Add voxelarea.lua helper to builtin

builtin/voxelarea.lua

@@ -0,0 +1,46 @@
+VoxelArea = {
+	MinEdge = {x=1, y=1, z=1},
+	MaxEdge = {x=0, y=0, z=0},
+	ystride = 0,
+	zstride = 0,
+}
+
+function VoxelArea:new(o)
+	o = o or {}
+	setmetatable(o, self)
+	self.__index = self
+
+	local e = o:getExtent()
+	o.ystride = e.x
+	o.zstride = e.x * e.y
+
+	return o
+end
+
+function VoxelArea:getExtent()
+	return {
+		x = self.MaxEdge.x - self.MinEdge.x + 1,
+		y = self.MaxEdge.y - self.MinEdge.y + 1,
+		z = self.MaxEdge.z - self.MinEdge.z + 1,
+	}
+end
+
+function VoxelArea:getVolume()
+	local e = self:getExtent()
+	return e.x * e.y * e.z
+end
+
+function VoxelArea:index(x, y, z)
+	local i = (z - self.MinEdge.z) * self.zstride +
+			  (y - self.MinEdge.y) * self.ystride +
+			  (x - self.MinEdge.x) + 1
+	return math.floor(i)
+end
+
+function VoxelArea:indexp(p)
+	local i = (p.z - self.MinEdge.z) * self.zstride +
+			  (p.y - self.MinEdge.y) * self.ystride +
+			  (p.x - self.MinEdge.x) + 1
+	return math.floor(i)
+end
+