions 3-5, zero-based).</li>
	* <li>If the MMSI starts with "00", "98", or "99", the MID is digits 3-5 (positions 2-4).</li>
	* <li>If the MMSI starts with "8" or "0", the MID is digits 2-4 (positions 1-3).</li>
	* <li>Otherwise, the MID is the first three digits (positions 0-2).</li>
	* </ul>
	* <p>
	* If the extracted MID does not match any known country, this method returns {@code null}.
	* <p>
	* @param mmsi the 9-digit MMSI string to parse
	* @return the corresponding {@link VesselFlag}, or {@code null} if no MID match is found
	* @throws NumberFormatException if the MMSI is not numeric or is too short to extract the MID
	*/

[Copilot]

Update country name to its current official name 'North Macedonia'.

Suggested change

	FLAGS_MAP.put(274, new VesselFlag("FYR Macedonia", "MK"));
	FLAGS_MAP.put(274, new VesselFlag("North Macedonia", "MK"));

[Copilot]

Update country name to its current official name 'Eswatini'.

Suggested change

	FLAGS_MAP.put(669, new VesselFlag("Swaziland", "SZ"));
	FLAGS_MAP.put(669, new VesselFlag("Eswatini", "SZ"));