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"use strict";
const assert = require('assert');
const lua = require('./lua.js');
const lapi = require('./lapi.js');
const lauxlib = require('./lauxlib.js');
const iscont = function(p) {
return p & 0xC0 === 0x80;
};
/* translate a relative string position: negative means back from end */
const u_posrelat = function(pos, len) {
if (pos >= 0) return pos;
else if (0 - pos > len) return 0;
else return len + pos + 1;
};
/*
** offset(s, n, [i]) -> index where n-th character counting from
** position 'i' starts; 0 means character at 'i'.
*/
const byteoffset = function(L) {
let s = lauxlib.luaL_checkstring(L, 1);
let n = lauxlib.luaL_checkinteger(L, 2);
let posi = n >= 0 ? 1 : s.length + 1;
posi = u_posrelat(lauxlib.luaL_optinteger(L, 3, posi), s.length);
lauxlib.luaL_argcheck(L, 1 <= posi && --posi <= s.length, 3, "position ot ouf range");
if (n === 0) {
/* find beginning of current byte sequence */
while (posi > 0 && iscont(s.slice(posi))) posi--;
} else {
if (iscont(s.slice(posi)))
lauxlib.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 && iscont(s.slice(posi)));
n++;
}
} else {
n--; /* do not move for 1st character */
while (n > 0 && posi < s.length) {
do { /* find beginning of next character */
posi++;
} while (iscont(s.slice(posi))); /* (cannot pass final '\0') */
n--;
}
}
}
if (n === 0) /* did it find given character? */
lapi.lua_pushinteger(L, posi + 1);
else /* no such character */
lapi.lua_pushnil(L);
return 1;
};
const funcs = {
"offset": byteoffset
};
/* pattern to match a single UTF-8 character */
const UTF8PATT = "[\0-\x7F\xC2-\xF4][\x80-\xBF]*";
const luaopen_utf8 = function(L) {
lauxlib.luaL_newlib(L, funcs);
lapi.lua_pushstring(L, UTF8PATT);
lapi.lua_setfield(L, -2, "charpattern");
return 1;
};
module.exports.luaopen_utf8 = luaopen_utf8;
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