Reconstruction of AddFlags/RemoveFlags/IsFlagSet on Garry's Mod Lua...

m_fFlags.lua

local ENTITY = FindMetaTable("Entity")
local Entity_GetInternalVariable = ENTITY.GetInternalVariable
local Entity_AddFlags, Entity_RemoveFlags = ENTITY.AddFlags, ENTITY.RemoveFlags
local bit_bor, bit_bnot, bit_band = bit.bor, bit.bnot, bit.band
function ENTITY.AddFlags(this, flagMask)
    assert(IsValid(this), "attempt to AddFlags on NULL entity")
    local m_fFlags = Entity_GetInternalVariable(this, "m_fFlags")
    Entity_AddFlags(this, bit_bor(m_fFlags, flagMask))
end
function ENTITY.RemoveFlags(this, flagMask)
    assert(IsValid(this), "attempt to RemoveFlags on NULL entity")
    local m_fFlags = Entity_GetInternalVariable(this, "m_fFlags")
    Entity_RemoveFlags(this, bit_band(m_fFlags, bit_bnot(flagMask)))
end
function ENTITY.IsFlagSet(this, flagMask)
    if IsValid(this) then
        local m_fFlags = Entity_GetInternalVariable(this, "m_fFlags")
        return bit_band(m_fFlags, flagMask) ~= 0
    end
    return false
end