lua-5.1.1中的字符串实现

lstring.c

void luaS_resize (lua_State *L, int newsize) {
  GCObject **newhash;
  stringtable *tb;
  int i;
  if (G(L)->gcstate == GCSsweepstring)
    return;  /* cannot resize during GC traverse */
  newhash = luaM_newvector(L, newsize, GCObject *);
  tb = &G(L)->strt;
  for (i=0; i<newsize; i++) newhash[i] = NULL;
  /* rehash */
  for (i=0; i<tb->size; i++) {
    GCObject *p = tb->hash[i];
    while (p) {  /* for each node in the list */
      GCObject *next = p->gch.next;  /* save next */
      unsigned int h = gco2ts(p)->hash;
      int h1 = lmod(h, newsize);  /* new position */
      lua_assert(cast_int(h%newsize) == lmod(h, newsize));
      p->gch.next = newhash[h1];  /* chain it */
      newhash[h1] = p;
      p = next;
    }
  }
  luaM_freearray(L, tb->hash, tb->size, TString *);
  tb->size = newsize;
  tb->hash = newhash;
}

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)];//如果这里已经有元素，则使用链表串接起来
       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 */
}

static TString *newlstr (lua_State *L, const char *str, size_t l,
                                       unsigned int h) {
  TString *ts;
  stringtable *tb;
  if (l+1 > (MAX_SIZET - sizeof(TString))/sizeof(char))
    luaM_toobig(L);
  ts = cast(TString *, luaM_malloc(L, (l+1)*sizeof(char)+sizeof(TString)));
  ts->tsv.len = l;
  ts->tsv.hash = h;
  ts->tsv.marked = luaC_white(G(L));
  ts->tsv.tt = LUA_TSTRING;
  ts->tsv.reserved = 0;
  memcpy(ts+1, str, l*sizeof(char));
  ((char *)(ts+1))[l] = '\0';  /* ending 0 */
  tb = &G(L)->strt;
  h = lmod(h, tb->size);
  ts->tsv.next = tb->hash[h];  /* chain new entry */
  tb->hash[h] = obj2gco(ts);
  tb->nuse++;
  if (tb->nuse > cast(lu_int32, tb->size) && tb->size <= MAX_INT/2)
    luaS_resize(L, tb->size*2);  /* too crowded */
  return ts;
}

lstring.h

/*
@@ LUAI_USER_ALIGNMENT_T is a type that requires maximum alignment.
** CHANGE it if your system requires alignments larger than double. (For
** instance, if your system supports long doubles and they must be
** aligned in 16-byte boundaries, then you should add long double in the
** union.) Probably you do not need to change this.
*/
#define LUAI_USER_ALIGNMENT_T	union { double u; void *s; long l; }

/* type to ensure maximum alignment */
typedef LUAI_USER_ALIGNMENT_T L_Umaxalign;

/*
** String headers for string table
*/
typedef union TString {
  L_Umaxalign dummy;  /* ensures maximum alignment for strings */
  struct {
    CommonHeader;
    lu_byte reserved;//是否是Lua虚拟机中的保留字符串，1不会在GC阶段被回收
    unsigned int hash;//字符串的散列值
    size_t len;//字符串长度 
  } tsv;
} TString;

union GCObject {
  GCheader gch;
  union TString ts;
};

typedef struct stringtable {
  GCObject **hash;//这是一个散列数组，专门用于存放字符串
  lu_int32 nuse;  /* number of elements */
  int size;
} stringtable;

typedef struct global_State {
  stringtable strt;  /* hash table for strings */
} global_State;

lstring.md

lstring

这是从lua-5.1.1中分离出来的字符串实现代码。很好地利用散列值优化。只需要一次整数的比较，即可实现字符串的比较。这个实现还有另一大好处，那就是空间优化，多份。

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lua字符串