Can we do enum (enum class in C++, table in lua) here? To accomplish that, I can think probably we can add an additional initializer function when we initialize the lua_state_. This initializer basically populate global state with e.g. a global table.

Hm, were you thinking of initializing it here? https://github.com/mindyor/envoy/blob/master/source/extensions/filters/http/lua/lua_filter.cc#L518

lua_newtable/lua_createtable want a lua_State, but I only see lua_state_ which is a ThreadLocalState. Is there a way to unwrap lua_State from here? I'm all ears!

I put a quick hack: https://github.com/envoyproxy/envoy/compare/master...dio:lua-enum?expand=1. Sure you can hack it up to create a registerEnum() function, better macros etc too.

By having that,

I managed to log the enums, within each global "hook" function that we have:

          - name: envoy.filters.http.lua
            typed_config:
              "@type": type.googleapis.com/envoy.config.filter.http.lua.v2.Lua
              inline_code: |
                function envoy_on_request(request_handle)
                  request_handle:logInfo(tostring(EnvoyTimestampResolution.MILLISECOND))
                  request_handle:logInfo(tostring(EnvoyTimestampResolution.NANOSECOND))
                end
                function envoy_on_response(response_handle)
                end

This is the way I register it. A bit ugly.

          [](lua_State* state) {
            lua_newtable(state);
            {
              LUA_ENUM(state, MILLISECOND, Timestamp::Resolution::Millisecond);
              LUA_ENUM(state, NANOSECOND, Timestamp::Resolution::Nanosecond);
            }
            lua_setglobal(state, "EnvoyTimestampResolution");
          },

Hm, I think I see what you're saying, let me give it a shot.

Pushing an implementation per your sketch (thank you for all of the detail!). I will try to come back to this PR a bit later this week (getting pulled into other things) to refactor it.

i think you can do absl::string_view unit_parameter = ... and not have to use the strcmp helpers, relying on empty() and operator== instead

that said id be curious to see if @dio's suggestion makes things cleaner, wdyt?

Haven't quite figured out the enum thing, pushed absl::string_view unit_paramter... change in the meantime.

Hum, should we do enumToInt() here?

if (resolution_as_int_from_state == enumToInt(Timestamp::Resolution::Millisecond)) {
  ...
}