Spring RTS Engine

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For instance, the code
for i = 1, 1000000 do
local x = math.sin(i)
end
runs 30% slower than this one:
local sin = math.sin
for i = 1, 1000000 do
local x = sin(i)
end

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Access to external locals (that is, variables that are local to an enclosing
function) is not as fast as access to local variables, but it is still faster than
access to globals. Consider the next fragment:
function foo (x)
for i = 1, 1000000 do
x = x + math.sin(i)
end
return x
end
print(foo(10))
We can optimize it by declaring sin once, outside function foo:
local sin = math.sin
function foo (x)
for i = 1, 1000000 do
x = x + sin(i)
end
return x
end
print(foo(10))
This second code runs 30% faster than the original one.

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A good place to look for chances of reducing garbage creation is in loops. For instance, if a constant table is created inside a loop, you can move it out the loop, or even out of the function enclosing its creation. Compare:
function foo (...)
for i = 1, n do
local t = {1, 2, 3, "hi"}
-- do something without changing ÔÇÖtÔÇÖ
...
end
end
local t = {1, 2, 3, "hi"} -- create ÔÇÖtÔÇÖ once and for all
function foo (...)
for i = 1, n do
-- do something without changing ÔÇÖtÔÇÖ
...
end
end

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local function myFunction()
local foo1, foo2, foo3
...do stuff with foo1, foo2, foo3
end

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local foo1, foo2, foo3
local function myFunction()
...do stuff with foo1, foo2, foo3
end

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function widget:DrawWorldPreUnit()
local glTexture = gl.Texture
glTexture(blah)
end

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local function Blah()
local X = 0
...do arithmetic that might change X's value, but cannot operate if it's nil
X = final value
end

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local X = 0
local function Blah()
...do arithmetic that might change X's value, but cannot operate if it's nil
X = final value
end

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local function Blah()
local X
if X ~= nil then
...do arithmetic that might change X's value, but cannot operate if it's nil
else
X = 0
return
end

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local function Blah()
local myVar
myVar = functionCall()
-->use myVar to draw something
myVar = anotherFunctionCall()
-->use myVar to tell end-user that "42" is the number
end

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local function Blah()
local myVarOne, myVarTwo
myVarOne = functionCall()
-->use myVarOne to draw something
myVarTwo = anotherFunctionCall()
-->use myVarTwo to tell end-user that "42" is the number
end

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function widget:GetInfo()
   return {
      name      = "Enemy Spotter v.2",
      desc      = "Draws smoothed octagon under enemy units",
      author    = "Argh,TradeMark,Trepan",
      date      = "December 17th, 2009",
      license   = "(C) WolfeGames, released under GNU GPL, v2 or later",
      layer     = 5,
      enabled   = false  --  loaded by default?
   }
end

--------------------------------------------------------------------------------
--------------------------------------------------------------------------------

-- Automatically generated local definitions

local glBeginEnd             = gl.BeginEnd
local glTexCoord =	gl.TexCoord
local glTexture = gl.Texture
local GL_QUADS = GL.QUADS
local glColor                = gl.Color
local glCreateList           = gl.CreateList
local glDeleteList           = gl.DeleteList
local glDepthTest            = gl.DepthTest
local glDrawListAtUnit       = gl.DrawListAtUnit
local glVertex               = gl.Vertex
local GetVisibleUnits     = Spring.GetVisibleUnits
local spGetUnitDefID         = Spring.GetUnitDefID
local spGetUnitTeam          = Spring.GetUnitTeam

--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
local LocalTeam = Spring.GetLocalTeamID()
local ENEMY_UNITS       = Spring.ENEMY_UNITS
local texture = "bitmaps/StaticCircle.tga"
local radiusDefs = {}

--------------------------------------------------------------------------------
--------------------------------------------------------------------------------

function widget:Initialize()
	for uDefID, uDef in pairs(UnitDefs) do
		radiusDefs[uDefID] = uDef.radius * 1.25
	end

