Here's a bunch of useful Lua snippets I keep always needing, so I decided to put them here!
I'll also probably make a sequel to this the moment I need more snippets

General Lua:

---

Lerp

function Lerp(t, a, b)
	return a * (1 - t) + b * t
end

Pixel grid loop

local w = 32
local h = 32
for i = 0, (w * h) do
	local xc = i % w
	local yc = math.floor(i / w)
	
	-- do stuff
end

Push/Pop Stack

local stack = {}
local currStack = nil
local currStackIdx = 0

local function pushToStack(var)
	currStackIdx = currStackIdx + 1

	stack[currStackIdx] = currStack
	currStack = var
end

local function popFromStack()
	currStack = stack[currStackIdx]

	-- uncomment for cleanup
	-- stack[currStackIdx] = nil

	currStackIdx = currStackIdx - 1
end

local function getActiveFromStack()
	return currStack
end

Rotate a point, then translate

local function rotatePoint(x, y, cX, cY, a)
	local angRad = math.rad(a)

	nX = x * math.cos(angRad) - y * math.sin(angRad) + cX
	nY = x * math.sin(angRad) + y * math.cos(angRad) + cY

	return nX, nY
end

Get calling script path

-- from some forum post
function GetCallingScriptPath()
	local str = debug.getinfo(2, "S").source:sub(2)
	return str:match("(.*/)")
end

GMod specific ones:

---

RT & Material pair

local w = 64
local h = 64
local rt = GetRenderTarget("rt_name_rt", w, h)
local mat = CreateMaterial("rt_name_mat", "UnlitGeneric", {
	["$basetexture"] = rt:GetName(),
	["$vertexcolor"] = 1,
})

Render 2D to a RenderTarget

local filterMode = TEXFILTER.POINT

function RenderOnRT(rt, func)
	local ow, oh = ScrW(), ScrH()
	render.SetViewPort(0, 0, rt:Width(), rt:Height())
	render.PushRenderTarget(rt)
	cam.Start2D()
	render.PushFilterMag(filterMode)
	render.PushFilterMin(filterMode)
	if system.IsLinux() then -- native Linux build fucks up transparency, this is a hack that semi-fixes it?
		render.OverrideAlphaWriteEnable(false, false)
	else
		render.OverrideAlphaWriteEnable(true, true)
	end
		local fine, ret = pcall(func)
		if not fine then
			ErrorNoHaltWithStack(ret)
		end
	render.OverrideAlphaWriteEnable(false)
	render.PopFilterMag()
	render.PopFilterMin()
	cam.End2D()
	render.PopRenderTarget()
	render.SetViewPort(0, 0, ow, oh)
end

Pixel function to a RT (depends on previous snippet!)

-- pixelfunc render
function PixelFuncOnRT(rt, func)
	RenderOnRT(rt, function()
		local oW, oH = ScrW(), ScrH()
		for i = 0, (oW * oH) -1 do
			local xc = i % oW
			local yc = math_floor(i / oW)

			local fine, r, g, b, a = pcall(func, xc, yc)
			if not fine then
				continue
			end

			render.SetViewPort(xc, yc, 1, 1)
			render.Clear(r, g, b, a or 255)
		end
		render.SetViewPort(0, 0, oW, oH)
	end)
end

Offset direction based on EyeAngles

local eyeAng  = LocalPlayer():EyeAngles()
local forward = eyeAng:Forward()
local right   = eyeAng:Right()
local up      = eyeAng:Up()


local offsetLeftRight = (math.random() - .5) * 2
local offsetUpDown = (math.random() - .5) * 2
local newDir = forward + right + up
newDir:Normalize()