Lua templating/Converting Wikitext templates

Collected here are tips for converting Wikitext-based templates to Lua.
==Switch statements==
One of the most common cases of a slow template is due to a large {{#switch:}} statement, which are often called multiple times on the same page. By making use of mw.loadData and a simple table of data, we can make these templates significantly faster. By using a simple Lua table of data, we move to using a roughly O(1) hash table lookup, and by using mw.loadData(), the data will only be loaded once in a page load no matter how many times Module:FooBar is invoked on a page. For a switch statement with 150 items, converting it to Lua in this way resulted in an approximately 50% improvement in parse time.
For example, if you have a template that uses a switch statement like so:
{{#switch:{{{1|}}}
| case 1 = Foo
| case 2 = Bar
| case 3
| case 4 = FooBar
| #default: {{{1|}}}
}}
To convert it to Lua, create two module pages.
Module:FooBar:
local p = {}
local fooBarData = mw.loadData( 'Module:FooBar/data' )
function p.fooBar( frame )
	local fooBarText = frame.args[1]
	return fooBarData[fooBarText] or fooBarData["#default"] or ""
end
return p
Module:FooBar/data:
return {
	["case 1"] = "Foo",
	["case 2"] = "Bar",
	["case 3"] = "FooBar",
	["case 4"] = "FooBar",
}
===== Using =====
 {{#invoke:fooBar|fooBar|case 1}}
Output:
 Foo
==Conditional  statements (#if)==
Many templates make use of "#if" parser functions resulting in a lot of nested functions, which may make the code very complex and hard to understand. This can be converted and made more efficient using Lua. See live demo.
For example, to create template that checks if a template argument contains "text", one would write something like:
{{#if:{{{1|}}}|You entered text|No text}}
This can be converted to: 
Module:if :
local p = {}
function p.main( frame )
	local tArgs = frame:getParent()
	if not(tArgs.args[1]) then
		tArgs = frame
	end
	local sText = tArgs.args[1]
	local sTrueAction = tArgs.args[2] or tArgs.args["true"]
	local sFalseAction = tArgs.args[3] or tArgs.args["false"]
	if sText == nil or sText == "" then
		sText = mw.getCurrentFrame():preprocess(sFalseAction)
	else
		sText = mw.getCurrentFrame():preprocess(sTrueAction)
	end
	return sText
end
return p
This can be applied to complex "#if" based parser functions, and would be much easier to understand.
===Check if article exists (#ifexists)===
Create Module:ifexists :
local p = {}
function p.exists( frame )
	local tArgs = frame:getParent()
	if not(tArgs.args[1]) then
		tArgs = frame
	end
	local sText = tArgs.args[1]
	local sTrueAction = tArgs.args[2] or tArgs.args["true"]
	local sFalseAction = tArgs.args[3] or tArgs.args["false"]
	if sText and sText ~= "" then
		if mw.title.new( sText ).exists then
			return sTrueAction
		else 
			return sFalseAction
		end
	end
end
return p
Then simply use {{#invoke:existpage|exists|pagename|yep|nop}} .
Note: This sadly still remains an expensive function.
One can remove the "expensive" nature of the above function by using the hack below. However, this hack can actually be worse because it transcludes a whole page.
local p = {}
function p.exists(frame)
 local sPage = frame.args[1]
 if sPage then 
 local num = frame:preprocess('{{:'..sPage..'}}')
 if string.sub(num,1,3) ~="[[:" then
 return frame.args[2] or frame.args["true"]
 end
 end
 return frame.args[3] or frame.args["false"]
end
return p
This method is not recommended.
However, yet another approach can be used to inexpensively check if a module exists:
local p = {}
function p.exists(frame)
 local sPage = frame.args[1]
 if sPage then 
 local _, val = pcall(package.loaders[2], sPage)
 if type(val) == "function" or type(val) == "string" then
 return frame.args[2] or frame.args["true"]
 end
 end
 return frame.args[3] or frame.args["false"]
end
return p
==Loop Statements (#for)==
One way to reduce the template size, and to make it load faster and more efficiently is to reduce repeated text. This can typically be achieved by using loops or using Lua.
A template that repeats some text can be created using the loops extension by using:
Template:foo
{{ #for: {{{n}}} | {{{list$n$}}} ♣ }}
Template use:
{{ foo: |n=4 | list1=a | list2=b | list3=c | list4=d }}
Output:
a♣b♣c♣d♣
This can be converted to Module:loop:
local p = {}
function p.appendloop( frame )
 local tArgs = frame:getParent()
 if not(tArgs.args[1]) then
 tArgs = frame
 end
	local sOutputText = ""
	for iKey,sValue in pairs(tArgs.args) do 
 sOutputText = sOutputText .. sValue .. "♣"
 end
	return sOutputText 
end
return p
Template:foo :
{{#invoke:loop|appendloop}}
Calling template foo with these arguments:
{{foo|a|b|c|d}}
Results in :
a♣b♣c♣d♣
==Time function (#time)==
Time functions can be converted with the following module:time :
local p = {}
 
