A curve metatable for lua.

curve.lua

local Curve = {}

function Curve:get(time) 
    for i=2,#self.keys,1 do    
        local nextKey = self.keys[i]
        local preKey = self.keys[i-1]

        if (nextKey.time == time) then
            return nextKey.value
        end

        if (nextKey.time > time) then    
            local f = Curve.InverseLerp(preKey.time,nextKey.time,time)

            return Curve.Lerp(preKey.value,nextKey.value,f)
        end   
    end      

    return 0
end
 
function Curve:addKey(time,value) 
    table.insert(self.keys,{
        time = time,
        value = value,
    })

    table.sort(self.keys,function(a,b)
        return a.time < b.time
    end)
end

function Curve:flat() 
    local values = {}

    for i=1,#self.keys,1 do
        table.insert(values,self.keys[i].value)
    end

    return values
end

Curve.Lerp = function(a,b,f)
    return a + ( b - a ) * Curve.Clamp01(f)
end

Curve.InverseLerp = function(a,b,f)
    return Curve.Clamp01((f - a) / (b - a))
end

Curve.Clamp01 = function(f)
    return math.min(1,math.max(0,f))
end

Curve.__index = Curve

setmetatable(Curve,{
    __call = function(self,t)
        local keys = t or {}

        table.sort(keys,function(a,b)
            return a.time < b.time
        end)

        return setmetatable({keys = keys},self)
    end
})

--[[

local curve = Curve()

curve:addKey(0.00, 0.00)
curve:addKey(1.00, 1.00)
curve:addKey(0.50, 0.75)

print( curve:get(0.00) )
print( curve:get(0.25) )
print( curve:get(0.50) )
print( curve:get(0.75) )
print( curve:get(1.00) )

> 0.0
> 0.375
> 0.75
> 0.875
> 1.0

--]]