/*
** Header for string value; string bytes follow the end of this structure
** (aligned according to 'UTString'; see next).
*/
typedef struct TString {
? CommonHeader;
? lu_byte extra;? /* reserved words for short strings; "has hash" for longs */
? lu_byte shrlen;? /* length for short strings */
? unsigned int hash;
? union {
? ? size_t lnglen;? /* length for long strings */
? ? struct TString *hnext;? /* linked list for hash table */
? } u;
} TString;
lua里的字符串表示，TString只記錄了字符串頭部，真正的字符內(nèi)容存儲(chǔ)在TString內(nèi)存對(duì)象后面扯夭，并以\0結(jié)尾，所以一個(gè)字符串的真正長(zhǎng)度是sizeof(TString) + strlen + 1咆爽，所以獲得字符串實(shí)際內(nèi)容的函數(shù) #define getstr(ts) cast(const char *, (ts) + 1)呛凶，TString地址+1即可。
為效率考慮，TString分長(zhǎng)短兩種情況：字符串長(zhǎng)度<=LUAI_MAXSHORTLEN(40)的被是短串，反之是長(zhǎng)串晾咪。短字符串存儲(chǔ)在global_State中的strt域中昼蛀，hash存儲(chǔ)叼旋，沖突的key采用油讯、可動(dòng)態(tài)增長(zhǎng)的鏈表由捎；長(zhǎng)字符串作為正常的GC對(duì)象存儲(chǔ)在global_State的allgc域中为居。
創(chuàng)建字符串前呜象，會(huì)先在 global_State的strcache中查找恭陡，這是一個(gè)記錄著TString地址的二維數(shù)組蹬音，x方向使用hash存儲(chǔ)，取模運(yùn)算休玩，如果找到，則直接返回罚随，否則接著創(chuàng)建字符串流程。短串創(chuàng)建前會(huì)首先在strt中查找趾疚，如果找到（此時(shí)會(huì)處理該字符串被標(biāo)記為可GC，但還未GC的情況赡磅，需要重置為白色changewhite）袒餐，則直接返回灸眼；否則創(chuàng)建短串泄隔，并掛載到strt域中。由此可見湾揽，短字符串是惟一的精居，不會(huì)存在重復(fù)的字符串，而長(zhǎng)串則會(huì)出現(xiàn)重復(fù)的情況搜贤。
CommonHeader：表明TString是個(gè)需要GC的結(jié)構(gòu)谆沃，
extra：在短字符串中只給系統(tǒng)保留關(guān)鍵字使用，記錄著對(duì)應(yīng)?luaX_tokens的索引值仪芒，其他短字符串值為0
保留關(guān)鍵字如下
/* ORDER RESERVED */
static const char *const luaX_tokens [] = {
? ? "and", "break", "do", "else", "elseif",
? ? "end", "false", "for", "function", "goto", "if",
? ? "in", "local", "nil", "not", "or", "repeat",
? ? "return", "then", "true", "until", "while",
? ? "http://", "..", "...", "==", ">=", "<=", "~=",
? ? "<<", ">>", "::", "<eof>",
? ? "<number>", "<integer>", "<name>", "<string>"
};
在長(zhǎng)字符串中唁影，字符串的hash值不是在創(chuàng)建時(shí)計(jì)算，而是在被當(dāng)做key來(lái)使用時(shí)計(jì)算掂名，extra==1則表示已經(jīng)計(jì)算過hash值据沈，否則計(jì)算hash值。
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