lua字符串类型
Lua中字符串结构体的定义是: typedef union TString { L_Umaxalign dummy; /* ensures maximum alignment for strings */ struct { CommonHeader; lu_byte reserved; unsigned int hash; size_t len; } tsv; } TString; 这里TString结构体是一个union,最开始的L_Umaxalign dummy;起到的是对齐作用.紧跟着是CommonHeader,可以看出TString也是可GC数据类型的一种. 在Lua中,字符串是一个保存在一个全局的地方,在globale_state的strt里面,这是一个hash数组,专门用于存放字符串: typedef struct stringtable { GCObject **hash; lu_int32 nuse; /* number of elements */ int size; } stringtable; 一个字符串TString,首先根据hash算法算出hash值,这就是stringtable中hash的索引值,如果这里已经有元素,则使用链表串接起来. 同时,TString中的字段reserved,表示这个字符串是不是保留字符串,比如Lua的关键字,在最开始赋值的时候是这么处理的: void luaX_init (lua_State *L) { int i; for (i=0; itsv.reserved = cast_byte(i+1); /* reserved word */ } } 这里存放的值,是数组luaX_tokens中的索引: const char *const luaX_tokens [] = { "and","break","do","else","elseif","end","false","for","function","if","in","local","nil","not","or","repeat","return","then","true","until","while","..","...","==",">=","<=","~=","",NULL }; 一方面可以迅速定位到是哪个关键字,另方面如果这个reserved字段不为0,则表示该字符串是不可自动回收的,在GC过程中会略过这个字符串的处理. 具体查找字符串时,首先计算出hash值,定位到所在的strt中的hash数组所在,再遍历hash桶所在链表,首先比较长度,如果相同再继续逐字节的比较字符串内容: TString *luaS_newlstr (lua_State *L,const char *str,size_t l) { GCObject *o; unsigned int h = cast(unsigned int,l); /* seed */ size_t step = (l>>5)+1; /* if string is too long,don‘t hash all its chars */ size_t l1; for (l1=l; l1>=step; l1-=step) /* compute hash */ h = h ^ ((h<<5)+(h>>2)+cast(unsigned char,str[l1-1])); for (o = G(L)->strt.hash[lmod(h,G(L)->strt.size)]; o != NULL; o = o->gch.next) { TString *ts = rawgco2ts(o); if (ts->tsv.len == l && (memcmp(str,getstr(ts),l) == 0)) { /* string may be dead */ if (isdead(G(L),o)) changewhite(o); return ts; } } return newlstr(L,str,l,h); /* not found */ } (编辑:李大同) 【声明】本站内容均来自网络,其相关言论仅代表作者个人观点,不代表本站立场。若无意侵犯到您的权利,请及时与联系站长删除相关内容! |