[mkgmap-dev] [PATCH v1] Reduce memory footprint of tags

8 Feb 2010

      The discussions around "Commit: r1566: Drop all tags from the osm file 
that are not used" inspired me to address the memory footprints of the tags.

The patch reduces the memory footprint by using the String.intern() 
method. This ensures that the String "highway" exists only once when 
stored as tag. Additionally the values of the most common tags with a 
limited number of different values are also interned. (It makes sense to 
intern the value of a highway tag but it does not make sense to intern 
the value of a name tag because the share rate of the name tag values is 
too low).

This all might sound embarrasing but the String objects that are 
returned from the XML reader are not interned. You can test this easily 
by exchanging the put method in class tag:

public String put(String key, String value) {
	if ("highway".equals(key) && "highway" != key) {
		log.error("Tag is not interned");
	}

	ensureSpace();
	Integer ind = keyPos(key);
	if (ind == null)
		assert false : "keyPos(" + key + ") returns null - size = " + size + 
", capacity = " + capacity;
	keys[ind] = key;

	String old = values[ind];
	if (old == null)
		size++;

	values[ind] = value;
	return old;
}

You will see lots of "Tag is not interned" errors.

I have seen memory reductions of > 10%.

Please test this patch against your well known tiles. It would be 
perfect if someone has a tile and knows its minimum memory requirement 
for mkgmap. This patch should lower it.

WanMil