lbitlib.c（5.3.4）解析

/*** $Id: lbitlib.c,v 1.30 2015/11/11 19:08:09 roberto Exp $** Standard library for bitwise operations** See Copyright Notice in lua.h*/#define lbitlib_c#define LUA_LIB#include "lprefix.h"#include "lua.h"#include "lauxlib.h"#include "lualib.h"#if defined(LUA_COMPAT_BITLIB)/* { */#define pushunsigned(L,n)lua_pushinteger(L, (lua_Integer)(n))#define checkunsigned(L,i)((lua_Unsigned)luaL_checkinteger(L,i))/* number of bits to consider in a number */#if !defined(LUA_NBITS)#define LUA_NBITS32#endif/*** a lua_Unsigned with its first LUA_NBITS bits equal to 1. (Shift must** be made in two parts to avoid problems when LUA_NBITS is equal to the** number of bits in a lua_Unsigned.)*/// ALLONES代表的是32位的1，其他位全是0// ALLONES的作用就是过滤32位之外的位，只保留32位数字#define ALLONES(~(((~(lua_Unsigned)0) << (LUA_NBITS - 1)) << 1))// 过滤32位数字/* macro to trim extra bits */#define trim(x)((x) & ALLONES)/* builds a number with 'n' ones (1 <= n <= LUA_NBITS) */#define mask(n)(~((ALLONES << 1) << ((n) - 1)))// 遍历lua栈里面的参数，依次进行与操作（&）// 这里之所以用到了r是为了截取32位之外的数字static lua_Unsigned andaux (lua_State *L) {  int i, n = lua_gettop(L);  lua_Unsigned r = ~(lua_Unsigned)0;  for (i = 1; i <= n; i++)    r &= checkunsigned(L, i);  return trim(r);}// 调用andaux就可以了，编译栈里面的参数，依次执行与（&）操作static int b_and (lua_State *L) {  lua_Unsigned r = andaux(L);  pushunsigned(L, r);  return 1;}// btest运算功能与band类似，不过其返回值为boolean型，用来将结果和0做对比。static int b_test (lua_State *L) {  lua_Unsigned r = andaux(L);  lua_pushboolean(L, r != 0);  return 1;}// b_or操作的核心在于或（|）操作，依然是采用遍历的思想// 再将运算出来的结果进行过滤入栈static int b_or (lua_State *L) {  int i, n = lua_gettop(L);  lua_Unsigned r = 0;  for (i = 1; i <= n; i++)    r |= checkunsigned(L, i);  pushunsigned(L, trim(r));  return 1;}// 和bor操作类似，执行异或（^）操作static int b_xor (lua_State *L) {  int i, n = lua_gettop(L);  lua_Unsigned r = 0;  for (i = 1; i <= n; i++)    r ^= checkunsigned(L, i);  pushunsigned(L, trim(r));  return 1;}// 用~取反就可以static int b_not (lua_State *L) {  lua_Unsigned r = ~checkunsigned(L, 1);  pushunsigned(L, trim(r));  return 1;}// i为负代表右移，为正代表左移// 关键操作是对r的移位赋值static int b_shift (lua_State *L, lua_Unsigned r, lua_Integer i) {  if (i < 0) {  /* shift right? */    i = -i;    r = trim(r);    if (i >= LUA_NBITS) r = 0;    else r >>= i;  }  else {  /* shift left */    if (i >= LUA_NBITS) r = 0;    else r <<= i;    r = trim(r);  }  pushunsigned(L, r);  return 1;}// 左移一个数字若干位static int b_lshift (lua_State *L) {  return b_shift(L, checkunsigned(L, 1), luaL_checkinteger(L, 2));}// 右移一个数字若干位static int b_rshift (lua_State *L) {  return b_shift(L, checkunsigned(L, 1), -luaL_checkinteger(L, 2));}// b_arshift返回a的算术位移，移位为b// i大于0表示右移，小于0表示左移// 右移的时候，如果i大于等于32，则r直接为ALLONES// 否则就分别用逻辑右移的结果和应该补充的1的位数进行或操作// tips：  算术左移同逻辑左移              // 算术右移移入的位用符号位填              // 逻辑右移移入的位用0填static int b_arshift (lua_State *L) {  lua_Unsigned r = checkunsigned(L, 1);  lua_Integer i = luaL_checkinteger(L, 2);  if (i < 0 || !