utf8.offset

1 files changed, 79 insertions, 0 deletions

diff --git a/src/lutf8lib.js b/src/lutf8lib.js
new file mode 100644
index 0000000..bc43d54
--- /dev/null
+++ b/src/lutf8lib.js
@@ -0,0 +1,79 @@
+"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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