Function byteoffset

third-party/lua-5.5.0/src/lutf8lib.c:178–225

** offset(s, n, [i]) -> indices where n-th character counting from ** position 'i' starts and ends; 0 means character at 'i'. */

**   position 'i' starts and ends; 0 means character at 'i'.
*/
static int byteoffset (lua_State *L) {
  size_t len;
  const char *s = luaL_checklstring(L, 1, &len);
  lua_Integer n  = luaL_checkinteger(L, 2);
  lua_Integer posi = (n >= 0) ? 1 : cast_st2S(len) + 1;
  posi = u_posrelat(luaL_optinteger(L, 3, posi), len);
  luaL_argcheck(L, 1 <= posi && --posi <= (lua_Integer)len, 3,
                   "position out of bounds");
  if (n == 0) {
    /* find beginning of current byte sequence */
    while (posi > 0 && iscontp(s + posi)) posi--;
  }
  else {
    if (iscontp(s + posi))
      return luaL_error(L, "initial position is a continuation byte");
    if (n < 0) {
      while (n < 0 && posi > 0) {  /* move back */
        do {  /* find beginning of previous character */
          posi--;
        } while (posi > 0 && iscontp(s + posi));
        n++;
      }
    }
    else {
      n--;  /* do not move for 1st character */
      while (n > 0 && posi < (lua_Integer)len) {
        do {  /* find beginning of next character */
          posi++;
        } while (iscontp(s + posi));  /* (cannot pass final '\0') */
        n--;
      }
    }
  }
  if (n != 0) {  /* did not find given character? */
    luaL_pushfail(L);
    return 1;
  }
  lua_pushinteger(L, posi + 1);  /* initial position */
  if ((s[posi] & 0x80) != 0) {  /* multi-byte character? */
    if (iscont(s[posi]))
      return luaL_error(L, "initial position is a continuation byte");
    while (iscontp(s + posi + 1))
      posi++;  /* skip to last continuation byte */
  }
  /* else one-byte character: final position is the initial one */
  lua_pushinteger(L, posi + 1);  /* 'posi' now is the final position */
  return 2;
}


static int iter_aux (lua_State *L, int strict) {