local Sod = {}
Sod.__index = Sod

function Sod:update(dt)
    local target_vel = (self.target-self.prev_target)/dt

    self.pos += self.vel*dt

    self.vel += (
        self.target
        +self.k3*target_vel
        -self.pos
        -self.k1*self.vel
    )*dt/self.k2

    self.prev_target = self.target
end

function Sod:config(frequency_rads,damping,response)
    local ifreq = 1/frequency_rads
    self.k1,self.k2,self.k3 =
        damping*ifreq*2,
        ifreq*ifreq,
        response*damping*ifreq
end

function sod(pos,frequency_rads,damping,response)
    local o = {
        pos = pos,
        prev_target = pos,
        target = pos,
        -- In case pos is a vector, or any other arithmetically capable data
        -- type, *0 guarantees we get the zero position of whatever type pos is.
        vel = pos*0,
    }
    setmetatable(o,Sod)
    o:config(frequency_rads,damping,response)
    return o
end