src/loslib.js: implement os.setlocale

As suggested in #136 (comment) by @Egor-Skriptunoff

src/loslib.js

@@ -29,6 +29,7 @@ const {
     luaL_argerror,
     luaL_buffinit,
     luaL_checkinteger,
+    luaL_checkoption,
     luaL_checkstring,
     luaL_checktype,
     luaL_error,
@@ -451,9 +452,28 @@ const os_difftime = function(L) {
     return 1;
 };
 
+const catnames = ["all", "collate", "ctype", "monetary", "numeric", "time"].map((lc) => to_luastring(lc));
+const C = to_luastring("C");
+const POSIX = to_luastring("POSIX");
+const os_setlocale = function(L) {
+    const l = luaL_optstring(L, 1, null);
+    luaL_checkoption(L, 2, "all", catnames);
+    /* It is not possible to set the JS-VM wide locale, so we say that we only
+       know the C locale. The "POSIX" locale is defined in
+       IEEE Std 1003.1-2017 Section 7.2 as equivalent to "C" */
+    lua_pushstring(L, (
+        l === null /* passing nil returns the current locale; which is "C" */
+        || l.length == 0 /* empty string resets to the default locale; which is "C" */
+        || luastring_eq(l, C) /* user passed "C" */
+        || luastring_eq(l, POSIX) /* user passed "POSIX", equivalent to "C" */
+    ) ? C : null);
+    return 1;
+};
+
 const syslib = {
     "date": os_date,
     "difftime": os_difftime,
+    "setlocale": os_setlocale,
     "time": os_time
 };
 

test/loslib.test.js

@@ -128,6 +128,23 @@ test('os.time normalisation of table', () => {
 });
 
 
+test('os.setlocale', () => {
+    let L = lauxlib.luaL_newstate();
+    if (!L) throw Error("failed to create lua state");
+
+    let luaCode = `
+        assert("C" == os.setlocale())
+        assert("C" == os.setlocale(""))
+        assert("C" == os.setlocale("C"))
+        assert("C" == os.setlocale("POSIX"))
+        assert(nil == os.setlocale("any_other_locale"))
+    `;
+    lualib.luaL_openlibs(L);
+    expect(lauxlib.luaL_loadstring(L, to_luastring(luaCode))).toBe(lua.LUA_OK);
+    lua.lua_call(L, 0, 0);
+});
+
+
 test('os.getenv', () => {
     let L = lauxlib.luaL_newstate();
     if (!L) throw Error("failed to create lua state");