Simple Health Regen

local RegenDelay = 3

local RegenAmount = {
    [10] = 15,
    [25] = 12.5,
    [40] = 10,
    [55] = 7.5,
    [70] = 5,
    [85] = 2.5,
    [100] = 1,
}

local RegenTimers = setmetatable({}, {
    __index = function(self, ply)
        self[ply] = util.Timer()
        return self[ply]
    end
})

local PreviousHealth = setmetatable({}, {
    __index = function(self, ply)
        self[ply] = ply:Health()
        return self[ply]
    end
})

local GetRegenPercent do
    local sorted_amounts = {} do
        for k, v in pairs(RegenAmount) do
            table.insert(sorted_amounts, {k / 100, v / 100})
        end

        table.sort(sorted_amounts, function(a, b)
            return a[1] < b[1]
        end)
    end

    function GetRegenPercent(health_frac)
        for i, v in ipairs(sorted_amounts) do
            if health_frac <= v[1] then
                return v[2]
            end
        end

        return 0
    end
end

hook.Add("PlayerTick", "Health Regen", function(ply)
    local regen_timer = RegenTimers[ply]

    local health = ply:Health()
    local max_health = ply:GetMaxHealth()
    local health_frac = health / max_health

    if health_frac <= 0 and regen_timer:Started() then
        regen_timer:Reset()
    elseif health_frac < 1 then
        if PreviousHealth[ply] < health then
            regen_timer:Start(RegenDelay)
            regen_timer.InitialDelay = true
        end

        if not regen_timer:Started() then
            regen_timer:Start(1 / (max_health * GetRegenPercent(health_frac)))
            regen_timer.InitialDelay = false
        end

        if regen_timer:Elapsed() then
            regen_timer:Reset()

            if not regen_timer.InitialDelay then
                ply:SetHealth(math.min(max_health, health + 1))
            end
        end
    end

    PreviousHealth[ply] = health
end)