function p.getdate( frame )
 local sText = frame.args[1]
 local sTimestamp = frame.args[2]
 local sLangCode = frame.args[3]
 local lang = mw.language.new(sLangCode or "en")
 
 if sText then 
 return lang:formatDate(sText, sTimestamp)
 end
end
return p
This can then be called using :
{{#invoke:time|getdate|Y-m-d|2013-10-04|fr}}
This shows the date, e.g. 2013-10-04. The special characters used to get this date can be found in mediawiki.
==Text manipulation functions ==
The table below lists some common text manipulation functions (StringFunctions) commonly used in wikis. Some of these are available in the Global Lua Modules/String.
===Length (#len)===
<tabber>
|-|Wikitext=
{{#len:Icecream}}
|-|Lua=
local text = "Icecream"
local results = string.len(text)
print(results)
</tabber>
Output: 8
===Position (#pos)===
<tabber>
|-|Wikitext=
{{#pos:Žmržlina|lina}}
|-|Lua=
local text = "Žmržlina"
local results, e = string.find(text, "lina")
print(#text + 1 - results)
</tabber>
Output: 4
===Substring (#sub)===
<tabber>
|-|Wikitext=
{{#sub:Icecream|0|3}}
|-|Lua=
local text = "Icecream"
local results = string.sub(text, 1, 3)
print(results)
</tabber>
Output: Ice
===Repeat (#pad)===
<tabber>
|-|Wikitext=
{{#pad:Ice|5|x|left}}
|-|Lua=
local text = "Ice"
local results = string.rep("x", 5 - #text) .. text
print(results)
</tabber>
Output: xxIce
===Replace (#replace)===
<tabber>
|-|Wikitext=
{{#replace:Žmržlina|ž|z}}
|-|Lua=
local text = "Žmržlina"
local results = string.gsub(text ,"ž","z")
print(results)
</tabber>
Output: Žmrzlina
===Explode (#explode)===
The "Explode" function can divide a piece of text and show the part needed. This is a parser function that was a subject of much argument and debate over close to a decade or more  and one of the reasons for existence of scribuntohttps://phabricator.wikimedia.org/T8455. This was primarily due to its useful ability to extract sections or parts of a text.
Its syntax is be something like:
{{#explode:And if you tolerate this| |2}} -> you
Scribunto is hundreds of times more efficienthttps://en.wikipedia.org/wiki/User:Dragons_flight/Lua_performance, and can be written as:
local text = "And if you tolerate this"
local tab = mw.text.split( text, " ")
print(tab[3])
Output: you
===Extracting text===
One can even do more complex things such as extracting arbitrary text using lua patterns :
local text = "Jameson, J. (2013). E-Leadership in higher education"
local surname,initials, year = string.match( text, "(%a*)%s*,%s*(%S*)%s*(%b())")
print( year, initials, surname)
(2013) J. Jameson
===Urlencode #urlencode===
<tabber>
|-|Wikitext=
{{urlencode:x:y/z á é}}
|-|Lua=
local results = mw.uri.encode(("x:y/z á é")
print(results)
</tabber>
Output: x%3Ay%2Fz+%C3%A1+%C3%A9
===Urldecode #urldecode===
<tabber>
|-|Wikitext=
{{urldecode:x%3Ay%2Fz+%C3%A1+%C3%A9}}
|-|Lua=
local results = mw.uri.decode("x%3Ay%2Fz+%C3%A1+%C3%A9","QUERY")
print(results)
</tabber>
Output: x:y/z á é
==Number formatting==
There are a number of parser functions (see this) to format numbers such as Extension:Formatnum and  Extension:Parser functions. Despite having the exact same name, they have different syntax and are largely incompatible. 
=== Formatnum ===
==== Wikitext ====
{{formatnum:1231231}}
==== Lua ====
local number = 1231231
local noCommas = false
local lang = mw.language.new("en")
local result = lang:formatNum(number, {noCommafy=noCommas})
mw.log(result)
Output : 1,231,231
==See also==
*Comparable Lua functions to wikitext
*Lua templating/Snippets
==References==
Converting Wikitext templates