Recently I've been working with a dataset which has many large polygons
with many holes (eg 100K+ vertices, 1000+ holes). This revealed a
performance issue with the Shapefile reading code that I'm using, which
is pretty much the same as the code in JUMP (and is descended from an
old GeoTools version). The issue is that the point-in-polygon test is
fairly slow when run against large shells and for many holes.
There's a few fixes required:
1. if only one shell is present, do not run the PIP code, but simply
assign the holes to the shell. (This fix has been made in the current
GeoTools PolygonHandler code, and could be copied from there)
2. this can be extended to checking for only one *candidate* shell (a
candidate shell is one whose envelope contains the hole envelope). This
code is not yet developed.
3. the code uses ArrayList.indexOf. This is inefficient, since it uses
equals(), which does a full equality comparison. Instead, an iterator
and == should be used instead (code below)
The performance improvement from just #1 was dramatic - reading a 1M
feature shapefile went from over 1000 s to 32 s.
----------------------------
PolygonHandler change:
((ArrayList) holesForShells.get(findIndex(shells,
minShell))).add(testHole);
/**
* Finds a object in a list. Should be much faster than indexof
*
* @param list
* @param o
* @return
*/
private static int findIndex(ArrayList list, Object o)
{
int n = list.size();
for (int i = 0; i < n; i++) {
if (list.get(i) == o)
return i;
}
return -1;
}
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