Sand particle simulation

local Vector2 = require"Vector2"
local Grid = require"Grid"
require"utility"

local particleSize = Vector2.new(2, 2)
local gridSize = Vector2.new(600, 500)
local gravity = Vector2.new(0, 30)
local gridRoot = Vector2.new()

local particleGrid = Grid.new("static", gridRoot, particleSize, gridSize); particleGrid:outline()
local gridCells = particleGrid.cells

local particleLists = {
    ["solid"] = {length = 0},
    ["liquid"] = {length = 0},
}

local solidParticlesList = particleLists["solid"]

local numberOfParticles = 0 --// All together

local particleDatabase = {
    ["sand"] = {
        possibleColors = {
            Color.new(255, 236, 26),
            Color.new(255, 239, 115),
            Color.new(255, 200, 61),
            Color.new(255, 183, 74),

        },
        type = "solid",

    }
}

local function isCellEmpty(cell)
    return cell and cell.particle == nil and cell
end

local function moveParticle(particle, newCell)
    local oldCell = particle.cell
    oldCell.particle = nil

    particle.cell = newCell
    newCell.particle = particle
end

local function getRandomDirection()
    local random = math.random()

    return random == 1 and 1 or -1
end

local function getCell(x, y)
    local cellX = gridCells[x]

    return cellX and cellX[y]
end

local function simulateSolidParticles()
    for i = solidParticlesList.length, 1, -1 do
        local particle = solidParticlesList[i]
        local cell = particle.cell
        local cellPos = cell.pos
        local freeFalling = particle.freeFalling

        local posX, posY = cellPos.x, cellPos.y

        local cellBellow = getCell(posX, posY+1)

        if isCellEmpty(cellBellow) then
            moveParticle(particle, cellBellow)

            freeFalling = true
        else
            freeFalling = false
            local diagonalCell = isCellEmpty(getCell(posX-1, posY+1)) or isCellEmpty(getCell(posX+1, posY+1))

            if diagonalCell then
                moveParticle(particle, diagonalCell)
            end

        end

        ::continue::
        particle.freeFalling = freeFalling
    end
end

local Particle = {
    grid = particleGrid,
    particleSize = particleSize,

}

function Particle.new(particleId, cellPos, ...)
    local particleData = particleDatabase[particleId]
    local particleType = particleData.type
    local possibleColors = particleData.possibleColors

    local particle = {
        type = particleType,
        cell = particleGrid:getCell(cellPos),
        color = possibleColors[math.random(1, #possibleColors)],

        freeFalling = true,
    }

    local particleList = particleLists[particleType]
    local listLength = particleList.length + 1; particleList.length = listLength

    --// insert into list for the particle to update
    particleList[listLength] = particle

    numberOfParticles = numberOfParticles + 1

    return particle
end

function Particle.update(timeElapsed)
    simulateSolidParticles()
end

function Particle.draw()
    love.graphics.print("Particles: "..numberOfParticles, 10, gridSize.y +20, 0)
    love.graphics.print("FPS: "..love.timer.getFPS(), 10, gridSize.y + 40, 0)

    for _, list in pairs(particleLists) do
        for i = 1, list.length do
            local particle = list[i]
            local worldPos = particleGrid:cellToWorldPos(particle.cell.pos)
            local color = particle.color

            love.graphics.setColor(color.r/255, color.g/255, color.b/255)
            love.graphics.rectangle("fill", worldPos.x + particleSize.x/2, worldPos.y + particleSize.y/2, particleSize.x, particleSize.y)
        end
    end
end

return Particle