defold-go-position-setter

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defold-go-position-setter

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# GoPositionSetter Native extension

Fast way to update position for thousands of game objects.

<strong style="font-size:20px;">30000 bunnies 53.71 fps</strong>

<strong style="font-size:32px;">30000 bunnies 67.69 fps</strong>

Tested on my PC.

## What changed and why it's faster

```lua
function update(self, dt)
	bunnymark.update()
	local bunnies = bunnymark.bunnies
	for i=#bunnymark.bunnies,1,-1 do
		local bunny = bunnies[i]
		bunny.velocity = bunny.velocity - 1200 * dt

local p = bunny.position
		p.y = p.y + bunny.velocity * dt
		if p.y < 50 then
			p.y = 50
			bunny.velocity = -bunny.velocity
		end
		--go.set_position(p, bunny.id)--this is used in update single
	end
	self.position_setter:update() --this is used in go_position_setter
	label.set_text("#label", ("Bunnies: %d FPS: %.2f. Click to add more"):format(bunnymark.get_bunny_count(), bunnymark.get_fps()))	
end
```

It is faster because:
1. It uses one C++ call instead of thousands of go.set_position calls.
2. It avoids some checks. You as the developer guarantee that the vector and gameobject are valid and available.

```cpp
//This is how go.set_position works:
static Instance* ResolveInstance(lua_State* L, int instance_arg)
{
    ScriptInstance* i = ScriptInstance_Check(L);
    Instance* instance = i->m_Instance;
    if (lua_gettop(L) == instance_arg && !lua_isnil(L, instance_arg)) {
        dmMessage::URL receiver;
        dmScript::ResolveURL(L, instance_arg, &receiver, 0x0);
        if (receiver.m_Socket != dmGameObject::GetMessageSocket(i->m_Instance->m_Collection->m_HCollection))
        {
            luaL_error(L, "function called can only access instances within the same collection.");
        }

instance = GetInstanceFromIdentifier(instance->m_Collection->m_HCollection, receiver.m_Path);
        if (!instance)
        {
            luaL_error(L, "Instance %s not found", lua_tostring(L, instance_arg));
            return 0; // Actually never reached
        }
    }
    return instance;
}

int Script_SetPosition(lua_State* L)
{
    Instance* instance = ResolveInstance(L, 2);
    dmVMath::Vector3* v = dmScript::CheckVector3(L, 1);
    dmGameObject::SetPosition(instance, dmVMath::Point3(*v));
    return 0;
}

//this is how go_position_setter.update() works:
//no checks. Just SetPosition to the instance
void PositionSetterUserdata::update() {
    for (int i = 0; i < instances.Size(); ++i) {
        InstancePositionData instancePositionData = instances[i];
        dmGameObject::SetPosition(instancePositionData.rootInstance, dmVMath::Point3(*instancePositionData.position));
    }
}

```

## How to use

1. Create a go_position_setter.new() in your collection
```lua
self.position_setter = go_position_setter.new()
```
2. Add bunny to position_setter
```lua
	self.position_setter:add(bunny.id, bunny.position)
```
3. Update bunny.position
```lua
	bunny.position.y = bunny.position.y + bunny.velocity * dt
```
4. Update all positions
```lua
	self.position_setter:update()
```

## Credits

Based on Defold bunnymark test https://github.com/britzl/defold-bunnymark
Bunny graphics from https://pixijs.github.io/bunny-mark/

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