string.byte

1 files changed, 33 insertions, 5 deletions

diff --git a/src/lstrlib.js b/src/lstrlib.js
index 3db3e5e..376d379 100644
--- a/src/lstrlib.js
+++ b/src/lstrlib.js
@@ -6,6 +6,13 @@ const lua     = require('./lua.js');
 const lapi    = require('./lapi.js');
 const lauxlib = require('./lauxlib.js');
 
+/* translate a relative string position: negative means back from end */
+const posrelat = function(pos, len) {
+    if (pos >= 0) return pos;
+    else if (0 - pos > len) return 0;
+    else return len + pos + 1;
+};
+
 const str_len = function(L) {
     lapi.lua_pushinteger(L, lauxlib.luaL_checkstring(L, 1).length);
     return 1;
@@ -47,13 +54,34 @@ const str_rep = function(L) {
     return 1;
 };
 
+const str_byte = function(L) {
+    let s = lauxlib.luaL_checkstring(L, 1);
+    s = L.stack[lapi.index2addr_(L, 1)].value;
+    let l = s.length;
+    let posi = posrelat(lauxlib.luaL_optinteger(L, 2, 1), l);
+    let pose = posrelat(lauxlib.luaL_optinteger(L, 3, posi), l);
+
+    if (posi < 1) posi = 1;
+    if (pose > l) pose = l;
+    if (posi > pose) return 0;  /* empty interval; return no values */
+    if (pose - posi >= Number.MAX_SAFE_INTEGER)  /* arithmetic overflow? */
+        return lauxlib.luaL_error(L, "string slice too long");
+
+    let n = (pose - posi) + 1;
+    lauxlib.luaL_checkstack(L, n, "string slice too long");
+    for (let i = 0; i < n; i++)
+        lapi.lua_pushinteger(L, s[posi + i - 1]);
+    return n;
+};
+
 const strlib = {
-    "char":  str_char,
-    "len":   str_len,
-    "lower": str_lower,
-    "rep": str_rep,
+    "byte":    str_byte,
+    "char":    str_char,
+    "len":     str_len,
+    "lower":   str_lower,
+    "rep":     str_rep,
     "reverse": str_reverse,
-    "upper": str_upper
+    "upper":   str_upper
 };
 
 const createmetatable = function(L) {