(r & ((lua_Unsigned)1 << (LUA_NBITS - 1))))    return b_shift(L, r, -i);  else {  /* arithmetic shift for 'negative' number */    if (i >= LUA_NBITS) r = ALLONES;    else      r = trim((r >> i) | ~(trim(~(lua_Unsigned)0) >> i));  /* add signal bit */    pushunsigned(L, r);    return 1;  }}// 这个旋转操作就是移位操作，然后把移出去的部分，在进行或运算补充进来// 核心操作在于 r = (r << i) | (r >> (LUA_NBITS - i));// 首先向左移i个单位，然后移出去的部分通过右移（LUA_NBITS - i）的方式保留下来，最后或运算就可以了static int b_rot (lua_State *L, lua_Integer d) {  lua_Unsigned r = checkunsigned(L, 1);  int i = d & (LUA_NBITS - 1);  /* i = d % NBITS */  r = trim(r);  if (i != 0)  /* avoid undefined shift of LUA_NBITS when i == 0 */    r = (r << i) | (r >> (LUA_NBITS - i));  pushunsigned(L, trim(r));  return 1;}static int b_lrot (lua_State *L) {  return b_rot(L, luaL_checkinteger(L, 2));}static int b_rrot (lua_State *L) {  return b_rot(L, -luaL_checkinteger(L, 2));}/*** get field and width arguments for field-manipulation functions,** checking whether they are valid.** ('luaL_error' called without 'return' to avoid later warnings about** 'width' being used uninitialized.)*/// f和w分别是bit32.extract的第二个参数和第三个参数，f默认大于等于0，w一定要大于0// 而且f和w的和不能超过32static int fieldargs (lua_State *L, int farg, int *width) {  lua_Integer f = luaL_checkinteger(L, farg);  lua_Integer w = luaL_optinteger(L, farg + 1, 1);  luaL_argcheck(L, 0 <= f, farg, "field cannot be negative");  luaL_argcheck(L, 0 < w, farg + 1, "width must be positive");  if (f + w > LUA_NBITS)    luaL_error(L, "trying to access non-existent bits");  *width = (int)w;  return (int)f;}// bit32.extract(x,f,w)，该运算返回的结果是从x的f位开始w位数。// 假如执行：bit32.extract(11,1,3)，运算过程是从11的二进制表示的1011第一位开始(整数部分的最右边指的是第0位)，至包含自身的3位，即101组成的整数，为5.// mask(w)的作用是构造一个低位为w个1，其余位为0的32位二进制数字，以此来保存低w位的数字static int b_extract (lua_State *L) {  int w;  lua_Unsigned r = trim(checkunsigned(L, 1));  int f = fieldargs(L, 2, &w);  r = (r >> f) & mask(w);  pushunsigned(L, r);  return 1;}// b_replace的意思是对一个数的某些位执行替换操作// 该运算有4个参数：第一个参数指的是我们要执行运算的数；第二个参数指的是要替换进去的数；后两个参数与extract的后两个参数意思一致：从f位开始共w位，即f至f+w-1位。// print(bit32.replace(11,6,1,3))          --13 // (r & ~(m << f)) 这部分算出的是r除了指定位的数字，此时指定为变为0// ((v & m) << f)  这部分算出的是替换后的r指定位替换后的数字// 然后或运算就可以了，不详细写了，没什么写的，分析一下就出来了static int b_replace (lua_State *L) {  int w;  lua_Unsigned r = trim(checkunsigned(L, 1));  lua_Unsigned v = trim(checkunsigned(L, 2));  int f = fieldargs(L, 3, &w);  lua_Unsigned m = mask(w);  r = (r & ~(m << f)) | ((v & m) << f);  pushunsigned(L, r);  return 1;}static const luaL_Reg bitlib[] = {  {"arshift", b_arshift},  {"band", b_and},  {"bnot", b_not},  {"bor", b_or},  {"bxor", b_xor},  {"btest", b_test},  {"extract", b_extract},  {"lrotate", b_lrot},  {"lshift", b_lshift},  {"replace", b_replace},  {"rrotate", b_rrot},  {"rshift", b_rshift},  {NULL, NULL}};LUAMOD_API int luaopen_bit32 (lua_State *L) {  luaL_newlib(L, bitlib);  return 1;}#else/* }{ */LUAMOD_API int luaopen_bit32 (lua_State *L) {  return luaL_error(L, "library 'bit32' has been deprecated");}#endif/* } */