NmeaMessageParser.java

package au.gov.amsa.util.nmea;

import java.util.Arrays;
import java.util.LinkedHashMap;

import org.apache.commons.lang3.StringUtils;

import com.google.common.collect.Maps;

/**
 * Parses NMEA messages.
 * 
 */
public class NmeaMessageParser {

    private static final String CHECKSUM_DELIMITER = "*";
    private static final String PARAMETER_DELIMITER = ",";
    private static final String CODE_DELIMITER = ":";

    /**
     * Return an {@link NmeaMessage} from the given NMEA line.
     * 
     * @param line
     * @return
     */
    public NmeaMessage parse(String line) {
        LinkedHashMap<String, String> tags = Maps.newLinkedHashMap();

        String remaining;
        if (line.startsWith("\\")) {
            int tagFinish = line.lastIndexOf('\\', line.length() - 1);
            if (tagFinish == -1)
                throw new NmeaMessageParseException(
                        "no matching \\ symbol to finish tag block: " + line);
            if (tagFinish == 0)
                throw new NmeaMessageParseException("tag block is empty or not terminated");
            tags = extractTags(line.substring(1, tagFinish));
            remaining = line.substring(tagFinish + 1);
        } else
            remaining = line;

        String[] items;
        String checksum;
        if (remaining.length() > 0) {
            if (!remaining.contains("*"))
                throw new NmeaMessageParseException("checksum delimiter * not found");
            items = getNmeaItems(remaining);
            // TODO validate message using checksum
            checksum = line.substring(line.indexOf('*') + 1);
        } else {
            items = new String[] {};
            // TODO decide what value to put here
            checksum = "";
        }

        return new NmeaMessage(tags, Arrays.asList(items), checksum);
    }

    /**
     * Returns the items from the comma delimited NMEA line. The last item
     * always contains a * followed by a checksum. If * is not present
     * {@link IndexOutOfBoundsException} will be thrown.
     * 
     * @param line
     * @return
     */
    private static String[] getNmeaItems(String line) {
        String[] items = StringUtils.splitByWholeSeparatorPreserveAllTokens(line,
                PARAMETER_DELIMITER);
        // remove the checksum from the end
        String last = items[items.length - 1];
        if (last.contains("*"))
            items[items.length - 1] = last.substring(0, last.lastIndexOf('*'));
        return items;
    }

    /**
     * Returns the tags from the tag block section of the message (NMEA v4.0).
     * If there is no tag block then returns an empty map.
     * 
     * @param s
     * @return
     */
    public static LinkedHashMap<String, String> extractTags(String s) {
        LinkedHashMap<String, String> map = Maps.newLinkedHashMap();
        int c = s.lastIndexOf(CHECKSUM_DELIMITER);
        if (c == -1) {
            return map;
        }
        s = s.substring(0, c);
        String[] items = s.split(PARAMETER_DELIMITER);
        for (String item : items) {
            int i = item.indexOf(CODE_DELIMITER);
            if (i == -1)
                throw new NmeaMessageParseException(
                        "TAG BLOCK parameter is not is format 'a:b' :" + s);
            map.put(item.substring(0, i), item.substring(i + 1));
        }
        return map;
    }
}