	SimpleCircle = glCreateList(function()
		glBeginEnd(GL_QUADS, function()
			glTexCoord(0.04,0.04)
			glVertex(-1,10,-1)
			glTexCoord(0.96,0.04)
			glVertex(1,10,-1)
			glTexCoord(0.96,0.96)
			glVertex(1,10,1)
			glTexCoord(0.04,0.96)
			glVertex(-1,10,1)
		end)
	end)
end

function widget:Shutdown()
	glDeleteList(SimpleCircle)
end


function widget:DrawWorldPreUnit()
	glDepthTest(true)
	glTexture(texture)
	glColor(1.0,0,0,1.0)
	local teamID, ud
	local visible = GetVisibleUnits(ENEMY_UNITS, nil, false)

	for i = 1,#visible do
		unitID = visible[i]
		teamID = spGetUnitTeam(unitID)
		if teamID ~= LocalTeam then
			ud = spGetUnitDefID(unitID)
			radius = radiusDefs[ud]
			glDrawListAtUnit(unitID, SimpleCircle, false, radius, 1.0, radius, 0, 0, 0, 0)
		end
	end
	glTexture(false)
end

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function widget:GetInfo()
   return {
      name      = "Enemy Spotter v.2",
      desc      = "Draws smoothed octagon under enemy units",
      author    = "Argh,TradeMark,Trepan",
      date      = "December 17th, 2009",
      license   = "(C) WolfeGames, released under GNU GPL, v2 or later",
      layer     = 5,
      enabled   = false  --  loaded by default?
   }
end

--------------------------------------------------------------------------------
--------------------------------------------------------------------------------

-- Automatically generated local definitions

local glBeginEnd             = gl.BeginEnd
local glTexCoord =   gl.TexCoord
local glTexture = gl.Texture
local GL_QUADS = GL.QUADS
local glColor                = gl.Color
local glCreateList           = gl.CreateList
local glDeleteList           = gl.DeleteList
local glDepthTest            = gl.DepthTest
local glDrawListAtUnit       = gl.DrawListAtUnit
local glVertex               = gl.Vertex
local GetVisibleUnits     = Spring.GetVisibleUnits
local spGetUnitDefID         = Spring.GetUnitDefID
local spGetUnitTeam          = Spring.GetUnitTeam

--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
local LocalTeam = Spring.GetLocalTeamID()
local ENEMY_UNITS       = Spring.ENEMY_UNITS
local texture = "bitmaps/StaticCircle.tga"
local radiusDefs = {}

--------------------------------------------------------------------------------
--------------------------------------------------------------------------------

function widget:Initialize()
   for uDefID, uDef in pairs(UnitDefs) do
      radiusDefs[uDefID] = uDef.radius * 1.25
   end

   SimpleCircle = glCreateList(function()
      glBeginEnd(GL_QUADS, function()
         glTexCoord(0.04,0.04)
         glVertex(-1,10,-1)
         glTexCoord(0.96,0.04)
         glVertex(1,10,-1)
         glTexCoord(0.96,0.96)
         glVertex(1,10,1)
         glTexCoord(0.04,0.96)
         glVertex(-1,10,1)
      end)
   end)
end

function widget:Shutdown()
   glDeleteList(SimpleCircle)
end


function widget:DrawWorldPreUnit()
   glDepthTest(true)
   glTexture(texture)
   glColor(1.0,0,0,1.0)
   local teamID, ud
   local visible = GetVisibleUnits(ENEMY_UNITS, nil, false)

   for i = 1,#visible do
      unitID = visible[i]
      ud = spGetUnitDefID(unitID)
      radius = radiusDefs[ud]
      glDrawListAtUnit(unitID, SimpleCircle, false, radius, 1.0, radius, 0, 0, 0, 0)
   end
   glTexture(false)
end

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if bannana == true
    eat pineapple
else
    for( i=0; i < 100000000000; i++)
        veryexpensiveoperation()
    end for
end if

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local X = 0
function Blah()
    --use X a little bit, so just leave it as an upvalue
end

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local X = 0
function Blah()
    local X_ = X
    --use X_ a *lot*, in a loop, needing a tiny performance tweak that makes the function two lines longer
    X = X_
end