rope = {}

rope.ActiveRopes = {}

local drag = 0.9
local delta = 1

local function add_vel(a, b)

    -- update the position
    a.pos.x = a.pos.x + a.vel.x
    a.pos.y = a.pos.y + a.vel.y
    a.pos.z = a.pos.z + a.vel.z

    local vx,vy,vz = b.pos.x-a.pos.x, b.pos.y-a.pos.y, b.pos.z-a.pos.z -- offset
    local len = vx*vx + vy*vy + vz*vz -- sqr len

    local force = len * 5

    vx = (vx/len) * force
    vy = (vy/len) * force
    vz = (vz/len) * force

    vx = vx * delta
    vy = vy * delta
    vz = (vz - 1) * delta -- is this the right place to add graivty?

    a.vel.x = a.vel.x * drag
    a.vel.y = a.vel.y * drag
    a.vel.z = a.vel.z * drag

    -- add the new velocity
    a.vel.x = a.vel.x + vx
    a.vel.y = a.vel.y + vy
    a.vel.z = a.vel.z + vz
end

function rope.Think()
    delta = FrameTime()

    for key, rope in pairs(rope.ActiveRopes) do
        local points = rope.points

        for i = 2, #points do
            add_vel(points[i - 1], points[i])
            add_vel(points[i], points[i - 1])
        end

        for i, var in pairs(rope.attachments) do
            local point = rope.points[i]

            if point then
                if type(var) == "function" then
                    local val = var()
                    point.pos.x = val.x
                    point.pos.y = val.y
                    point.pos.z = val.z
                else
                    local val = var
                    point.pos.x = val.x
                    point.pos.y = val.y
                    point.pos.z = val.z
                end

                point.vel.x = 0
                point.vel.x = 0
                point.vel.x = 0
            end
        end
    end
end

function rope.Draw()
    for key, rope in pairs(rope.ActiveRopes) do
        local count = #rope.points

        render.SetMaterial(rope.mat)

        render.StartBeam(count)
            for i = 1, count do
                local v = rope.points[i].pos
                render.AddBeam(Vector(v.x, v.y, v.z), 20, i/count, color_white)
            end
        render.EndBeam()
    end
end

function rope.Create(points, attachments, mat)
    points = points or 128
    mat = mat or Material("cable/rope")
    attachments = attachments or {[1] = function() return LocalPlayer():EyePos() end, [#points] = function() return Vector(0,0,0) end}

    local data = {}

    data.mat = mat
    data.points = {}
    data.attachments = attachments

    local  _,pos = next(attachments)

    if type(pos) == "function" then pos = pos() end

    for i = 1, points do
        data.points[i] = {pos = {x=pos.x+math.Rand(-10,10),y=pos.y+math.Rand(-10,10),z=pos.z+math.Rand(-10,10)}, vel = {x=0,y=0,z=0}}
    end

    local id = table.insert(rope.ActiveRopes, data)

    data.Remove = function() table.remove(rope.ActiveRopes, id) end

    return data
end

hook.Add("Think", "rope", rope.Think)
hook.Add("PostDrawTranslucentRenderables", "rope", rope.Draw)

rope.Create(100, {[1] = function() return Entity(473):GetPos() end, [100] = function() return Entity(474):GetPos() end})