add square:intersects()

2 files changed, 52 insertions, 3 deletions

diff --git a/city/geometry-test.lua b/city/geometry-test.lua
index 758ce95..d48abbc 100644
--- a/city/geometry-test.lua
+++ b/city/geometry-test.lua
@@ -132,6 +132,43 @@ test(
    end
 )
 
+test(
+   'intersect various squares',
+   function()
+      local center = geom.point(0, 0)
+      local square = geom.square(center, 2)
+
+      -- overlap x/y
+      assert(square:intersects(
+		geom.square(geom.point(1, 1), 2)))
+
+      -- nest inside
+      assert(square:intersects(
+		geom.square(geom.point(0, 0), 1)))
+
+      -- nest outside
+      assert(square:intersects(
+		geom.square(geom.point(0, 0), 5)))
+
+      -------- non-intersections --------
+
+      -- share x on left
+      assert(not square:intersects(
+		geom.square(geom.point(-2, 0), 2)))
+
+      -- share x on right
+      assert(not square:intersects(
+		geom.square(geom.point(2, 0), 2)))
+
+      -- share y on top
+      assert(not square:intersects(
+		geom.square(geom.point(0, -2), 2)))
+
+      -- share y on bottom
+      assert(not square:intersects(
+		geom.square(geom.point(0, 2), 2)))
+   end
+)
 
 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 --
diff --git a/city/geometry.lua b/city/geometry.lua
index 5893731..a24e508 100644
--- a/city/geometry.lua
+++ b/city/geometry.lua
@@ -62,15 +62,27 @@ geom.square = class{
       return false
    end,
 
+   -- check if the square intersects the given square
+   intersects = function(self, square)
+      local overlap = function(a, b)
+	 return math.max(a.min, b.min) < math.min(a.max, b.max)
+      end
+      local x_overlap = overlap(self.x, square.x)
+      local y_overlap = overlap(self.y, square.y)
+
+      if x_overlap and y_overlap then 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),
@@ -105,7 +117,7 @@ geom.qt_node = class{
 	 -- point isn't even in this region
 	 return false
       end
-      
+
       if self:is_leaf() and not self.point then
 	 -- no preexisting point, just store it
 	 self.point = point