add square:divide() function

2 files changed, 57 insertions, 0 deletions

diff --git a/city/geometry-test.lua b/city/geometry-test.lua
index f85bec0..74ddb3a 100644
--- a/city/geometry-test.lua
+++ b/city/geometry-test.lua
@@ -93,3 +93,40 @@ test(
       end
    end
 )
+
+test(
+   'subdivide a square',
+   function()
+      local center = geom.point(2, 2)
+      local square = geom.square(center, 4)
+
+      local children = square:divide()
+      assert(
+	 #children == 4,
+	 string.format(
+	    'incorrect number of children: %d',
+	    #children
+	 )
+      )
+
+      local aa = children[1]
+      assert(aa.center.x == 1)
+      assert(aa.center.y == 1)
+      assert(aa.span == 2)
+
+      local ba = children[2]
+      assert(ba.center.x == 3)
+      assert(ba.center.y == 1)
+      assert(ba.span == 2)
+
+      local ab = children[3]
+      assert(ab.center.x == 1)
+      assert(ab.center.y == 3)
+      assert(ab.span == 2)
+
+      local bb = children[4]
+      assert(bb.center.x == 3)
+      assert(bb.center.y == 3)
+      assert(bb.span == 2)
+   end
+)
diff --git a/city/geometry.lua b/city/geometry.lua
index 1632393..13088d1 100644
--- a/city/geometry.lua
+++ b/city/geometry.lua
@@ -47,6 +47,9 @@ geom.square = class{
 
    axis_range = function(c, r) return {min=c-r, max=c+r} end,
 
+   -- returns true if a point is inside of the square
+   -- and false otherwise
+   -- (x/y mininum exclusive, maximum inclusive)
    contains = function(self, point)
       local x_overlap =
 	 (point.x > self.x.min) and (point.x <= self.x.max)
@@ -57,6 +60,23 @@ geom.square = class{
 	 return true
       end
       return false
+   end,
+
+   -- return an array of four squares covering the same
+   -- area as their parent
+   divide = function(self)
+      local this = getmetatable(self).__index
+      
+      local x = self.center.x
+      local y = self.center.y
+      local d = self.span / 4
+      
+      return {
+	 this(geom.point(x-d, y-d), d*2),
+	 this(geom.point(x+d, y-d), d*2),
+	 this(geom.point(x-d, y+d), d*2),
+	 this(geom.point(x+d, y+d), d*2),
+      }
    end
 }