MT

minetest.calculate_knockback(function(player, hitter, time_from_last_punch, tool_capabilities, dir, distance, damage)
    local in_bed = beds.player
    local name = player:get_player_name()

    if damage == 0 or player:get_armor_groups().immortal or in_bed[name] then
        return 0.0
    end

    local m = 8
    -- solve m - m*e^(k*4) = 4 for k
    local k = -0.17328
    local res = m - m * math.exp(k * damage)

    if distance < 2.0 then
        res = res * 1.1 -- more knockback when closer
    elseif distance > 4.0 then
        res = res * 0.9 -- less when far away
    end
    return res
end)