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util.strbitop: Add common_prefix_bits() method

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This returns the number of bits that two strings have in common. It is
significantly more efficient than similar calculations in Lua.
This commit is contained in:
Matthew Wild 2024-02-23 12:08:37 +00:00
parent 1606675762
commit 7f748556a2
2 changed files with 81 additions and 0 deletions
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37
util-src/strbitop.c
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@ -8,6 +8,8 @@
#include <lua.h>
#include <lauxlib.h>
#include <sys/param.h>
#include <limits.h>
/* TODO Deduplicate code somehow */
@ -74,11 +76,46 @@ static int strop_xor(lua_State *L) {
return 1;
}
unsigned int clz(unsigned char c) {
#if __GNUC__
return __builtin_clz((unsigned int) c) - ((sizeof(int)-1)*CHAR_BIT);
#else
if(c & 0x80) return 0;
if(c & 0x40) return 1;
if(c & 0x20) return 2;
if(c & 0x10) return 3;
if(c & 0x08) return 4;
if(c & 0x04) return 5;
if(c & 0x02) return 6;
if(c & 0x01) return 7;
return 8;
#endif
}
LUA_API int strop_common_prefix_bits(lua_State *L) {
size_t a, b, i;
const char *str_a = luaL_checklstring(L, 1, &a);
const char *str_b = luaL_checklstring(L, 2, &b);
size_t min_len = MIN(a, b);
for(i=0; i<min_len; i++) {
if(str_a[i] != str_b[i]) {
lua_pushinteger(L, i*8 + (clz(str_a[i] ^ str_b[i])));
return 1;
}
}
lua_pushinteger(L, i*8);
return 1;
}
LUA_API int luaopen_prosody_util_strbitop(lua_State *L) {
luaL_Reg exports[] = {
{ "sand", strop_and },
{ "sor", strop_or },
{ "sxor", strop_xor },
{ "common_prefix_bits", strop_common_prefix_bits },
{ NULL, NULL }
};