## Using recursive fill to count maps

In the Empire game (yes, just one last mention!) during the map generation the program counts up the size of individual islands and sea areas. This is don by recursion and quite useful. Otherwise, the usual place you see recursion mentioned is in calculating factorials and Fibonacci sequences.

The map struct holds not only the terrain type and other information but a continent number. This is set initially to 0.  After the land and seas have been generated, the program picks a random land point on the map (just keep picking a random location until it has land with a continent number of 0).

It then counts by calling itself 8 times. It calls itself recursively at location y,x with directions (y-1,x-1) that’s NorthWest, (y-1,x)  and so on round to West (y,x-1).  But it only does it as long as each of those locations is land type with a continent number of 0. And as it does it, it sets the continent numbers in each location to 1 for the first continent and so on.

This recursive algorithm means that every contiguous land location gets reached. A similar thing is done with seas and lakes but you have to be careful that the stack is large enough.

``````void FillIn(int x,int y,int locis) {
if onMap(x,y) {
if (locis==map[x][y].locis && map[x]
[y].continent==UNALLOCATED) {
map[x][y].continent=numconts;
allconts[numconts].count++;
FillIn(x-1,y,locis);
FillIn(x+1,y,locis);
FillIn(x,y-1,locis);
FillIn(x,y+1,locis);
FillIn(x-1,y-1,locis);
FillIn(x+1,y-1,locis);
FillIn(x+1,y+1,locis);
FillIn(x-1,y+1,locis);
}
}
}``````

This is used for both land and sea, so locis can be either. It’s an efficient algorithm and quite quick.

Those graphics? They were the initial hexagon graphics…