text: /*
  ** $Id: ltm.c,v 2.8.1.1 2007/12/27 13:02:25 roberto Exp $
  ** Tag methods
  ** See Copyright Notice in lua.h
  */
  
  
  #include <string.h>
  
  #define ltm_c
  #define LUA_CORE
  
  #include "lua.h"
  
  #include "lobject.h"
  #include "lstate.h"
  #include "lstring.h"
  #include "ltable.h"
  #include "ltm.h"
  
  
  
  const char *const luaT_typenames[] = {
    "nil", "boolean", "userdata", "number",
    "string", "table", "function", "userdata", "thread",
    "proto", "upval"
  };
  
  
  void luaT_init (lua_State *L) {
    static const char *const luaT_eventname[] = {  /* ORDER TM */
      "__index", "__newindex",
      "__gc", "__mode", "__eq",
      "__add", "__sub", "__mul", "__div", "__mod",
      "__pow", "__unm", "__len", "__lt", "__le",
      "__concat", "__call"
    };
    int i;
    // 初始化meta method
    for (i=0; i<TM_N; i++) {
      G(L)->tmname[i] = luaS_new(L, luaT_eventname[i]);
      luaS_fix(G(L)->tmname[i]);  /* never collect these names CommonHeader.marked 的第FIXEDBIT 5位设置为1  */
    }
  }
  
  
  /*
  ** function to be used with macro "fasttm": optimized for absence of
  ** tag methods
  */
  const TValue *luaT_gettm (Table *events, TMS event, TString *ename) {
    const TValue *tm = luaH_getstr(events, ename);
    
    // 这里这个条件是什么原因?? => 应该是只运行 TM_EQ 及小于 这个的 event 才能使用 快速查找  因为flag只有8bit
    lua_assert(event <= TM_EQ);
    
    if (ttisnil(tm)) {  /* no tag method? */
  	// 如果没有这个meta method，那么标记上，避免下一次再次进入这个函数
      events->flags |= cast_byte(1u<<event);  /* cache this fact */
      return NULL;
    }
    else return tm;
  }
  
  // 只有table和userdata有metatable  然后再看metatable是否有event 比如 __INDEX 
  const TValue *luaT_gettmbyobj (lua_State *L, const TValue *o, TMS event) {
    Table *mt;
    switch (ttype(o)) {
      case LUA_TTABLE:
        mt = hvalue(o)->metatable;
        break;
      case LUA_TUSERDATA:
        mt = uvalue(o)->metatable;
        break;
      default:
        mt = G(L)->mt[ttype(o)];
    } 
    // luaH_getstr 在table里面 找指定key为字符串的value   G(L)->tmname[event]是全局字符串表中的
    return (mt ? luaH_getstr(mt, G(L)->tmname[event]) : luaO_nilobject);
  }
  
  